JP2011151913A - Power supply circuit and lighting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a power loss in a power supply circuit. <P>SOLUTION: A choke coil L22 (coil) gains an input voltage via a pair of input terminals. A smoothing capacitor C26 is charged with a coil current passing through the choke coil L22. A switching element Q21 (first switching circuit) is placed between the input terminals and the choke coil L22 and interrupts a voltage applied from the input terminals to the choke coil L22. A rectifying device D23 is placed between the choke coil L22 and the smoothing capacitor C26 and interrupts a current flowing from the choke coil L22 to the smoothing capacitor C26 when the switching element Q21 is in conduction. A switching element Q25 (second switching circuit) is electrically connected in parallel with the rectifying device D23, and interrupts a current charging the smoothing capacitor C26 from the choke coil L22 when the rectifying device D23 is interrupted. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a power supply circuit for driving a load circuit having a light emitting diode or the like.

A load circuit having an LED or the like needs constant current driving. In addition, electrical products that consume more than a certain amount of power are subject to harmonic regulation in order to suppress power supply distortion of commercial power supplies.
For this reason, a two-stage power supply circuit is used in which a power factor correction circuit is provided at the front stage and a constant current drive circuit is provided at the rear stage.
In order to reduce the power loss of the power supply circuit, a one-stage power supply circuit having a circuit that has both a function as a power factor correction circuit and a function as a constant current drive circuit has been proposed.

Special table 2008-537459 gazette

Power loss in the power supply circuit can be reduced by configuring the power supply circuit in one stage, but in order to prevent global warming, reduce carbon dioxide emissions, and save energy, power loss in the power supply circuit Further reduction is required.
The present invention has been made, for example, in order to solve the above-described problems, and an object thereof is to further reduce power loss in a one-stage power supply circuit.

A power supply circuit according to the present invention includes:
A pair of input terminals for inputting an input voltage;
A coil to which an input voltage input by the pair of input terminals is applied;
A smoothing capacitor charged by a coil current flowing through the coil;
A first switching circuit that is interposed between the pair of input terminals and the coil and blocks a voltage applied to the coil from the pair of input terminals;
A rectifying element that is interposed between the coil and the smoothing capacitor and cuts off a current flowing from the coil to the smoothing capacitor when the first switching circuit is in a conductive state;
A second switching circuit that is electrically connected in parallel with the rectifying element, and that cuts off a current that charges the smoothing capacitor from the coil when the rectifying element is in an interrupted state;
And a pair of output terminals for outputting a capacitor voltage charged in the smoothing capacitor.

  According to the power supply circuit of the present invention, since the current flowing when the rectifying element is in the conductive state is diverted to the second switching circuit, power loss in the rectifying element can be reduced.

FIG. 2 is a system configuration diagram illustrating an example of an overall configuration of a lighting apparatus 800 according to Embodiment 1. FIG. 3 is a circuit configuration diagram illustrating an example of a circuit configuration of the lighting device 800 in Embodiment 1. FIG. 3 is a waveform diagram showing an example of voltage values / current values of each part of the step-up / down circuit 120 according to the first embodiment. FIG. 3 shows an example of an equivalent circuit of an nMOSFET used as a switching element Q25 in the first embodiment. FIG. 5 is a diagram illustrating an example of a voltage-current characteristic between a source and a drain of an nMOSFET used as a switching element Q25 in the first embodiment. FIG. 6 is a circuit configuration diagram illustrating another example of the control signal generation circuit 125 according to the first embodiment. FIG. 7 is a waveform diagram showing an example of voltage values at various parts of the control signal generation circuit 125 shown in FIG. 6. FIG. 6 is a circuit configuration diagram illustrating an example of a circuit configuration of a lighting device 800 in Embodiment 2.

Embodiment 1 FIG.
The first embodiment will be described with reference to FIGS.

FIG. 1 is a system configuration diagram showing an example of the overall configuration of an illumination device 800 according to this embodiment.
The lighting device 800 receives power from an AC power source AC such as a commercial power source, and turns on the light source by the input power. The light source is a light emitting element such as a light emitting diode (hereinafter referred to as “LED”) or organic electroluminescence (hereinafter referred to as “organic EL”).
The lighting device 800 includes a power supply circuit 100 and a light source circuit 810.
The power supply circuit 100 converts power input from the AC power supply AC into power supplied to the light source circuit 810. The light source circuit 810 (load circuit) receives the supply of power converted by the power supply circuit 100 and turns on the light source. For example, the power supply circuit 100 supplies the light source circuit 810 with electric power that is direct current and has a desired current value.

The power supply circuit 100 includes a full-wave rectifier circuit 110, a step-up / step-down circuit 120, and a current detection circuit 150.
The full-wave rectifier circuit 110 (rectifier circuit) performs full-wave rectification on the power input from the AC power supply AC and converts it into a pulsating voltage waveform.
The step-up / step-down circuit 120 converts power having a pulsating waveform voltage converted by the full-wave rectifier circuit 110 into power supplied to the light source circuit 810. The step-up / down circuit 120 is, for example, a buck-boost converter circuit.
The current detection circuit 150 detects a current value of power supplied from the step-up / down circuit 120 to the light source circuit 810.

The buck-boost circuit 120 has two roles. One is a role as a power factor correction circuit. The other is a role as a constant current drive circuit.
The power factor correction circuit is a circuit whose purpose is to bring the power factor of the power input by the power supply circuit 100 close to 1. The step-up / step-down circuit 120 makes the power factor of the power input by the power supply circuit 100 close to 1 by making the current waveform of the power input by the power supply circuit 100 a waveform that approximates the voltage waveform.
Further, the step-up / step-down circuit 120 serves as a constant current driving circuit, and sets the current value of power supplied to the light source circuit 810 to a desired value. The step-up / down circuit 120 adjusts the voltage value of the power to be converted based on the current value detected by the current detection circuit 150 so that the current value of the power supplied to the light source circuit 810 becomes a desired value.

FIG. 2 is a circuit configuration diagram illustrating an example of a circuit configuration of the illumination device 800 according to this embodiment.
The light source circuit 810 is a circuit in which a plurality of light sources 811 such as LEDs and organic ELs are electrically connected in series. For example, the light source circuit 810 is a circuit in which 24 LEDs having a forward drop voltage of 3V are electrically connected in series. In this case, in order to light the LED, it is necessary to apply a voltage of 72 V across the light source circuit 810.
The brightness of the light source 811 changes depending on the current flowing through the light source circuit 810. In order to light the light source 811 with a desired brightness, the current flowing through the light source circuit 810 is set to a value corresponding to the brightness. The relationship between the current flowing through the light source circuit 810 and the voltage across the light source circuit 810 varies depending on the temperature characteristics of the light source 811 and the ambient temperature. Further, the forward voltage drop of each light source 811 varies. For this reason, the light source circuit 810 is operated not by constant voltage drive but by constant current drive.
The light source circuit 810 may be configured to be detachably connected to the power supply circuit 100, for example, as a light emitting element unit, or may be configured to be fixed to the power supply circuit 100.

  In the power supply circuit 100, the full-wave rectifier circuit 110 has four terminals. Two of the four terminals are AC side terminals. The AC side terminal of the power supply circuit 100 is electrically connected to the AC power supply AC. The remaining two terminals are pulsating flow side terminals. The two terminals on the pulsating side have a distinction between a positive voltage side and a negative voltage side. The power supply circuit 100 includes, for example, a diode bridge DB. The diode bridge DB is a circuit in which four rectifier elements such as diodes are bridge-connected.

The current detection circuit 150 detects a current flowing through the light source circuit 810. The current detection circuit 150 generates a voltage proportional to the detected current. The current detection circuit 150 outputs the generated voltage (hereinafter referred to as “current detection voltage”). The current detection circuit 150 includes a current detection resistor R51, for example. The current detection resistor R51 is provided at a position where it is electrically connected in series with the light source circuit 810, for example. Thereby, the same current as the current flowing through the light source circuit 810 flows through the current detection resistor R51. A voltage proportional to the flowing current is generated at both ends of the current detection resistor R51. The current detection circuit 150 outputs the voltage across the current detection resistor R51 as a current detection voltage.
The current detection circuit 150 is not limited to the current detection resistor R51, and may be a circuit configured using a semiconductor element and other electronic components.
Further, the current detection circuit 150 may be configured not to directly detect a current flowing through the light source circuit 810 but to detect a current for charging the smoothing capacitor C26 from the choke coil L22, for example. The current for charging the smoothing capacitor C26 from the choke coil L22 varies according to the instantaneous value of the voltage of the full-wave rectified by the full-wave rectifier circuit 110 and the on / off of the switching element Q21. By averaging over a longer period (milliseconds to 20 milliseconds), the same value as the current flowing through the light source circuit 810 can be obtained.

  The step-up / step-down circuit 120 includes, for example, a harmonic current cutoff capacitor C11, a control IC 121, a DC / DC conversion circuit 122, a feedback circuit 123, two voltage dividing resistors R31 and R32, an input current detection resistor R33, and an auxiliary winding L41.

  The harmonic current cutoff capacitor C11 is electrically connected between the pulsating current side terminals of the diode bridge DB. The electrostatic capacitance of the harmonic current cutoff capacitor C11 is very small as the frequency of the AC power supply AC (for example, 50 Hz [Hz] or 60 Hz), and the switching frequency of the switching element Q21 described later (for example, several tens to several hundreds of kilohertz). [KHz]) is a very large capacitance, for example, several thousand picofarads [pF] to 1 microfarad [μF]. The harmonic current cut-off capacitor C11 cuts off the high-frequency current generated by the switching operation of the switching element Q21 and prevents leakage to the AC power supply AC side.

The control IC 121 (control circuit) controls the operation of the DC / DC conversion circuit 122.
The direct current direct current conversion circuit 122 converts electric power whose voltage waveform is direct current or pulsating current into electric power whose voltage waveform is direct current in accordance with control by the control IC 121. The DC-DC conversion circuit 122 can set the voltage value of the power to be converted to a voltage value higher than the voltage value (or peak value) of the input power, or can be set to a lower voltage value. The DC / DC conversion circuit 122 is, for example, a buck-boost converter circuit. The DC / DC conversion circuit 122 includes, for example, a switching element Q21, a choke coil L22, a rectifying element D23, an auxiliary winding L71, a switching element Q25, and a smoothing capacitor C26.

The switching element Q21 is repeatedly turned on and off at a high frequency in accordance with an instruction from the control IC 121. Switching element Q21 has three terminals. One of the three terminals is a terminal (hereinafter referred to as “control input terminal”) for inputting a control signal instructing on / off of the switching element Q21. The remaining two terminals are terminals (hereinafter referred to as “controlled terminals”) that conduct or block between the two terminals in accordance with a control signal input by the control input terminal. One of the two controlled terminals of the switching element Q21 is electrically connected to one of the pulsating flow side terminals of the diode bridge DB.
The switching element Q21 is, for example, an enhancement type nMOS field effect transistor (hereinafter referred to as “FET”). That is, the drain terminal and the source terminal of the FET are controlled terminals, and the gate terminal is a control input terminal. The control signal is represented by a voltage applied to the gate terminal. The reference potential of the control signal is the potential of the source terminal. The nMOSFETs are connected so that the potential of the drain terminal is higher than the potential of the source terminal. For example, if the pulsating current side terminal of the diode bridge DB is electrically connected to the negative voltage side terminal, the source terminal of the nMOSFET is electrically connected to the diode bridge DB. On the contrary, if the pulsating current side terminal of the diode bridge DB is electrically connected to the positive voltage side terminal, the drain terminal of the nMOSFET is electrically connected to the diode bridge DB.
Note that the switching element Q21 is not limited to an enhancement type FET, but may be a depletion type FET, or is not limited to an nMOSFET, and may be a pMOSFET. Further, the switching element Q21 is not limited to a MOSFET but may be another FET such as a junction FET. Further, the switching element Q21 is not limited to an FET, and may be an npn-type or pnp-type bipolar transistor, or may be another electronic component. The switching element Q21 is not limited to an electronic switch such as a transistor, but may be a switch including a relay and other mechanical elements.

The choke coil L22 has two terminals (hereinafter referred to as “coil terminals”). One of the two coil terminals is electrically connected to one of the pulsating flow side terminals of the diode bridge DB (which is not electrically connected to the switching element Q21). The other one of the two coil terminals is electrically connected to one of the controlled terminals of the switching element Q21 (the one not electrically connected to the diode bridge DB). Thus, a closed circuit (hereinafter referred to as “input-side closed circuit”) is constituted by the diode bridge DB (its pulsating current side terminal), the switching element Q21 (its controlled terminal), and the choke coil L22. To do.
An auxiliary winding L71 and an auxiliary winding L41 are wound around the iron core around which the choke coil L22 is wound. The choke coil L22 (hereinafter also referred to as “main winding” or “coil”), the auxiliary winding L71, and the auxiliary winding L41 constitute one transformer.

  Smoothing capacitor C26 has two terminals (hereinafter referred to as “capacitor terminals”). One of the two capacitor terminals is electrically connected to one of the coil terminals of the choke coil L22. The smoothing capacitor C26 is, for example, an electrolytic capacitor. The smoothing capacitor C26 is a capacitor for smoothing the voltage applied to the light source circuit 810. The capacitance of the smoothing capacitor C26 is, for example, several hundred to several thousand microfarads [μF]. Since the output voltage is further smoothed by increasing the capacitance of the smoothing capacitor C26, the fluctuation of the output voltage due to the voltage fluctuation of the commercial power supply frequency 50/60 Hz can be reduced.

The rectifying element D23 has two terminals (hereinafter referred to as “rectifying terminals”). The rectifying element D23 is an element having a property that current flows only in one direction. The rectifying element D23 is, for example, a diode. That is, the anode terminal and the cathode terminal of the diode are two rectification terminals. In the diode, current flows from the anode terminal side to the cathode terminal side, but does not flow from the cathode terminal side to the anode terminal side.
One of the two rectifying terminals is electrically connected to the coil terminal of the choke coil L22 (which is not electrically connected to the smoothing capacitor C26). The other of the two rectifying terminals is electrically connected to the capacitor terminal (not electrically connected to the choke coil L22) of the smoothing capacitor C26. Thus, a closed circuit (hereinafter referred to as an “output side closed circuit”) is constituted by the choke coil L22, the rectifying element D23, and the smoothing capacitor C26.

  The direction of the rectifying element D23 is a direction that blocks when the switching element Q21 is on. The direction of the smoothing capacitor C26 is a direction in which the smoothing capacitor C26 is charged with a current flowing through the rectifying element D23.

The switching element Q25 is electrically connected in parallel with the rectifying element D23. The switching element Q25 conducts when the rectifying element D23 is on, and shuts off when the rectifying element D23 is off. That is, the switching element Q25 is turned on and off in the same manner in synchronization with the rectifying element D23. Thereby, the electric current which flows through the rectifier D23 is reduced, and the power loss in the rectifier D23 is reduced. The two controlled terminals of the switching element Q25 are electrically connected to the two rectifying terminals of the rectifying element D23, respectively. Note that the switching element Q25 may be configured to be always turned off when the rectifying element D23 is turned off. For example, the switching element Q25 may be turned on later than the rectifying element D23 after the rectifying element D23 is turned on, or may be turned off earlier than the rectifying element D23 before the rectifying element D23 is turned off.
The switching element Q25 is, for example, an enhancement type nMOSFET. A positive voltage or a negative voltage may be applied between the controlled terminals of the switching element Q25. The source terminal of the nMOSFET is electrically connected to the anode terminal of the rectifying element D23. The drain terminal of the nMOSFET is electrically connected to the cathode terminal of the rectifying element D23. In the nMOSFET, when the potential of the source terminal is higher than the potential of the drain terminal, a diode formed at the boundary between the p region of the sub slate and the n region to which the drain terminal is connected conducts and current flows. Further, in order to prevent the MOSFET from being destroyed by this current, the potential of the source terminal is higher than the potential of the drain terminal separately from the diode formed at the boundary between the p region of the sub slate and the n region to which the drain terminal is connected. There may be a built-in protective diode that conducts when high. Therefore, the source terminal is electrically connected to the side to be conducted when the potential is high, and the drain terminal is electrically connected to the side to be conducted when the potential is low.
Switching element Q25 may be a pMOSFET instead of an nMOSFET. Further, the rectifying element D23 may be a diode built in the MOSFET, or may be a separate component from the switching element Q25.

  The auxiliary winding L71 detects the voltage across the choke coil L22 and generates a control signal for turning on and off the switching element Q25. As described above, the switching element Q25 is turned on / off in synchronization with the rectifying element D23 by the control signal generated by the auxiliary winding L71. The auxiliary winding L71 is configured to have a polarity opposite to that of the main winding (choke coil L22).

Based on the current detection voltage generated by the current detection circuit 150, the feedback circuit 123 generates a feedback signal indicating whether to increase or decrease the voltage value of the power converted by the step-up / down circuit 120. The feedback circuit 123 includes, for example, a reference voltage source V61, an error amplifier A62, and a photocoupler PC.
The reference voltage source V61 (reference power supply) generates a reference voltage for determining whether the current flowing through the light source circuit 810 is larger or smaller than the current detection voltage generated by the current detection circuit 150. To do.
The error amplifier A62 (error amplifier) compares the current detection voltage generated by the current detection circuit 150 with the reference voltage generated by the reference voltage source V61, and indicates a larger signal (hereinafter referred to as “feedback signal”). .) Is generated.
The photocoupler PC transmits a feedback signal while being electrically insulated. The control IC 121 receives the feedback signal transmitted from the photocoupler PC.

  Note that the error amplifier A62 may have an integration function. By averaging with a cycle longer than the commercial frequency 50/60 Hz by the integration function, a desired current can be flowed to the light source circuit 810 on average even if the output voltage varies.

  The two voltage dividing resistors R31 and R32 are electrically connected in series with each other, and are electrically connected between the pulsating flow side terminals of the diode bridge DB. The voltage dividing resistors R31 and R32 divide the voltage waveform of the electric power converted and output by the diode bridge DB, and convert it to a voltage level that can be input to the control IC 121. The control IC 121 inputs the voltage across the voltage dividing resistor R32 as a signal (hereinafter referred to as “input voltage signal”) representing the voltage instantaneous value (waveform information) of the power input by the step-up / step-down circuit 120.

  The auxiliary winding L41 detects the voltage across the choke coil L22 and generates a signal (hereinafter referred to as “zero current detection signal”) that is input to the control IC 121. The control IC 121 receives the zero current detection signal generated by the auxiliary winding L41. The auxiliary winding L41 is configured to have a polarity opposite to that of the main winding (choke coil L22).

  The input current detection resistor R33 detects a current flowing through the switching element Q21 and generates a signal (hereinafter referred to as “input current detection signal”) input to the control IC 121. The control IC 121 receives the input current detection signal generated by the input current detection resistor R33.

Based on the input signal group (feedback signal, input voltage signal, zero current detection signal, input current detection signal), the control IC 121 switches the switching element so that the step-up / step-down circuit 120 operates as a power factor correction circuit / constant current drive circuit. A control signal for instructing on / off of Q21 is generated. As the control IC 121, for example, a commercially available control IC for a power factor correction circuit (PFC circuit) is used.
The control IC 121 switches the switching element so that the waveform of the current flowing into the DC / DC conversion circuit 122 (the line connecting the envelope of the input current detection signal and the current peak value for each switching) is similar to the waveform of the input voltage. Adjust the timing to turn on and off Q21. A waveform similar to the waveform of the input voltage is a waveform in which the waveform excluding the harmonic component substantially matches the input voltage. The harmonic component of the input current is removed by the common mode filter (not shown) and the harmonic current cutoff capacitor C11 and does not go out of the power supply circuit 100. Therefore, the waveform of the current flowing into the DC / DC conversion circuit 122 ( If the envelope of the input current detection signal and the line connecting the current peak values for each switching are made similar to the waveform of the input voltage, the power factor of the power input by the power supply circuit 100 becomes close to 1.
The control IC 121 generates a control signal that turns on the switching element Q21. The control IC 121 detects the instantaneous input voltage based on the input voltage signals input from the two voltage dividing resistors R31 and R32, and the value of the input current detected by the input current detection resistor R33 (envelope of the input current detection signal) The length of the period during which the switching element Q21 is turned on is set so that the line or the line connecting the current peak values for each switching becomes a value proportional to the input voltage. The control IC 121 generates a control signal for turning off the switching element Q21 after the set period has elapsed. Based on the zero current detection signal input from the auxiliary winding L41, the control IC 121 determines whether or not the current flowing through the choke coil L22 has become zero. The control IC 121 operates the DC / DC conversion circuit 122 in the critical mode. That is, the control IC 121 does not turn on the switching element Q21 until the current flowing through the choke coil L22 becomes zero. After determining that the current flowing through the choke coil L22 has become 0, the control IC 121 generates a control signal for turning on the switching element Q21. The control IC 121 basically repeats this.

Further, the control IC 121 adjusts the voltage value of the electric power converted by the DC / DC conversion circuit 122 based on the feedback signal input from the feedback circuit 123 so that the current flowing through the light source circuit 810 becomes a desired current value.
The current flowing through the light source circuit 810 is determined by the voltage value of the converted power.
When the current flowing through the light source circuit 810 is larger than the desired current value, the control IC 121 decreases the average value of the current flowing into the DC / DC conversion circuit 122 and decreases the voltage value of the electric power converted by the DC / DC conversion circuit 122. . For example, the control IC 121 reduces the overall length of the period during which the switching element Q21 is turned on. Since the current flowing into the DC / DC conversion circuit 122 increases in proportion to the length of the period during which the switching element Q21 is on, the DC / DC conversion circuit 122 can be reduced by shortening the period during which the switching element Q21 is on. The current flowing into the becomes smaller. That is, the control IC 121 decreases the proportionality constant while maintaining the proportional relationship between the instantaneous value of the input voltage and the maximum input current in one on period. As a result, the on-duty is reduced and the average value of the current flowing into the DC / DC conversion circuit 122 is reduced.
When the current flowing through the light source circuit 810 is less than the desired current value, the control IC 121 conversely increases the average value of the current flowing into the DC / DC conversion circuit 122 and converts the voltage of the power converted by the DC / DC conversion circuit 122. Increase the value. For example, the control IC 121 increases the overall length of the period during which the switching element Q21 is turned on.

FIG. 3 is a waveform diagram showing an example of the voltage value / current value of each part of the step-up / down circuit 120 in this embodiment.
The horizontal axis represents time. The vertical axis represents voltage or current. Note that the horizontal scales of the curves 711 and 712 and the curves 721 to 735 are different. The curves 721 to 735 are enlarged and displayed for a time shorter than the curves 711 and 712. This is because the frequency is greatly different.

A curve 711 represents a voltage waveform of power input from the AC power supply AC by the power supply circuit 100. The voltage waveform of the power input from the power supply circuit 100 from the AC power supply AC is, for example, a sine wave. The frequency is, for example, 50 Hz or 60 Hz. The effective value is, for example, 85 volts [V] to 265V.
A curve 712 represents a voltage waveform of power that is full-wave rectified by the full-wave rectifier circuit 110. The voltage waveform of the electric power that is full-wave rectified by the full-wave rectifier circuit 110 is a pulsating current. The minimum value is, for example, 0V. The maximum value is, for example, 120V to 375V.

Curves 721a and 721b represent the voltage across the choke coil L22. Curves 722a and 722b represent the voltage across the smoothing capacitor C26. Curves 731a and 731b represent the current flowing through switching element Q21. Curves 732a and 732b represent the current flowing through the choke coil L22. Curves 733a and 733b represent the total value of currents flowing through the rectifying element D23 and the switching element Q25. Curves 734a and 734b represent currents that charge and discharge the smoothing capacitor C26. Curves 735a and 735b represent the current flowing through the light source circuit 810.
Curves 721a to 735a represent when the instantaneous value of the voltage of the pulsating current that is full-wave rectified by the full-wave rectifier circuit 110 is relatively high. Curves 712b to 735b represent when the instantaneous value of the voltage of the pulsating current that has been full-wave rectified by the full-wave rectifier circuit 110 is relatively low.

At time t ₁ (or time t ₅ ), the control IC 121 generates a control signal that turns on the switching element Q21. According to the control signal generated by the control IC 121, the switching element Q21 is turned on.
A pulsating voltage that is full-wave rectified by the full-wave rectifier circuit 110 is applied to both ends of the choke coil L22. Accordingly, the voltage across the choke coil L22 varies depending on the instantaneous value of the pulsating voltage that is full-wave rectified by the full-wave rectifier circuit 110.
The current flowing through the choke coil L22 gradually increases from zero. The increasing rate (differentiation) of the current flowing through the choke coil L22 is proportional to the voltage across the choke coil L22. Therefore, if the instantaneous value of the pulsating voltage obtained by full-wave rectification by the full-wave rectifier circuit 110 is high, the current flowing through the choke coil L22 increases rapidly, and if it is low, it increases slowly.
The smoothing capacitor C26 is charged with a voltage having a polarity opposite to that of the full-wave rectified circuit 110. For this reason, regardless of whether the voltage charged in the smoothing capacitor C26 is higher or lower than the instantaneous value of the full-wave rectified voltage by the full-wave rectifier circuit 110, the rectifier element D23 is off. Therefore, the switching element Q25 is also off. The currents flowing through the rectifying element D23 and the switching element Q25 are both 0, and all the current flowing through the choke coil L22 flows through the switching element Q21.
As a result, a coil current flows through the loop of harmonic current cutoff capacitor C11 (full wave rectifier circuit 110) → main winding (choke coil L22) → switching element Q21 so as to charge the main winding with electromagnetic energy.

At time t ₂ (or time t ₆ ), the current flowing through the choke coil L22 reaches a value proportional to the instantaneous value of the pulsating voltage that is full-wave rectified by the full-wave rectifier circuit 110, and therefore the input current detection resistor R33 Based on the input current detection signal detected by the control IC 121, the control IC 121 generates a control signal for turning off the switching element Q21. The switching element Q21 is turned off in accordance with the control signal generated by the control IC 121.
When the switching element Q21 is turned off, the current flowing through the choke coil L22 loses its place and is likely to decrease rapidly. Back electromotive force is generated at both ends of the choke coil L22, and exceeds the voltage at both ends of the smoothing capacitor C26. The rectifying element D23 is turned on, and the current flowing through the choke coil L22 flows through the rectifying element D23 (and switching element Q25) as it is.
The current that flows through the rectifying element D23 (and the switching element Q25) flows in the direction in which the smoothing capacitor C26 is charged.
A voltage across the smoothing capacitor C26 is applied across the choke coil L22. The polarity of the voltage across, and the period from time t ₁ to t ₂ because it is opposite, the current flowing through the choke coil L22 is gradually decreased. The decreasing rate (differentiation) of the current flowing through the choke coil L22 is proportional to the voltage across the choke coil L22. Therefore, if the voltage charged in the smoothing capacitor C26 is high, the current flowing through the choke coil L22 decreases quickly, and if it is low, it decreases slowly. If the voltage charged in the smoothing capacitor C26 is the same, the current flowing through the choke coil L22 has the same speed (same slope) regardless of the instantaneous value of the pulsating voltage that is full-wave rectified by the full-wave rectifier circuit 110. Decrease. Therefore, the time taken for the current flowing through the choke coil L22 to become zero is proportional to the value of the current flowing through the choke coil L22 at time t ₂ (or time t ₆ ).
As a result, the coil current that releases the electromagnetic energy accumulated in the main winding (choke coil L22) in the loop of the main winding (choke coil L22) → diode (rectifier element D23) and switching element Q25 → smoothing capacitor C26 is the return current. Flowing as.

At time t ₃ (or time t ₇ ), when the current flowing through the choke coil L22 becomes 0, the rectifying element D23 (and switching element Q25) is turned off. Although the reverse current is about to flow through the choke coil L22, since the switching element Q21 and the rectifying element D23 (and the switching element Q25) are both off, there is no current supply source and the current flowing through the choke coil L22 remains zero. Become. Since the current flowing through the choke coil L22 does not change, there is no back electromotive force, and the voltage across the choke coil L22 becomes zero.

Since the auxiliary winding L41 is wound so as to have a polarity opposite to that of the choke coil L22, a positive voltage is generated when the coil current returns from the choke coil L22 to the smoothing capacitor C26.
The voltage of the zero current detection signal generated by the auxiliary winding L41 is proportional to the voltage across the choke coil L22. The control IC 121 determines that the return current flowing through the choke coil L22 has ended when the potential of the zero current detection signal generated by the auxiliary winding L41 becomes 0 or less than a threshold value close thereto. Thereafter, at time t ₄ (or time t ₈ ), the control IC 121 generates a control signal for turning on the switching element Q21 again.

The step-up / step-down circuit 120 repeats this to charge the smoothing capacitor C26. Thus, the period when the step-up / step-down circuit 120 turns on / off the switching element Q21 varies depending on the situation and is not constant.
A voltage charged in the smoothing capacitor C26 is applied to both ends of the light source circuit 810. A current determined by the applied voltage flows through the light source circuit 810. The current flowing through the smoothing capacitor C26 is a current corresponding to the difference obtained by subtracting the current flowing through the light source circuit 810 from the current flowing through the rectifying element D23 (and the switching element Q25). The smoothing capacitor C26 is charged if the current flowing through the rectifying element D23 (and the switching element Q25) is larger than the current flowing through the light source circuit 810, and the current flowing through the light source circuit 810 causes the rectifying element D23 (and switching element Q25) to flow. If it is larger than the flowing current, the smoothing capacitor C26 is discharged. When the instantaneous value of the pulsating voltage rectified by the full-wave rectifier circuit 110 is high, the amount of charge is larger, and when it is low, the amount of discharge is larger. If the capacitance of the smoothing capacitor C26 is sufficiently large, the voltage across the smoothing capacitor C26 is stabilized at a voltage that balances the amount of charge and the amount of discharge on average. At this time, the voltage between both ends of the smoothing capacitor C26 may be larger or smaller than the maximum value of the pulsating voltage that is full-wave rectified by the full-wave rectifier circuit 110.
The current flowing through the light emitting element unit (light source circuit 810) flows in a loop of the smoothing capacitor C26 → the light emitting element unit → the current detection circuit 150 regardless of whether the switching element Q21 is on or off.

  The control IC 121 adjusts the timing for turning on and off the switching element Q21 so that the current flowing through the light source circuit 810 becomes a desired current value, and changes the voltage charged in the smoothing capacitor C26. The voltage across the smoothing capacitor C26 may be larger or smaller than the maximum value of the pulsating voltage that is full-wave rectified by the full-wave rectifier circuit 110. For example, it is assumed that when the voltage charged in the smoothing capacitor C26 is 200 V, the current flowing through the light source circuit 810 has a desired current value. When the effective voltage of the AC power supply AC is 85V, the maximum value is about 120V, so the step-up / step-down circuit 120 performs a step-up operation to generate a DC voltage of 200V. Further, when the effective voltage of the AC power supply AC is 265V, the maximum value is about 375V, so the step-up / step-down circuit 120 performs a step-down operation to generate a DC voltage of 200V. The step-up / step-down circuit 120 can perform both step-up and step-down operations only by adjusting the timing at which the switching element Q21 is turned on / off.

FIG. 4 is a diagram showing an example of an equivalent circuit of the nMOSFET used as the switching element Q25 in this embodiment.
In an equivalent circuit of an nMOSFET, a variable resistor and a diode whose resistance value changes based on a potential difference between a gate terminal G and a source terminal S are connected in parallel between the drain terminal D and the source terminal S. .

FIG. 5 is a diagram showing an example of the voltage-current characteristics between the source and drain of the nMOSFET used as the switching element Q25 in this embodiment.
The horizontal axis represents the source-drain voltage. The vertical axis represents the drain current.
A curve 741 represents a voltage-current characteristic when an off voltage is applied between the gate and the source. A curve 742 represents a voltage-current characteristic when an on-voltage is applied between the gate and the source.
In the region where the drain potential is higher than the source potential, the voltage-current characteristic at the time of off is substantially a straight line, and the slope thereof is off resistance. The voltage-current characteristics at the on time can be approximated by a straight line 743 in the linear region, and the slope is the on-resistance. The on-resistance is, for example, several milliohms [mΩ] to several hundred mΩ. When the drain potential is lower than the source potential, in the region where the diode between the source and the drain is turned on, the potential difference between the source and the drain is determined by the forward voltage drop of the diode regardless of the on-resistance and the off-resistance. On the other hand, even when the drain potential is lower than the source potential, when the potential difference between the source and drain is lower than the forward voltage drop of the diode, the diode is turned off, and the characteristics are almost the same as the region where the drain potential is higher than the source potential. Indicates.

For this reason, even when the same drain current flows in the reverse direction, the source-drain voltage 752 when the on-voltage is applied between the gate and the source is greater than the source-drain when the off-voltage is applied between the gate and the source. It becomes smaller than the inter-voltage (forward voltage drop 751).
The forward voltage drop 751 is determined by the energy level difference of the pn junction of the diode, and is about 0.6V to 1V, for example. On the other hand, for example, when the on-resistance is 50 mΩ and the drain current is, for example, 500 milliamps [mA], the source-drain voltage at the on time is 25 millivolts [mV].

The forward drop voltage of the rectifying element D23 is also about the same, for example, about 0.6V to 1V. When the rectifying element D23 is on and the switching element Q25 is off, the voltage across the rectifying element D23 is equal to the forward voltage drop. The power loss in the rectifying element D23 is the product of the both-end voltage and the flowing current. For example, when the forward voltage drop of the rectifying element D23 is 1 V and the flowing current is 500 mA, the power loss in the rectifying element D23 is 0.5 watt [W].
On the other hand, when the switching element Q25 is turned on, the voltage at both ends when the same current of 500 mA is applied is lowered to 25 mV, so the total power loss in the rectifying element D23 and the switching element Q25 is 12.5 milliwatts [mW]. Become.

  Thus, the switching element Q25 is provided in parallel with the rectifying element D23, and is turned on / off in synchronization with the rectifying element D23. As a result, a large current bypasses the rectifying element D23 and flows through the switching element Q25. By preventing a large current from flowing through a path (rectifier element D23) having a pn junction, power loss can be reduced.

The auxiliary winding L71 generates a control signal that instructs the switching element Q25 to turn on and off. The control signal means, for example, on when it is a positive voltage higher than a predetermined voltage, and off when it is not.
The auxiliary winding L71 is coupled to the choke coil L22 by electromagnetic induction. A voltage proportional to the voltage across the choke coil L22 is generated at both ends of the auxiliary winding L71. The voltage ratio is determined by the turn ratio. The polarity of the voltage is determined by the winding direction of the winding. In this example, the auxiliary winding L71 is wound in such a direction that a voltage having a polarity opposite to that of the both ends of the choke coil L22 is generated at both ends of the auxiliary winding L71.
Thus, during the period from time t ₂ in FIG. 3 to time t ₃ (or from time t ₆ to time t _7), the voltage across the auxiliary winding L71 is a positive voltage proportional to the voltage across the smoothing capacitor C26 become. That is, during this period, the auxiliary winding L71 generates a control signal for turning on the switching element Q25. As a result, the switching element Q25 is turned on when the rectifying element D23 is on.
In other periods, the voltage across the auxiliary winding L71 is 0 or a negative voltage. That is, in the other period, the auxiliary winding L71 generates a control signal for turning off the switching element Q25. Thereby, the switching element Q25 is turned off when the rectifying element D23 is turned off.

  Thus, by turning on / off the switching element Q25 in synchronization with the rectifying element D23, the power loss can be reduced without changing the operation of the entire step-up / down circuit 120.

  For comparison, consider a case where the on / off control of the switching element Q25 is performed in synchronization with the switching element Q21, not in synchronization with the rectifying element D23. That is, the switching element Q25 is turned on when the switching element Q21 is off, and the switching element Q25 is turned off when the switching element Q21 is on. In such a manner, a signal obtained by inverting the control signal generated by the control IC 121 for controlling the switching element Q21 by a pulse transformer or the like can be used as the control signal for the switching element Q25.

The method of the comparative example works well if the operation mode of the DC / DC conversion circuit 122 is a continuous mode. However, this embodiment cannot be applied when the operation mode of the DC / DC conversion circuit 122 is a critical mode. This is because the switching element Q25 remains on while the switching element Q21 is off, and therefore does not turn off even at time t ₃ (or time t ₇ ) in FIG. 3, and at time t ₄ (or time t ₈ ). Turned off for the first time. As a result, the current flowing through the choke coil L22 becomes 0 at time t ₃ (or time t ₇ ) and then further decreases and flows in the reverse direction until time t ₄ (or time t ₈ ). This current passes through the switching element Q25 and discharges the smoothing capacitor C26. That is, the energy accumulated in the smoothing capacitor C26 flows backward, resulting in poor energy efficiency.

On the other hand, in the system of this embodiment, the switching element Q25 is turned off at time t ₃ (or time t ₇ ), so that there is no back flow of energy. For this reason, the energy efficiency of the whole power supply circuit 100 becomes high by reducing the power loss in the rectifying element D23.

In the DC / DC conversion circuit 122 of the power supply circuit 100 in this embodiment, a pair of input terminals (one of the coil terminals of the choke coil L22 and one of the controlled terminals of the switching element Q21) inputs an input voltage. The main winding (choke coil L22) is applied with the input voltage input by the pair of input terminals. The smoothing capacitor C26 is charged by a coil current flowing through the main winding. The first switching circuit (switching element Q21) is interposed between the pair of input terminals and the main winding, and cuts off a voltage applied to the main winding from the pair of input terminals. The rectifying element D23 is interposed between the main winding and the smoothing capacitor C26, and cuts off a current flowing from the main winding to the smoothing capacitor C26 when the first switching circuit is conductive. The second switching circuit (switching element Q25) is electrically connected in parallel with the rectifying element D23, and when the rectifying element D23 is in a cut-off state, cuts off a current for charging the smoothing capacitor C26 from the main winding. A pair of output terminals (two capacitor terminals of the smoothing capacitor C26) outputs a capacitor voltage charged in the smoothing capacitor C26.
As a result, the current flowing when the rectifying element D23 is in a conductive state is diverted to the second switching circuit, so that power loss in the rectifying element D23 can be reduced. If the power loss of the second switching circuit (power consumed by the on-resistance) is smaller than the power loss of the rectifying element D23, the power loss of the entire power supply circuit 100 can be reduced.

One terminal of the main winding (choke coil L22) is electrically connected to one of the pair of input terminals. In the first switching circuit (switching element Q21), one terminal is electrically connected to the input terminal of the pair of input terminals that is not electrically connected to the main winding. In the first switching circuit, the other terminal is electrically connected to a terminal that is not electrically connected to the pair of input terminals of the main winding. The first switching circuit cuts off the conduction. One terminal of the smoothing capacitor C26 is electrically connected to one of the terminals of the main winding and one of the pair of output terminals. The other end of the smoothing capacitor C26 is electrically connected to the other output terminal of the pair of output terminals. One terminal of the rectifying element D23 is electrically connected to a terminal that is not electrically connected to the smoothing capacitor C26 of the main winding. The other terminal of the rectifying element D23 is electrically connected to the terminal that is not electrically connected to the main winding of the smoothing capacitor C26. The rectifying element D23 is electrically connected in such a direction that no current flows when the first switching circuit is in a conducting state.
One terminal of the second switching circuit (switching element Q25) is electrically connected to one terminal of the rectifying element D23. The other terminal of the second switching circuit is electrically connected to the other terminal of the rectifying element D23. The second switching circuit shuts off when the rectifying element D23 is in a shut-off state.
As a result, the current flowing when the rectifying element D23 is in a conductive state is diverted to the second switching circuit, so that power loss in the rectifying element D23 can be reduced. If the power loss of the second switching circuit (power consumed by the on-resistance) is smaller than the power loss of the rectifying element D23, the power loss of the entire power supply circuit 100 can be reduced.

In the second switching circuit (switching element Q25), the auxiliary winding L71 is wound so that a voltage having a polarity opposite to that of the main winding (choke coil L22) is generated. The main winding is charged with energy by a coil current that flows when the first switching circuit (switching element Q21) is conducting.
The energy charged in the main winding charges the smoothing capacitor C26 through the rectifying element D23 as a coil current that flows when the first switching circuit (switching element Q21) is cut off.
At this time, the auxiliary winding L71 generates 0 or a negative voltage when the first switching circuit (switching element Q21) is conductive, and a positive voltage when it is cut off.
The voltage generated in the auxiliary winding L71 is input to the second switching circuit (switching element Q25).
Accordingly, the switching element Q25 can be turned on / off in synchronization with the on / off of the rectifying element D23.

In addition, the second switching circuit (switching element Q25) is configured such that when the voltage generated at both ends of the auxiliary winding L71 is equal to or higher than a predetermined threshold voltage, the smoothing capacitor C26 from the main winding (choke coil L22) Conducting current to charge.
Accordingly, the switching element Q25 can be turned on / off in synchronization with the on / off of the rectifying element D23.

Based on the input voltage signal detected by the voltage dividing resistors R31 and R32 and the input current detection signal detected by the input current detection resistor R33, the control IC 121 has a waveform of the coil current flowing through the main winding (choke coil L22). The first switching circuit (switching element Q21) is controlled so that (envelope curve connecting peak values) has a waveform similar to the input voltage.
Thereby, the power factor of the power supply circuit 100 can be made close to 1.

The control IC 121 inputs a signal (zero current detection signal) for detecting that the voltage of the auxiliary winding L41 is equal to or less than a threshold value close to 0, whereby the coil current from the main winding (choke coil L22) is input. It is determined that the recirculation is completed, and the first switching circuit (switching element Q21) is controlled to be turned on again.
Thereby, the step-up / step-down circuit 120 operates in the critical mode, and the power loss due to the reverse recovery current in the rectifying element D23 can be reduced.

The control IC 121 is applied to the coil from the pair of input terminals so that the value of the output current flowing out from the pair of output terminals becomes a predetermined value based on the feedback signal generated by the error amplifier A62. The first switching circuit is controlled by changing the length of the period during which the voltage is conducted, and the output voltage is adjusted.
Accordingly, an output voltage that allows a predetermined output current to flow through the light source circuit 810 can be generated, and the power supply circuit 100 performs a constant current control operation. Therefore, it is not necessary to provide a constant current control circuit in the subsequent stage. Therefore, power loss in the power supply circuit 100 can be reduced.

In the lighting device 800, the full-wave rectification circuit 110 performs full-wave rectification on the AC power from the AC power supply AC and inputs it to the pair of input terminals of the power supply circuit (buck-boost circuit 120). The light source circuit 810 includes a light source 811 that is turned on by power supplied from the power supply circuit. The light source circuit 810 is electrically connected between a pair of output terminals of the power supply circuit.
Thus, both the power factor correction operation and the constant current control operation can be performed with a single-stage configuration of the step-up / step-down circuit 120. Since the circuit configuration of the power supply circuit 100 is simplified and the number of parts of the power supply circuit 100 is reduced, the manufacturing cost of the lighting device 800 can be suppressed, and the reliability of the lighting device 800 is improved. Further, since power loss in the power supply circuit 100 is reduced, the energy efficiency of the lighting device 800 is increased.

The power supply circuit 100 described above supplies power to the load circuit (light source circuit 810) operated with a constant current. The power supply circuit 100 includes a rectifier circuit (full-wave rectifier circuit 110), a transformer (choke coil L22, auxiliary winding L71, auxiliary winding L41), a first switching element Q21, a control circuit (control IC 121), It has a diode (rectifier element D23), a second switching element Q25, a capacitor (smoothing capacitor C26), and a current detection circuit 150.
The rectifier circuit rectifies a commercial power supply (AC power supply AC) to generate an input voltage.
The transformer has a main winding (choke coil L22) and two auxiliary windings L71 and L41.
The switching element Q21 is connected in series with the rectifier circuit and the main winding. The switching element Q21 is driven by a control circuit. The switching element Q21 applies an input voltage to both ends of the main winding during the ON period.
The current detection circuit 150 detects a current flowing through the load circuit or the main winding and transmits current information to the control circuit.
The control circuit switches the input voltage so that the envelope of the peak or average current waveform of the current flowing through the first switching element Q21 approaches the voltage waveform of the input voltage by dividing or directly inputting the input voltage. While performing the on / off control of the element Q21, the current of the load circuit is subjected to constant current control based on the current information detected by the current detection circuit 150.
The main winding causes a coil current to flow so as to charge electromagnetic energy while the first switching element Q21 is on. The main winding allows a coil current to flow so as to emit electromagnetic energy when the first switching element Q21 is off. The main winding transmits power from the input voltage to the load circuit side by turning on / off the first switching element Q21.
The diode is connected in series with the main winding and the capacitor. The diode causes the coil current discharged from the main winding to flow back to the capacitor when the first switching element Q21 is off.
The capacitor is charged when the coil current circulates while the first switching element Q21 is off.
The second switching element Q25 is connected in parallel with the diode. The second switching element Q25 performs synchronous rectification of the diode by turning on the section in which the coil current discharged from the main winding is recirculated.

When the coil current discharged from the main winding (choke coil L22) finishes circulating, the voltage waveform of the auxiliary winding L41 decreases. The control IC 121 detects the timing, and turns on the switching element Q21 thereafter.
Further, the control IC 121 uses the input current detection resistor R33 to make the peak value of the current flowing through the switching element Q21 so that the current flowing through the switching element Q21 has a full-wave rectified sine wave shape same as the input voltage. Or the average value of the peak value is controlled.
Therefore, the operating frequency of the switching element Q21 is not constant. For example, the lower the full-wave rectified sine wave voltage, the lower the frequency (longer cycle), and the higher the sine wave voltage, the higher the frequency (shorter cycle). The operating frequency of the switching element Q21 also depends on the voltage of the commercial power supply (AC power supply AC) to be input. Since the inductance value of the main winding is a fixed value, the current increases in a shorter time as the voltage at both ends is higher. For this reason, the operating frequency is high (the cycle is short).
In this way, a power supply circuit in which the operating frequency varies depending on the operation of the control IC determining the operating frequency of the switching element by determining the end of the current flow, that is, the operation of the power supply circuit itself, is generally used. This is called excitation oscillation.
The power supply circuit 100 is a self-oscillation circuit. The power supply circuit 100 determines the operating frequency of the switching element by detecting the end of the return current based on the voltage generated in the auxiliary winding L41.

  The diode (rectifier element D23) is connected to the main winding (choke coil L22) side in the direction of the anode terminal and the smoothing capacitor C26 side in the direction of the cathode terminal. As a result, the diode passes only a current in a direction in which the energy charged in the main winding is returned to the smoothing capacitor C26 in one way. The diode serves as a free wheeling diode that returns the coil current discharged from the main winding to the smoothing capacitor C26 while the switching element Q21 is off. In addition, since the diode is connected in this way, even when the energy charged in the main winding is small at the timing when the input sine wave voltage is low, the energy can be sent in one way, and the main winding Even after the energy is released, it does not flow backward.

The auxiliary winding L71 is connected to the gate terminal of the switching element Q25 used as a synchronous rectification semiconductor element, and the other is connected to the source terminal. That is, the switching element Q25 is connected in parallel with the diode.
Similar to the auxiliary winding L41, the auxiliary winding L71 of the transformer has a polarity opposite to that of the main winding. For this reason, a positive voltage is generated in the auxiliary winding L71 while the switching element Q21 is off. The voltage value generated in the auxiliary winding L71 is determined by the current value flowing through the main winding and the turn ratio of the main winding and the auxiliary winding L71.
Since the input voltage input to the power supply circuit 100 is a sine wave voltage that is full-wave rectified by the full-wave rectifier circuit 110, the input voltage value varies depending on the timing when the switching element Q21 is turned on in the cycle. For example, the input voltage value periodically varies at a frequency of 50 Hz or 60 Hz.
Therefore, the voltage value applied to the main winding is different depending on the timing when the switching element Q21 is turned on, and the current value flowing through the main winding is also different.
In a section where the full-wave rectified sine wave voltage is high, the voltage value applied to the main winding and the flowing current value increase. In the voltage value section where the sine wave voltage is low, the voltage value applied to the main winding and the flowing current value decrease. Further, near the peak of the sine wave voltage, the voltage value applied to the main winding and the flowing current value are maximized.

  Since the main winding and the auxiliary winding L71 have opposite polarities, the switching element Q21 is turned on and the section opposite to the section in which the current flows through the main winding, that is, the switching element Q21 is turned off. A positive voltage is generated in the auxiliary winding L71 in a section where the coil current discharged from the main winding through the diode to the smoothing capacitor C26 is circulating. The voltage value generated in the auxiliary winding L71 is determined by the winding ratio between the main winding and the auxiliary winding L71 and the voltage value applied to the main winding, and varies according to the period of the sine wave voltage. In a section where the full-wave rectified sine wave voltage is high, the voltage value generated in the auxiliary winding L71 increases. In the voltage value interval where the sine wave voltage is low, the voltage value generated in the auxiliary winding L71 decreases. Further, the voltage value generated in the auxiliary winding L71 is maximized near the peak of the sine wave voltage. As described above, the auxiliary winding L71 generates a voltage that changes according to the full-wave rectified sinusoidal voltage in the off-period of the switching element Q21.

The switching element Q25 inputs a voltage generated in the auxiliary winding L71. Switching element Q25 is turned on only when the input voltage exceeds the gate threshold voltage of the MOSFET. Switching element Q25 remains off when the input voltage is lower than the gate threshold voltage.
In a section where the full-wave rectified sine wave voltage is high, that is, in a section where a large amount of current flows through the main winding, the switching element Q25 is turned on, bypassing the current flowing through the diode (rectifier element D23). In a section where the full-wave rectified sine wave voltage is low, that is, a section where the current flowing through the main winding is small (or hardly flows), the switching element Q25 remains off, bypassing the current flowing through the diode. do not do.
As a result, when the full-wave rectified sine wave voltage is high, a large amount of current flows through the main winding, and a large amount of return current flows through the diode, synchronous rectification is performed. Also, when the full-wave rectified sine wave voltage is low and a small amount of current flows through the main winding and the return current flowing through the diode is small, synchronous rectification is not performed. Synchronous rectification can be effectively performed when the current is large.

  By supplying the gate voltage applied to the switching element Q25 from the voltage generated in the auxiliary winding L71 that changes depending on the value of the current flowing in the main winding, the section in which there is a large amount of current flowing in the voltage range not higher than necessary is the gate. The gate voltage can be lowered in the section where the voltage is high and the flowing current is small.

By using a MOSFET as the switching element Q25, when the gate voltage is increased, the on-resistance between the drain and the source is lowered, so that the electrical loss is further reduced.
Since the time required to turn off increases as the gate voltage increases, the turn ratio is set so as not to increase more than necessary. Thereby, it can be turned off at a timing until the return current ends. Since the current decreases as the full-wave rectified sine wave voltage decreases, the gate voltage decreases following the current. For this reason, the switching element Q25 is easily turned off. In the section where the full-wave rectified sine wave voltage is low, the current further decreases, so that the gate voltage is further reduced. Since the gate threshold voltage is not exceeded, switching element Q25 is not turned on. As a result, an unnecessary gate voltage is not applied to the switching element Q25, so that the time until turning off can be shortened. The switching element Q25 is turned off at the timing when the return of the coil current discharged from the main winding is finished, and has the same function as the original function of the diode that functions as the return diode.

In addition, since the diode (rectifier element D23) is connected in parallel with the switching element Q25, switching is performed earlier than the timing when the diode acting as the free wheel diode turns off, that is, the timing when the discharge of the coil current from the main winding ends. Element Q25 may be turned off.
When the switching element Q25 is a MOSFET, the diode (rectifier element D23) may not be provided. This is because the body diode of the MOSFET itself can be used.

  Synchronous rectification is stopped by turning off the switching element Q25 at the timing when the return of the coil current discharged from the main winding ends (or earlier). Since the operation is the same as the timing when the original diode (rectifier element D23) is turned off, it is possible to prevent the current from flowing backward from the smoothing capacitor C26 to the main winding.

The power supply circuit 100 operates while controlling the waveform and phase of the current flowing from the commercial power supply (AC power supply AC) so as to match the full-wave rectified sine wave voltage. A lighting fixture (lighting device 800) that consumes a certain amount of power is subject to harmonic regulation. The power supply circuit 100 can improve the electric efficiency by keeping the harmonics within the regulation value and bringing the power factor close to 1.
The switching element Q25 is turned on at the timing when the switching element Q21 is turned off in a section where the full-wave rectified sine wave voltage is high. In the section where the sine wave voltage is low, the ON operation is not performed even when the switching element Q21 is turned OFF. The switching element Q25 is driven by a voltage generated in the auxiliary winding L41 in accordance with the current flowing in the main winding. Thereby, without applying a driving voltage more than necessary, it is possible to turn on in a section where the current of the main winding is large and not turn on in a section where the current is small. Since the switching element Q25 is not driven with a voltage higher than necessary, the switching element Q25 is turned off until the coil current discharged from the main winding to the smoothing capacitor C26 finishes circulating. Accordingly, the timing at which the switching element Q25 is turned off is delayed, so that the current flows even though the coil current discharged from the main winding to the smoothing capacitor C26 has circulated, that is, from the output side to the input side. It is possible to prevent the occurrence of a reverse flow condition.

The control IC 121 uses the fact that the coil current released from the main winding (choke coil L22) recirculates and the voltage of the auxiliary winding L41 decreases as the recirculation current decreases, thereby terminating the recirculation current. To detect. The control IC 121 turns on the switching element Q21 after a while. Thereby, the step-up / step-down circuit 120 operates in a critical mode which is a boundary state where the continuous mode and the discontinuous mode are switched.
When the full-wave rectified sine wave voltage is in a high section, the voltage generated in the auxiliary winding L71 is high and exceeds the gate threshold voltage of the switching element Q25. For this reason, the switching element Q25 is turned ON. Since the return current flowing through the diode (rectifier element D23) is bypassed toward the switching element Q25, the electric efficiency is improved.
When the full-wave rectified sine wave voltage is in a low section, the voltage generated in the auxiliary winding L71 is low and does not exceed the gate threshold voltage of the switching element Q25. For this reason, the switching element Q25 remains off, and a reflux current flows through the diode (rectifier element D23). By stopping the synchronous rectification, the backflow of the current from the smoothing capacitor C26 to the main winding is prevented. Although the return current flows through a diode having a loss larger than that of the MOSFET, since the current flowing in the section where the sine wave voltage is low is small, if the power supply circuit 100 is viewed as a whole, the electrical efficiency is affected without performing synchronous rectification. Don't give.

According to the power supply circuit 100 according to the present invention, the electric efficiency can be improved by simultaneously performing the two functions of power factor improvement and power conversion used in the lighting device 800 and synchronous rectification of the freewheeling diode.
By driving the switching element Q25 by the auxiliary winding L71, synchronous rectification of the return diode (rectifier element D23) is performed only in a section where the voltage is high in the period of the sine wave voltage that has been full-wave rectified.
The current flowing through the main winding (choke coil L22) increases and decreases following the period of the sine wave voltage subjected to full wave rectification. The voltage generated in the auxiliary winding L71 is determined by the current flowing in the main winding and the turn ratio of the main winding and the auxiliary winding L71, and becomes higher in the section where the full-wave rectified sine wave voltage is high, and in the low section At low.
By driving the switching element Q25 with the voltage generated in the auxiliary winding L71, synchronous rectification is performed only in a section where a high current flows in a full-wave rectified sine wave voltage cycle, and full-wave rectification is performed. Synchronous rectification is not performed in a section where the voltage is low and the current flowing in the sine wave voltage cycle is small.
The switching element Q25 is turned off when the coil current discharged from the main winding has been recirculated so that the switching element Q25 is driven at an unnecessarily high voltage and performs the same function as the original freewheeling diode. It is possible to prevent backflow of current flowing from the output side (smoothing capacitor C26) to the input side (main winding).
Thereby, the electrical efficiency of the power supply circuit 100 can be improved.

The turn ratio of the main winding and the auxiliary winding L71 is set so that the voltage generated in the auxiliary winding L71 does not exceed the gate threshold voltage of the switching element Q25 in the section where the full-wave rectified sine wave voltage is low. As a result, the switching element Q25 is turned on in a section where the input full-wave rectified sine wave voltage is high, and the switching element Q25 is turned off in a section where the input full-wave rectified sine wave voltage is low.
Thus, since the voltage applied to the switching element Q25 is generated by the auxiliary winding L71, a voltage more than necessary is not applied to the gate, and the turn-off timing is not delayed. Therefore, while performing synchronous rectification, it is possible to maintain the function of preventing conduction in the reverse direction as a free-wheeling diode that is the role of the original diode. Therefore, even though the coil current discharged from the main winding has been recirculated, the current continues to flow, and the current flows from the smoothing capacitor C26 to the main winding in the reverse direction to charge the main winding. Does not happen.
As a result, power loss due to backflow can be prevented and the current flowing from the main winding to the smoothing capacitor C26 can be bypassed in a section where the current of the diode is large, so that the power loss can be effectively reduced.

The voltage generated in the auxiliary winding L71 is determined by the voltage value applied to the main winding that changes within the period of the full-wave rectified sine wave voltage and the turns ratio of the main winding and the auxiliary winding L71. The voltage generated in the auxiliary winding L71 is higher than the gate threshold voltage of the switching element Q25 in a section where the input full-wave rectified sine wave voltage is high, and the input full-wave rectified sine wave voltage is low. The turn ratio of the main winding and the auxiliary winding L71 is set so as to be lower than the gate threshold voltage of the switching element Q25 in the section.
For example, when the effective voltage of the commercial power supply (AC power supply AC) is 100 V, the input voltage after the full-wave rectifier circuit 110 fluctuates in a range of about 0 to 140 V at a frequency of 50 Hz or 60 Hz. Among these, for example, the turn ratio of the main winding and the auxiliary winding L71 is set so that the voltage generated in the auxiliary winding L71 in the section of 20 to 140 V is higher than the gate threshold voltage of the switching element Q25.
Thereby, it is prevented that the current due to the delay of turning off of the switching element Q25 flows backward from the output side to the input side. Moreover, electrical efficiency can be improved by performing synchronous rectification.

Synchronous rectification of the diode (rectifier element D23) with the switching element Q25 has the following power loss reduction effect.
The power loss of the diode is forward voltage × forward current. For example, if the forward voltage is 1.0 V and the forward current is 0.5 A, the power loss of the diode is 0.5 W.
The power loss of the MOSFET is the on resistance times the square of the current. For example, if the on-resistance is 0.05Ω and the current is 0.5A, the power loss of the MOSFET is 0.0125W.
For this reason, the power loss can be significantly reduced by making another semiconductor element such as a MOSFET conductive instead of the diode and returning the energy stored in the transformer (main winding).

  The control IC 121 turns on the switching element Q21 at the timing when the return of the coil current of the main winding is completed, that is, at the zero cross timing. Thereby, the step-up / step-down circuit 120 can reduce the switching loss in the switching element Q21 and reduce the recovery loss due to the reverse recovery current flowing through the diode (rectifier element D23). Thereby, the electrical efficiency of the power supply circuit 100 can be improved.

  The power supply circuit 100 makes the voltage waveform of the commercial power supply (AC power supply AC) and the current waveform flowing from the commercial power supply to the full-wave rectifier circuit 110 substantially the same waveform, and the power factor approaches 1, so that the electric efficiency is improved. Since the power supply circuit 100 can suppress harmonics, the power supply circuit 100 can be used as a power supply circuit for lighting fixtures that require countermeasures against power supply harmonics. Since the power supply circuit 100 can improve the power factor, it can be used as a power supply circuit for a lighting fixture that requires a high power factor.

  Thus, since the power supply circuit 100 is not a two-stage configuration of the power factor correction circuit and the power conversion circuit for driving the LEDs, but has a single-stage configuration, the power loss of the power supply circuit 100 itself is reduced and the electric efficiency is reduced. Will improve. Further, the power supply circuit 100 can be reduced in size and cost can be reduced. Furthermore, the power supply circuit 100 further improves power supply efficiency by performing synchronous rectification.

FIG. 6 is a circuit configuration diagram showing another example of the control signal generation circuit 125 in this embodiment.
The control signal generation circuit 125 is a circuit that generates a control signal for turning on and off the switching element Q25. In the example described above, the auxiliary winding L71 functions as the control signal generation circuit 125. The control signal generation circuit 125 of this example includes a capacitor C72, a rectifier element D73, and two resistors R74 and R75 in addition to the auxiliary winding L71.
Capacitor C72 passes only the AC component of the input pulsed voltage waveform. The resistor R74 suppresses the peak of current flowing into the gate. The AC component current that has passed through the resistor R74 charges the input capacitance between the gate and source of the switching element Q25. The resistor R75 and the rectifier element D73 (diode) accelerate the extraction of the gate charge when the switching element Q25 is turned off.
The control signal generation circuit 125 uses the voltage across the resistor R75 as a control signal. The switching element Q25 inputs the voltage across the resistor R75 as a control signal. Therefore, when the voltage generated in the auxiliary winding L71 is a positive voltage, the input capacitance of the switching element Q25 is charged. When the voltage across the resistor R75 is higher than a predetermined threshold voltage, the switching element Q25 is turned on. Become. When the voltage generated in the auxiliary winding L71 is a negative voltage, the rectifier element D73 conducts, so that the charge charged in the input capacitance of the switching element Q25 is drawn, and the voltage across the resistor R75 decreases. Switching element Q25 is turned off.

FIG. 7 is a waveform diagram showing an example of voltage values at various parts of the control signal generation circuit 125 shown in FIG.
The horizontal axis represents time. The vertical axis represents voltage.
Curves 723a and 723b represent the voltage across the auxiliary winding L71. Curves 724a and 724b represent the voltage across resistor R75. A straight line 725 represents a threshold voltage at which the switching element Q25 is turned on.

A voltage proportional to the voltage across the choke coil L22 is generated at both ends of the auxiliary winding L71.
When the voltage across the auxiliary winding L71 is positive, the rectifier element D73 is turned off, so that an AC component current flows through the capacitor C72 and charges the input capacitance of the switching element Q25. Switching element Q25 is turned on when the input capacitance is charged and the gate voltage exceeds the threshold voltage. After that, when the charging is finished and the voltage generated in the auxiliary winding L71 is divided by the resistor R74 and the resistor R75, the AC component disappears, so that it cannot pass through the capacitor C72.
When the voltage across the auxiliary winding L71 is negative, the rectifying element D73 is turned on and both ends of the capacitor C72 are short-circuited. The voltage between both ends of the capacitor C72 becomes almost zero, and the voltage between both ends of the resistor R75 becomes a negative voltage proportional to the voltage between both ends of the auxiliary winding L71. Since zero or a negative voltage is applied to the gate terminal, the switching element Q25 is quickly turned off.

When the switching element Q21 is turned off at time t ₂ (or time t ₆ ), the voltage across the choke coil L22 becomes negative, the voltage across the auxiliary winding L71 becomes positive, and the voltage across the resistor R75 is switched. The switching element Q25 is turned on exceeding the threshold voltage of the element Q25.
Thereafter, the voltage across the resistor R75 continues to be charged to the input capacitance of the switching element Q25, and stops at a voltage obtained by dividing the voltage generated in the auxiliary winding L71 by the resistor R74 and the resistor R75.

  As described above, in the configuration of FIG. 6, when the switching element Q25 is on, the voltage is divided by the resistor R74 and the resistor R75, so that an excessive voltage is not applied to the gate, and when the switching element Q25 is off, the rectifier element D73 is conductive. By doing so, the negative voltage is applied quickly. By doing so, the switching element Q25 can be quickly turned off.

As described above, the control signal generation circuit 125 may have a configuration in which the voltage generated at both ends of the auxiliary winding L71 is directly used as a control signal, or is generated based on the voltage generated at both ends of the auxiliary winding L71. A configuration may be adopted in which the controlled voltage is used as a control signal.
For example, the control signal generation circuit 125 may include a constant voltage diode such as a Zener diode, and may be configured to clamp a constant voltage so that a voltage higher than a predetermined voltage value is not applied to the gate terminal of the switching element Q25.
Alternatively, the control signal generation circuit 125 may have a configuration in which the voltage generated at both ends of the auxiliary winding L71 is passed through a pulse transformer or the like, electrically insulated, and applied to the switching element Q25.

  The power supply circuit 100 is a one-stage power supply circuit that simultaneously performs two functions of power factor improvement and power conversion, and can drive the load circuit (light source circuit 810) at a constant current. By performing synchronous rectification of the diode (rectifier element D23) at the timing when the return current flows, the electric efficiency can be further improved.

  In the power supply circuit 100, the transformer has a main winding (choke coil L22) and at least two auxiliary windings L41 and L71. One of the auxiliary windings (auxiliary winding L71) is connected to the second switching element Q25. The auxiliary winding L71 generates a voltage that fluctuates following the current flowing through the main winding. The second switching element Q25 is connected in parallel with the diode (rectifier element D23). The second switching element Q25 is connected to the auxiliary winding L71 only in a section where the sine wave voltage of the input voltage rectified by the full-wave rectifier circuit 110 from the main winding is high, and in the section where the released coil current flows back. By driving with the generated voltage, synchronous rectification of the diode is performed.

  By driving the switching element Q25 by the auxiliary winding L71, synchronous rectification of the return diode is performed only in a section where the full-wave rectified sine wave voltage is high. This prevents the switching element Q25 from slowing off when the freewheeling diode is synchronously rectified without driving the switching element Q25 at an unnecessarily high voltage, and reverses the current flow from the output side to the input side. Can be prevented.

  The lighting device 800 can improve electrical efficiency while improving the power factor by mounting such a power supply circuit.

Embodiment 2. FIG.
The second embodiment will be described with reference to FIG.
In addition, about the part which is common in Embodiment 1, the same code | symbol is attached | subjected and description is abbreviate | omitted.

FIG. 8 is a circuit configuration diagram showing an example of a circuit configuration of lighting apparatus 800 in this embodiment.
Compared with the first embodiment, the positions of the switching element Q21, the rectifying element D23 (and the switching element Q25), and the current detection circuit 150 are different, but this is not an essential difference.
In the first embodiment, the switching element Q21 is electrically connected to the lower potential side than the choke coil L22. In this embodiment, the switching element Q21 is electrically connected to the higher potential side. In any case, the diode bridge DB (the pulsating current side terminal), the switching element Q21 (the controlled terminal thereof), and the choke coil L22 constitute the input side closed circuit.
In the first embodiment, the rectifier element D23 (and the switching element Q25) is electrically connected to the high potential side of the output voltage of the smoothing capacitor C26. In this embodiment, the rectifier element D23 (and switching element Q25) is electrically connected to the low potential side of the output voltage. Connected. In any case, the point that the choke coil L22, the rectifying element D23, and the smoothing capacitor C26 form an output side closed circuit is the same.
Similarly, in the first embodiment, the current detection circuit 150 is electrically connected to the low potential side of the output voltage of the light source circuit 810. However, in this embodiment, the current detection circuit 150 is electrically connected to the high potential side of the output voltage. .

The feedback circuit 123 does not include the photocoupler PC, and includes a reference voltage source V61 and an error amplifier A62. This is because the position of the current detection circuit 150 is different so that the feedback signal generated by the feedback circuit 123 can be directly input to the control IC 121 without using the photocoupler PC.
Further, the step-up / step-down circuit 120 has a level conversion circuit 124. The level conversion circuit 124 changes the potential as a reference of the control signal generated by the control IC 121 and converts it to a level that can be input to the switching element Q21. The level conversion circuit 124 is configured by, for example, a pulse transformer. The level conversion circuit 124 is a circuit that is required because the position of the switching element Q21 is different from that of the first embodiment.

  As in the first embodiment, the switching element Q25 is connected for the purpose of synchronously rectifying the rectifying element D23, and the switching element Q25 is driven by the auxiliary winding L71.

Embodiment 3 FIG.
A third embodiment will be described.

  In the first embodiment, the configuration for generating the control signal from the auxiliary winding L71 of the choke coil L22 has been described as the configuration for generating the control signal for turning on and off the switching element Q25. In the second embodiment, the configuration in which the switching element Q21 is connected to the high potential side has been described. In addition to the configuration described above, for example, the following configuration is conceivable as a configuration for generating a control signal for turning on / off the switching element Q25 different from the first embodiment.

  First, the control signal is generated by detecting the current flowing through the rectifying element D23. For example, a resistor (hereinafter referred to as “rectified current detection resistor”) is electrically connected in series with the rectifier element D23. The current flowing through the rectifying element D23 is detected by the voltage generated across the rectifying current detection resistor. The voltage across the rectified current detection resistor is used as it is, or is amplified by an amplifier circuit or the like, and used as a control signal. Thus, when the rectifying element D23 is turned on, the switching element Q25 is also turned on.

  Second, the control signal is generated by detecting the current flowing through the smoothing capacitor C26. For example, a resistor (hereinafter referred to as “capacitor current detection resistor”) is electrically connected in series with the smoothing capacitor C26. The current flowing through the smoothing capacitor C26 is detected by the voltage generated across the capacitor current detection resistor. The voltage across the capacitor current detection resistor is used as it is or amplified by an amplifier circuit or the like to be used as a control signal. Thereby, the switching element Q25 is turned on during the period in which the smoothing capacitor C26 is charged.

  Third, the control signal is generated by detecting the current flowing through the choke coil L22. For example, a resistor (hereinafter referred to as “coil current detection resistor”) is electrically connected in series with the choke coil L22. The current flowing through the choke coil L22 is detected by the voltage generated across the coil current detection resistor. In the case of this configuration, the choke coil L22 is provided with a mask circuit that masks the control signal during the period when the switching element Q21 is on since the current flows even while the switching element Q21 is on. The mask circuit outputs the voltage across the coil current detection resistor as it is or amplifies it as a control signal only when the switching element Q21 is off. The mask circuit does not output a control signal while the switching element Q21 is on. Thereby, the switching element Q25 is turned on only when a current flows through the choke coil L22 and the switching element Q21 is off.

  In any of the configurations including the configurations described in the first embodiment and the second embodiment, a control signal for turning on / off the switching element Q25 is autonomously generated in the output side closed circuit. As a result, an existing IC for controlling the power factor correction circuit can be used as it is as the control IC 121. Therefore, it is not necessary to develop a new control IC, and the manufacturing cost of the power supply circuit 100 can be suppressed.

In the case where the coil current detection resistor is configured to detect the coil current, the current detection circuit 150 may detect the current flowing through the light source circuit 810 based on the current detected by the coil current detection resistor. For example, an integration circuit that integrates the voltage across the coil current detection resistor is provided. The current detection circuit 150 detects the current flowing through the light source circuit 810 based on the result of integration by the integration circuit. This is because if the current flowing through the choke coil L22 is averaged, a value corresponding to the current flowing through the light source circuit 810 can be obtained. As a result, the coil current detection resistor also serves as the current detection resistor R51, and the current detection resistor R51 is not necessary.
Similarly, the current flowing through the light source circuit 810 can be detected when the rectified current detection resistor and the capacitor current detection resistor are configured to detect the current flowing through the rectifier element D23 and the capacitor current.

  DESCRIPTION OF SYMBOLS 100 Power supply circuit, 110 Full wave rectifier circuit, 120 Buck-boost circuit, 121 Control IC, 122 DC-DC conversion circuit, 123 Feedback circuit, 124 Level conversion circuit, 125 Control signal generation circuit, 150 Current detection circuit, 800 Illumination device, 810 Light source circuit, 811 light source, A62 error amplifier, AC AC power supply, C11 harmonic current cutoff capacitor, C26 smoothing capacitor, C72, C76 capacitor, D23, D73 rectifier, DB diode bridge, L22 choke coil, L41, L71 Auxiliary winding , PC photocoupler, Q21, Q25 switching element, R31, R32 voltage dividing resistor, R33 input current detection resistor, R51 current detection resistor, R74, R75, R77 resistor, V61 reference voltage source.

Claims (8)

A pair of input terminals for inputting an input voltage;
A coil to which an input voltage input by the pair of input terminals is applied;
A smoothing capacitor charged by a coil current flowing through the coil;
A first switching circuit that is interposed between the pair of input terminals and the coil and blocks a voltage applied to the coil from the pair of input terminals;
A rectifying element that is interposed between the coil and the smoothing capacitor and cuts off a current flowing from the coil to the smoothing capacitor when the first switching circuit is in a conductive state;
A second switching circuit that is electrically connected in parallel with the rectifying element, and that cuts off a current that charges the smoothing capacitor from the coil when the rectifying element is in an interrupted state;
And a pair of output terminals for outputting a capacitor voltage charged in the smoothing capacitor. A pair of input terminals;
A pair of output terminals;
A coil having one terminal electrically connected to any one of the pair of input terminals;
One terminal is electrically connected to the input terminal that is not electrically connected to the coil of the pair of input terminals, and the other terminal is not electrically connected to the pair of input terminals of the coil A first switching circuit that electrically connects to and interrupts conduction;
One terminal is electrically connected to one terminal of the coil and one output terminal of the pair of output terminals, and the other terminal is the other output terminal of the pair of output terminals. A smoothing capacitor electrically connected to
One terminal is electrically connected to a terminal of the coil that is not electrically connected to the smoothing capacitor, and the other terminal is electrically connected to a terminal of the smoothing capacitor that is not electrically connected to the coil, A rectifying element electrically connected in a direction in which no current flows when the first switching circuit is in a conductive state;
A second switching circuit in which one terminal is electrically connected to one of the terminals of the rectifying element, the other terminal is electrically connected to the other terminal of the rectifying element, and is cut off when the rectifying element is in a cut-off state. A power supply circuit comprising:   The second switching circuit has a detection circuit that detects a voltage or a current across the coil, the rectifier element, or the smoothing capacitor, and charges the smoothing capacitor from the coil based on a result detected by the detection circuit. The power supply circuit according to claim 1, wherein a current to be cut off is cut off. The coil has an auxiliary winding,
The said 2nd switching circuit interrupts | blocks the electric current which charges the said smoothing capacitor from the said coil, when the voltage which generate | occur | produced at the both ends of the said auxiliary | assistant winding is below a predetermined threshold voltage. The power supply circuit described.   The first switching circuit has a voltage applied to the coil from the pair of input terminals when the value of the coil current flowing through the coil reaches a value proportional to the input voltage between the pair of input terminals. The power supply circuit according to claim 1, wherein the power supply circuit is cut off.   The first switching circuit conducts after the voltage applied to the coil from the pair of input terminals is cut off and the value of the coil current flowing through the coil becomes zero. The power supply circuit according to claim 1.   The first switching circuit changes a length of a period in which a voltage applied to the coil from the pair of input terminals is conducted so that a value of an output current flowing out from the pair of output terminals becomes a predetermined value. The power supply circuit according to any one of claims 1 to 6, wherein the power supply circuit is provided. A power supply circuit according to any one of claims 1 to 7,
Full-wave rectification of alternating-current power from an alternating-current power supply and input to a pair of input terminals of the power supply circuit; and
An illumination device, comprising: a light source that is turned on by power supplied from the power supply circuit; and a light source circuit that is electrically connected between a pair of output terminals of the power supply circuit.

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