JP2014072916A - Power supply device and display apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply device that can suppress noise caused when a load is driven by PWM control, and a display apparatus that has the power supply device.SOLUTION: A power supply section 10 includes a transformer and a switching element, and converts an AC voltage supplied from a commercial power supply 1 to a DC voltage and outputs the converted DC voltage to an LED driver 20. The LED driver 20 PWM-controls the output of the power supply section 10 in accordance with a PWM signal to drive a backlight 110. A control section 30 controls the output of the power supply section 10 in response to an output fluctuation of the power supply section 10. An adjustment section 40 acquires the PWM signal output from a microcomputer 120, and adjusts the responsiveness of the control section 30 in accordance with a duty ratio of the acquired PWM signal.

Description

  The present invention relates to a power supply device that drives a load by PWM control and a display device including the power supply device.

  Many electric devices or electronic devices including a liquid crystal monitor (display device) use a power supply device to supply a required voltage or current to a load. For example, an AC voltage is induced on the secondary side of the transformer by connecting a transistor to the primary side of the switching transformer and switching the transistor by applying a pulse signal with a pulse width controlled to the base of the transistor. A power supply device that supplies a DC voltage to a load by rectifying and smoothing the induced AC voltage is disclosed (Patent Document 1). Such a power supply device can be used for a display device that dimmes the backlight of a liquid crystal monitor using a PWM control method.

  FIG. 9 is a block diagram showing an example of the configuration of a conventional display device 300. As shown in FIG. 9, the power supply device includes a switching transformer 313, and a transistor 314 is connected to the primary side of the transformer 313. A pulse signal having a required pulse width output from the control IC 335 is applied to the base of the transistor 314, and the transistor 314 performs a switching operation at a predetermined frequency. A diode 315 and a smoothing capacitor 316 are connected to the secondary side of the transformer 313, and an LC filter including a coil 320 and a capacitor 330 is connected to the subsequent stage. An LED driver 20 and a backlight 110 are connected to the output side of the power supply device. When the user operates to set the brightness of the liquid crystal monitor, a brightness value corresponding to the user setting is output to the microcomputer 120, and the microcomputer 120 generates a PWM signal corresponding to the input brightness value to generate an LED driver. 20 output.

Utility Model Registration No. 2513741

  In the conventional power supply device, when the backlight 110 is dimmed by the PWM control method, the load current of the power supply device fluctuates greatly. As a result, the transistor 314 performs a switching operation in an attempt to follow the fluctuation of the load current. Therefore, the magnetic flux change of the transformer 313 becomes large, and the core or winding of the transformer 313 vibrates due to the magnetic flux change, and abnormal noise is generated from the transformer 313. For this reason, in order to reduce the abnormal noise generated from the transformer 313, an LC filter is provided at the output end of the power supply device to reduce the change in the current flowing through the transformer 313. However, the parts used in the conventional LC filter are considerably large, and it is difficult to reduce the size of the power supply device.

  The present invention has been made in view of such circumstances, and provides a power supply device capable of suppressing abnormal noise generated when a load is driven by PWM control, and a display device including the power supply device. It is in.

  A power supply device according to a first aspect of the present invention includes a power supply unit, a drive unit that drives a load by PWM-controlling the output of the power supply unit in accordance with a PWM signal, and the power supply unit in response to output fluctuations of the power supply unit. A power supply apparatus including a control unit that controls output includes an adjustment unit that adjusts a response speed of the control unit in accordance with a duty ratio of the PWM signal.

  According to a second aspect of the present invention, there is provided the power supply device according to the first aspect, wherein the power supply unit is connected to the transformer on the secondary side and the primary side of the transformer, and is based on the frequency of the PWM signal. A switching element that repeats an on / off operation at a higher frequency, and the control unit is configured to control an on period of the switching element according to an output variation of the power supply unit, and the adjustment unit includes: According to the duty ratio of the PWM signal, the change rate of the ON period of the switching element controlled by the control unit is adjusted.

  According to a third aspect of the present invention, there is provided the power supply device according to the second aspect, wherein the adjustment unit adjusts the change rate of the on period of the switching element from the on / off time of the PWM signal according to the duty ratio of the PWM signal. It is made to do so.

  According to a fourth aspect of the present invention, there is provided the power supply device according to the third aspect, wherein the adjustment unit includes the switching element when the duty ratio is greater than a first threshold value (<50%) and smaller than a second threshold value (> 50%). The on period from the on / off time of the PWM signal when the duty ratio is smaller than the first threshold value or larger than the second threshold value. It is characterized in that it is adjusted so as to be slower than the rate of change of.

  According to a fifth aspect of the present invention, there is provided a power supply apparatus according to the third aspect, further comprising a smoothing circuit that rectifies and smoothes the PWM signal, and the adjustment unit has a voltage smoothed by the smoothing circuit greater than a first threshold voltage, The rate of change of the ON period from the ON / OFF time point of the PWM signal of the switching element when the voltage is smaller than the threshold voltage (> the first threshold voltage), when the voltage is smaller than the first threshold voltage or the second The PWM signal is adjusted so as to be slower than the rate of change of the ON period from the ON / OFF point of the PWM signal when it is larger than the threshold voltage.

  A display device according to a sixth aspect of the invention includes the power supply device according to any one of the above-described inventions, and a backlight driven by a drive unit of the power supply device.

  In the first invention, the power supply unit, the drive unit that drives the load by PWM control of the output of the power supply unit according to the PWM signal, and the control that controls the output of the power supply unit in response to the output fluctuation of the power supply unit A part. The power supply unit includes, for example, a switching transformer and a switching element. The control unit controls the output so as to respond to the output fluctuation (for example, current fluctuation, voltage fluctuation, etc.) of the power supply section. For example, when the load current of the power supply unit increases, the control unit controls the switching operation of the switching element of the power supply unit to increase the current output from the power supply unit.

  The adjustment unit adjusts the response speed of the control unit in accordance with the duty ratio of the PWM signal. The PWM signal is a signal in which an ON / OFF pulse waveform is repeated at a predetermined frequency, and the duty ratio α can be a ratio of an ON period in one cycle of the PWM signal. When the duty ratio α is, for example, 10%, the current output from the power supply unit is small, and the change in current flowing through the transformer of the power supply unit (for example, the difference between the maximum value and the minimum value of the current waveform) is small. Further, when the duty ratio α is 90%, for example, the current output from the power supply unit is large, but the current change flowing through the transformer of the power supply unit (for example, the difference between the maximum value and the minimum value of the current waveform) is small. On the other hand, when the duty ratio α is, for example, about 50%, the change in current flowing through the transformer of the power supply unit (for example, the difference between the maximum value and the minimum value of the current waveform) becomes large, and the magnetic flux change of the transformer becomes large. It becomes large and causes abnormal noise in the transformer.

  The adjustment unit adjusts the response of the control unit to be delayed when the duty ratio α is such that a change in current flowing through the transformer of the power supply unit increases. By slowing down the response of the control unit, the current output from the power supply unit is prevented from abruptly changing following the output fluctuation of the power supply unit, and the transformer magnetic flux is prevented from changing suddenly. Generation of sound can be reduced. In addition, since an LC filter for reducing the current change flowing through the transformer is not required, relatively large parts used for the LC filter are not required, and the board area for mounting the LC filter can be reduced. The power supply device can be reduced in size and cost.

  In the second invention, the power supply unit is connected to the primary side of the transformer having the drive unit connected to the secondary side, and is turned on / off at a frequency higher than the frequency of the PWM signal. A repeated switching element. The frequency of the PWM signal can be set to 200 Hz, for example, and the operating frequency of the switching element can be set to 100 kHz, for example. The control unit performs control so that the ON period of the switching element is lengthened / shortened according to the magnitude of the output of the power source unit. For example, when the load current of the power supply unit increases, the control unit increases the current output from the power supply unit by extending the ON period of the switching element of the power supply unit.

  The adjustment unit adjusts the change rate of the ON period of the switching element controlled by the control unit according to the duty ratio of the PWM signal. When the duty ratio α is, for example, 10% or 90%, the change in current flowing through the transformer of the power supply unit (for example, the difference between the maximum value and the minimum value of the current waveform) is small, and the transformer Since abnormal noise is small, there is no need to slow down the response of the control unit. Therefore, the adjustment unit does not adjust the change rate of the ON period of the switching element controlled by the control unit. On the other hand, when the duty ratio α is, for example, about 50%, a change in current flowing through the transformer of the power supply unit becomes large, and a change in magnetic flux of the transformer becomes large, causing abnormal noise generation in the transformer. Therefore, in order to slow down the response of the control unit, the adjustment unit adjusts so that the change rate of the ON period of the switching element controlled by the control unit becomes slow. By slowing down the change rate of the ON period of the switching element, the current flowing in the transformer is suppressed from increasing rapidly, preventing the transformer magnetic flux from changing suddenly and reducing the occurrence of abnormal noise. can do.

  In the third invention, the adjustment unit adjusts the changing speed of the ON period of the switching element from the ON / OFF time point of the PWM signal according to the duty ratio of the PWM signal. The on / off time of the PWM signal is a time when the power supply unit starts / ends current output to the drive unit, and is a time when output fluctuation of the power supply unit becomes large.

  When the duty ratio α is, for example, about 50%, a change in current flowing through the transformer of the power supply unit becomes large, a change in magnetic flux of the transformer becomes large, and abnormal noise is generated in the transformer. Therefore, in order to slow down the response of the control unit, the adjustment unit adjusts the change rate of the ON period of the switching element controlled by the control unit from the time when the PWM signal is turned on / off. Slowing the change rate of the ON period of the switching element prevents the current flowing in the transformer from increasing suddenly, prevents the transformer magnetic flux from changing suddenly, and reduces the occurrence of abnormal noise. can do.

  In the fourth aspect of the invention, the adjustment unit is configured to start from the ON / OFF time point of the PWM signal of the switching element when the duty ratio is larger than the first threshold value (<50%) and smaller than the second threshold value (> 50%). The change rate of the ON period is adjusted to be slower than the change rate of the ON period from the ON / OFF point of the PWM signal when the duty ratio is smaller than the first threshold value or larger than the second threshold value. The first threshold value Th1 can be set to a value such as 30% or 40%, for example. The second threshold Th2 can be set to a value such as 70% or 60%, for example. As a result, when the duty ratio α (Th1 <α <Th2) is such that the magnetic flux change of the transformer becomes large, the change rate of the on period of the transistor at the time of turning on / off the PWM signal is slowed down. It is possible to suppress a sudden increase in the current flowing through the transformer, to prevent the magnetic flux of the transformer from changing abruptly, and to reduce the occurrence of abnormal noise.

  In the fifth invention, the smoothing circuit rectifies and smoothes the PWM signal. That is, the voltage smoothed by the smoothing circuit is high or low according to the duty ratio of the PWM signal. The adjusting unit changes the on period from the on / off time of the PWM signal of the switching element when the voltage smoothed by the smoothing circuit is larger than the first threshold voltage and smaller than the second threshold voltage (> first threshold voltage). Is adjusted to be slower than the rate of change of the on period from the on / off time point of the PWM signal when the voltage is smaller than the first threshold voltage or larger than the second threshold voltage. The first threshold voltage corresponds to the first threshold Th1 of the duty ratio, and the second threshold voltage corresponds to the second threshold Th2 of the duty ratio. Thereby, it can suppress that the electric current which flows into a transformer increases rapidly, it can prevent that the magnetic flux of a transformer changes rapidly, and generation | occurrence | production of abnormal noise can be reduced.

  According to the sixth aspect of the invention, it is possible to provide a display device that can suppress abnormal noise that occurs when the load is driven by PWM control.

  ADVANTAGE OF THE INVENTION According to this invention, the noise which generate | occur | produces from a power supply device resulting from PWM control can be suppressed in the power supply device which drives a load by PWM control, and a display apparatus provided with this power supply device.

It is a block diagram which shows an example of a structure of the display apparatus of this Embodiment. It is explanatory drawing which shows an example of the circuit structure of the display apparatus of this Embodiment. It is explanatory drawing which shows the relationship between duty ratio (alpha) of a PWM signal, and the magnetic flux change of the transformer of a power supply part. It is a schematic diagram which shows the relationship between a PWM signal and the switching waveform of a transistor. It is explanatory drawing which shows an example of operation | movement of an adjustment part. It is explanatory drawing which shows an example of the pulse signal applied to the base of a transistor in case the duty ratio (alpha) of a PWM signal is (alpha) <Th1 or (alpha)> Th2. It is explanatory drawing which shows an example of the pulse signal applied to the base of a transistor in case the duty ratio (alpha) of a PWM signal is Th1 <(alpha) <Th2. It is explanatory drawing which shows an example of the switching waveform whose duty ratio (alpha) of a PWM signal is 50% vicinity. It is a block diagram which shows an example of a structure of the conventional display apparatus.

  Hereinafter, a power supply device and a display device according to the present invention will be described with reference to the drawings illustrating embodiments. FIG. 1 is a block diagram illustrating an example of a configuration of a display device 200 according to the present embodiment, and FIG. 2 is an explanatory diagram illustrating an example of a circuit configuration of the display device 200 according to the present embodiment. As shown in FIG. 1, the display device 200 includes a power supply device 100, a backlight 110, a microcomputer 120, a liquid crystal panel 130, and the like. The power supply apparatus 100 includes a power supply unit 10, an LED driver 20 as a drive unit, a control unit 30, an adjustment unit 40, and the like.

  When the user performs a brightness setting operation to set the brightness of the liquid crystal panel 130, a brightness value corresponding to the user setting is input to the microcomputer 120. The microcomputer 120 generates a PWM signal corresponding to the input brightness value, and outputs the generated PWM signal to the LED driver 20 and the adjustment unit 40.

  As will be described later, the power supply unit 10 includes a switching transformer 13 and a transistor 14 as a switching element, converts an AC voltage supplied from the commercial power supply 1 into a DC voltage, and converts the converted DC voltage to the LED driver 20. Output to. Details of the power supply unit 10 will be described later.

  The LED driver 20 drives the backlight 110 as a load by PWM-controlling the output of the power supply unit 10 according to the PWM signal. That is, the backlight 110 can be dimmed by the PWM control method.

  The control unit 30 controls the output of the power supply unit 10 to respond to output fluctuations (for example, current fluctuation, voltage fluctuation, etc.) of the power supply unit 10. For example, when the load current of the power supply unit 10 increases, the control unit 30 controls the switching operation of the switching element of the power supply unit 10 to increase the current output from the power supply unit 10. That is, the control unit 30 feeds back an output (for example, current, voltage, etc.) of the power supply unit 10 and controls the switching operation of the power supply unit 10. Details of the control unit 30 will be described later.

  The adjustment unit 40 acquires the PWM signal output from the microcomputer 120 and adjusts the response speed of the control unit 30 according to the duty ratio of the acquired PWM signal. The PWM signal is a signal in which an ON / OFF pulse waveform is repeated at a predetermined frequency, and the duty ratio α can be a ratio of an ON period in one cycle of the PWM signal. The frequency of the PWM signal can be, for example, 200 Hz, but is not limited thereto.

  FIG. 3 is an explanatory diagram showing the relationship between the duty ratio α of the PWM signal and the change in magnetic flux of the transformer 13 of the power supply unit 10. The magnetic flux change is a magnetic flux change of the transformer 13 that is caused by fluctuations in the load current. 3A, 3B, and 3C schematically show how the PWM signal and the magnetic flux change of the transformer 13 of the power supply unit 10 change when the duty ratio α of the PWM signal is 10%, 50%, and 90%, respectively.

  As shown in FIG. 3A, when the duty ratio α is, for example, 10%, the current output from the power supply unit 10 is small, and the change in current flowing through the transformer 13 of the power supply unit 10 (for example, the maximum value of the current waveform) The difference from the minimum value is small. Therefore, the difference between the maximum value and the minimum value of the magnetic flux of the transformer 13 is also small.

  As shown in FIG. 3C, when the duty ratio α is 90%, for example, a large amount of current is output from the power supply unit 10, but a change in current flowing through the transformer 13 of the power supply unit 10 (for example, the current waveform) The difference between the maximum and minimum values is small. Therefore, the difference between the maximum value and the minimum value of the magnetic flux of the transformer 13 is also small.

  On the other hand, as shown in FIG. 3B, when the duty ratio α is about 50%, for example, the change in the current flowing through the transformer 13 of the power supply unit 10 (for example, the maximum value and the minimum value of the current waveform) (Difference) increases, and the magnetic flux change of the transformer 13 increases, causing abnormal noise in the transformer 13. The 10%, 50%, and 90% described above are examples, and the numerical value of the duty ratio α is not limited to these.

  The adjustment unit 40 adjusts the response of the control unit 30 to be slow when the duty ratio α is such that a change in the current flowing through the transformer 13 of the power supply unit 10 increases. By slowing down the response of the control unit 30, the current output from the power supply unit 10 is suppressed from changing rapidly following the output fluctuation of the power supply unit 10, and the magnetic flux of the transformer 13 is changed rapidly. This can prevent the occurrence of abnormal noise. In addition, since an LC filter for reducing the change in the current flowing through the transformer 13 is not required, relatively large components used for the LC filter are not required, and the board area for mounting the LC filter can be reduced. Therefore, it is possible to reduce the size and cost of the power supply device.

  Next, the power supply device 100 of the present embodiment will be described in detail. As shown in FIG. 2, the power supply unit 10 includes a rectifying element 11, a capacitor 12, a switching transformer 13, a transistor 14 as a switching element, a diode 15, a capacitor 16, and the like.

  The control unit 30 includes resistors 31, 32, a photocoupler 33 having a light emitting diode 331 and a phototransistor 332, a shunt regulator 34, a control IC 35, a capacitor 36, and the like.

  The adjustment unit 40 includes diodes 41 and 42, resistors 43 and 44, capacitors 45 and 46, resistors 47, 48, 49, 51, 52, 54, 55 and 56, transistors 50, 53 and 58, light emitting diodes 571 and a phototransistor. The photocoupler 57 having the 572, the capacitor 59, and the like are provided.

  First, the power supply unit 10 will be described. As shown in FIG. 2, the collector of the transistor 14 is connected to the primary side of the transformer 13, and the LED driver 20 as a drive unit is connected to the secondary side of the transformer 13 after the diode 15 and the capacitor 16. Is connected. The LED driver 20 supplies required power (current, voltage) to the backlight 110 by performing PWM control on the output of the power supply unit 10 based on the PWM signal from the microcomputer 120.

  The drive terminal D of the control IC 35 is connected to the base of the transistor 14. By outputting to the base of the transistor 14 a pulse signal having a required width which is repeatedly turned on and off at a predetermined frequency (for example, 100 kHz) from the drive terminal D of the control IC 35, the transistor 14 has a frequency higher than the frequency of the PWM signal. The on / off operation is repeated at. Note that the switching operation frequency of the transistor 14 is not limited to 100 kHz, and may be other frequencies.

  FIG. 4 is a schematic diagram showing the relationship between the PWM signal and the switching waveform of the transistor 14. As shown in FIG. 4, the transistor 14 performs a switching operation with a frequency of 100 kHz, for example. The frequency of the PWM signal is, for example, 200 Hz. During the on period of the PWM signal, a current is output to the backlight 110 via the LED driver 20 by the switching operation of the transistor 14. In addition, during the OFF period of the PWM signal, no current is output to the backlight 110 regardless of whether or not the transistor 14 performs a switching operation. During the OFF period of the PWM signal, the switching operation of the transistor 14 may be stopped or the switching operation may be continued.

  Next, the control unit 30 will be described. Resistors 31 and 32 connected in series are connected between the power supply unit 10 and the LED driver 20, and a series circuit of a photocoupler 33 (light emitting diode 331) and a shunt regulator 34 is connected in parallel with the resistors 31 and 32. Connected. The reference of the shunt regulator 34 is connected to the connection node of the resistors 31 and 32.

  The collector of the phototransistor 332 is connected to the feedback terminal FB of the control IC 35, and a capacitor 36 for determining the responsiveness of the control unit 30 is connected to the feedback terminal FB.

  The control IC 35 changes the pulse width of the pulse signal output from the drive terminal D according to the voltage value of the feedback terminal FB. More specifically, the control IC 35 increases the ON width of the pulse signal output from the drive terminal D as the voltage value of the feedback terminal FB is higher. As the ON width of the pulse signal is increased, the ON period of the transistor 14 becomes longer, and the power supply unit 10 can output more current.

  With the above configuration, the control IC 35 can control the ON period of the transistor 14 in accordance with the output fluctuation of the power supply unit 10. For example, when the load current of the power supply unit 10 decreases, the voltage divided by the resistors 31 and 32, that is, the reference voltage of the shunt regulator 34 increases, and the current flowing through the light emitting diode 331 increases. Then, the phototransistor 332 causes a large amount of current to flow out from the feedback terminal FB of the control IC 35 and lowers the voltage value of the FB terminal. Therefore, the on period of the transistor 14 of the power supply unit 10 is shortened and the current output from the power supply unit 10 is reduced. Decrease.

  Note that, during the off period of the PWM signal, the output of the power supply unit 10 is zero or extremely small, so that the current flowing through the light emitting diode 331 increases and the current that the phototransistor 332 flows out from the feedback terminal FB of the control IC 35 increases. , Decrease the voltage value of the FB terminal. In this case, the control IC 35 can stop the switching operation of the transistor 14 or operate it with a very short on-time by reducing the on width of the pulse signal output from the drive terminal D to zero or extremely short.

  Next, the adjustment unit 40 will be described. A series circuit of a diode 41 and a resistor 43 is connected to the output terminal P of the PWM signal of the microcomputer 120, and a capacitor 45 is connected to the subsequent stage of the resistor 43. Similarly, a series circuit of a diode 42 and a resistor 44 is connected to the output terminal P of the PWM signal of the microcomputer 120, and a capacitor 46 and a resistor 47 are connected to the subsequent stage of the resistor 44. The diode 41, the resistor 43, and the capacitor 45 constitute a smoothing circuit. The diode 42, the resistors 44 and 47, and the capacitor 46 also form a smoothing circuit. With this configuration, the PWM signal of the microcomputer 120 is smoothed (integrated) by the capacitors 45 and 46. In the configuration shown in FIG. 2, the output end P of the PWM signal is separated from the output end to the LED driver, but the output end P of the PWM signal may also serve as the output end to the LED driver.

  A base of the transistor 50 is connected to a connection node between the capacitor 46 and the resistor 44 through a resistor 49. A resistor 48 is connected between the base and emitter of the transistor 50. Similarly, the base of the transistor 53 is connected to the connection node between the capacitor 45 and the resistor 43 via the resistor 51. A resistor 52 is connected between the base and emitter of the transistor 53.

  The emitter of the transistor 50 and the collector of the transistor 53 are connected to the base of the transistor 58. A voltage obtained by dividing the required voltage Vcc by resistors 54 and 55 is applied to the base of the transistor 58. A series circuit of a resistor 56 and a photocoupler 57 (light emitting diode 571) is connected to the collector of the transistor 58. A series circuit of a capacitor 59 and a photocoupler 57 (phototransistor 572) is connected to the feedback terminal FB of the control IC 35.

  Next, the operation of the adjustment unit 40 will be described. FIG. 5 is an explanatory diagram showing an example of the operation of the adjustment unit 40. FIG. 5 illustrates how the adjustment unit 40 operates in accordance with the duty ratio α of the PWM signal. In FIG. 5, Th1 is a first threshold value and Th2 is a second threshold value. For example, the first threshold Th1 can be set to 30% and the second threshold Th2 can be set to 70%. Alternatively, the first threshold Th1 may be 40% and the second threshold Th2 may be 60%. The values of the first threshold value and the second threshold value are not limited to these numerical values.

  As shown in FIG. 5, when the duty ratio α of the PWM signal output from the microcomputer 120 is small (α <Th1), the voltages V1 and V2 obtained by smoothing the PWM signal are small, so that the transistor 50 is turned on and the transistor 53 is turned on. Turn off. Since the transistor 50 is turned on, the base potential of the transistor 58 is at the reference level (ground level), and the transistor 58 is turned off. Since the transistor 58 is off, the current flowing through the light-emitting diode 571 is zero or extremely small (about the leakage current), and the phototransistor 572 is turned off. For this reason, only the capacitor 36 is connected to the feedback terminal FB of the control IC 35, and the capacitance of the capacitor of the feedback terminal FB becomes relatively small.

  As shown in FIG. 5, when the duty ratio α of the PWM signal output from the microcomputer 120 is large (α> Th2), the voltages V1 and V2 obtained by smoothing the PWM signal are large, the transistor 50 is turned off, and the transistor 53 Is turned on. Since the transistor 53 is turned on, the base potential of the transistor 58 is at the reference level (ground level), and the transistor 58 is turned off. Since the transistor 58 is off, the current flowing through the light-emitting diode 571 is zero or extremely small (about the leakage current), and the phototransistor 572 is turned off. For this reason, only the capacitor 36 is connected to the feedback terminal FB of the control IC 35, and the capacitance of the capacitor of the feedback terminal FB becomes relatively small.

  Further, as shown in FIG. 5, when the duty ratio α of the PWM signal output from the microcomputer 120 is Th1 <α <Th2, the voltages V1 and V2 obtained by smoothing the PWM signal are medium voltages, and the transistor 50 53 are turned off. This case corresponds to a case where the voltage smoothed by the smoothing circuit is larger than the first threshold voltage and smaller than the second threshold voltage (> first threshold voltage). Since both the transistors 50 and 53 are turned off, a voltage obtained by dividing the voltage Vcc is applied to the base of the transistor 58, and the transistor 58 is turned on. Since the transistor 58 is turned on, a current flows through the light emitting diode 571, and the phototransistor 572 is turned on. Therefore, the capacitor 59 is connected to the feedback terminal FB of the control IC 35 in parallel with the capacitor 36, and the capacitance of the capacitor of the feedback terminal FB becomes relatively large.

  When the phototransistor 332 of the photocoupler 33 is turned on, the change rate of the voltage value of the feedback terminal FB becomes slower as the capacitance of the feedback terminal FB of the control IC 35 is larger. The control IC 35 shortens the ON width of the pulse signal output from the drive terminal D as the voltage value of the feedback terminal FB is lower. As the ON width of the pulse signal is shortened, the ON period of the transistor 14 is shortened, and the power supply unit 10 reduces the output current.

  Further, when the phototransistor 332 of the photocoupler 33 is turned on, the change rate of the voltage value of the feedback terminal FB increases as the capacitance of the capacitor of the feedback terminal FB of the control IC 35 decreases. The control IC 35 increases the ON width of the pulse signal output from the drive terminal D as the voltage value of the feedback terminal FB is higher. As the ON width of the pulse signal is increased, the ON period of the transistor 14 is increased, and the power supply unit 10 increases the output current.

  As described above, the adjustment unit 40 adjusts the change rate of the ON period of the transistor 14 controlled by the control unit 30 according to the duty ratio of the PWM signal. When the duty ratio α is, for example, 10% (α <Th1) or 90% (α> Th2), a change in current flowing through the transformer 13 of the power supply unit 10 (for example, the maximum value of the current waveform) (The difference from the minimum value) is small and the noise of the transformer 13 is small, so there is no need to slow down the response of the control unit 30. Therefore, the adjustment unit 40 does not adjust the change rate of the ON period of the transistor 14 controlled by the control unit 30. That is, the capacitor 59 is not connected to the feedback terminal FB of the control IC 35.

  On the other hand, when the duty ratio α is, for example, about 50% (Th1 <α <Th2), a change in current flowing through the transformer 13 of the power supply unit 10 becomes large, and a change in magnetic flux of the transformer 13 becomes large. Cause abnormal noise. Therefore, in order to slow down the response of the control unit 30, the adjustment unit 40 adjusts so that the change rate of the ON period of the transistor 14 controlled by the control unit 30 is slow. That is, the capacitor 59 is connected to the feedback terminal FB of the control IC 35. By slowing down the change rate of the ON period of the transistor 14, it is possible to suppress a sudden increase in the current flowing through the transformer 13, thereby preventing the magnetic flux of the transformer 13 from changing suddenly and generating abnormal noise. Can be reduced.

  Further, the adjustment unit 40 adjusts the change rate of the ON period of the transistor 14 from the ON point of the PWM signal according to the duty ratio of the PWM signal. The time when the PWM signal is turned on is the time when the power supply unit 10 starts to output current to the LED driver 20, and the time when the output fluctuation of the power supply unit 10 increases.

  When the duty ratio α is, for example, about 50% (Th1 <α <Th2), a change in current flowing through the transformer 13 of the power supply unit 10 becomes large, a change in magnetic flux of the transformer 13 becomes large, and a difference in the transformer 13 occurs. Causes sound generation. Therefore, in order to slow down the response of the control unit 30, the adjustment unit 40 adjusts so that the change rate of the ON period of the transistor 14 controlled by the control unit 30 becomes slow from the time when the PWM signal is turned on. By slowing down the change rate of the ON period of the transistor 14, it is possible to suppress a sudden increase in the current flowing through the transformer 13, thereby preventing the magnetic flux of the transformer 13 from changing suddenly and generating abnormal noise. Can be reduced.

  FIG. 6 is an explanatory diagram showing an example of a pulse signal applied to the base of the transistor 14 when the duty ratio α of the PWM signal is α <Th1 or α> Th2, and FIG. 7 shows the duty ratio α of the PWM signal. It is explanatory drawing which shows an example of the pulse signal applied to the base of the transistor in the case of Th1 <α <Th2.

  As shown in FIG. 6, when the duty ratio α of the PWM signal is α <Th1 or α> Th2, as described above, the change in the current flowing through the transformer 13 of the power supply unit 10 is small. Since the abnormal noise is small, there is no need to delay the response of the control unit 30. Therefore, by setting the capacitor 59 not to be connected to the feedback terminal FB of the control IC 35, the adjustment unit 40 can change the change rate of the ON period of the transistor 14 controlled by the control unit 30 from the ON time of the PWM signal at time t0. Do not make adjustments. In this case, the ON width of the pulse signal applied to the base of the transistor 14 is T1, and the load current rises at time t0.

  As shown in FIG. 7, when the duty ratio α of the PWM signal is Th1 <α <Th2, the change in current flowing through the transformer 13 of the power supply unit 10 is large as described above, and Since abnormal noise becomes large, the response of the control unit 30 is delayed. Therefore, by setting the capacitor 59 to be connected to the feedback terminal FB of the control IC 35, the adjustment unit 40 can change the ON period of the transistor 14 controlled by the control unit 30 from the ON point of the PWM signal at time t0. Make adjustments. In this case, the ON width of the pulse signal applied to the base of the transistor 14 is T2 (<T1).

  As described above, the adjustment unit 40 determines the ON period from the ON point of the PWM signal of the transistor 14 when the duty ratio α is larger than the first threshold Th1 (<50%) and smaller than the second threshold Th2 (> 50%). Is adjusted to be slower than the change rate of the ON period from the ON point of the PWM signal when the duty ratio α is smaller than the first threshold value Th1 or larger than the second threshold value Th2. As a result, when the duty ratio α (Th1 <α <Th2) is such that the magnetic flux change of the transformer 13 increases, the change rate of the ON period of the transistor 14 from the ON point of the PWM signal is slowed down. It is possible to suppress a sudden increase in the current flowing through the transformer 13, prevent the magnetic flux of the transformer 13 from changing suddenly, and reduce the occurrence of abnormal noise.

  Similarly, the adjustment unit 40 adjusts the change rate of the ON period of the transistor 14 from the OFF time of the PWM signal according to the duty ratio of the PWM signal. The time when the PWM signal is turned off is the time when the power supply unit 10 finishes outputting the current to the LED driver 20, and the time when the output fluctuation of the power supply unit 10 becomes large.

  When the duty ratio α is, for example, about 50% (Th1 <α <Th2), a change in current flowing through the transformer 13 of the power supply unit 10 becomes large, a change in magnetic flux of the transformer 13 becomes large, and a difference in the transformer 13 occurs. Causes sound generation. Therefore, in order to slow down the response of the control unit 30, the adjustment unit 40 adjusts the rate of change of the ON period of the transistor 14 controlled by the control unit 30 from the time when the PWM signal is turned off. By slowing down the change rate of the ON period of the transistor 14, it is possible to suppress a sudden increase in the current flowing through the transformer 13, thereby preventing the magnetic flux of the transformer 13 from changing suddenly and generating abnormal noise. Can be reduced.

  FIG. 8 is an explanatory diagram showing an example of a switching waveform when the duty ratio α of the PWM signal is around 50%. 8A shows the PWM signal, FIG. 8B shows the conventional switching waveform, and FIG. 8C shows the switching waveform of the present embodiment. As shown in FIG. 8B, in the conventional switching waveform, the switching waveform changes sharply when the PWM signal is turned on / off. On the other hand, in the present embodiment, as shown in FIG. 8C, the change of the switching waveform when the PWM signal is turned on / off is gradual.

  As described above, in the present embodiment, the response performance of the power supply unit 10 is degraded at the point where the output current of the power supply unit 10 changes suddenly, that is, at the point where the magnetic flux of the transformer 13 swings to the maximum and abnormal noise increases (specifically). Specifically, by increasing the capacitance of the feedback terminal FB of the control IC 35, the feedback gain is lowered and the follow-up performance with respect to the output fluctuation is lowered), so that the magnetic flux change of the transformer 13 is softened and abnormal noise is generated. Can be suppressed.

  Further, the magnitude of the magnetic flux change of the transformer 13 of the power supply unit 10 is determined by the backlight control PWM signal, and the response (following performance) to the load fluctuation of the power supply unit 10 is controlled, so that the transformer 13 The change in magnetic flux can be softened and the generation of abnormal noise from the transformer 13 can be suppressed. Further, since the generation of abnormal noise from the transformer 13 can be suppressed (reduced or miniaturized), the LC filter required in the conventional switching power supply can be omitted, and the apparatus can be reduced in size and cost. Can be planned. Note that an LC filter may be provided in the power supply unit 10 also in the present embodiment. In this case, the LC filter can be made smaller than the conventional one.

  In the above-described embodiment, two capacitors are connected to the feedback terminal FB of the control IC 35, and one capacitor is connected or disconnected. However, the number of capacitors is limited to two. However, three or more can be provided. When three or more capacitors are provided, the number of capacitors to be connected can be increased or decreased stepwise according to the duty ratio α of the PWM signal.

  In the above-described embodiment, a database in which relationships such as the brightness value, the PWM signal, and the pulse width of the pulse signal are previously stored may be stored, and the microcomputer 120 may control the switching operation of the transistor 14. . In this case, the microcomputer 120 performs the functions of the control unit 30 and the adjustment unit 40.

10 Power Supply Unit 13 Transformer 14 Transistor (Switching Element)
20 LED driver (drive unit)
30 Control Unit 31, 32 Resistor 33 Photocoupler 34 Shunt Regulator 35 Control IC
36 capacitor 40 adjustment unit 41, 42 diode (smoothing circuit)
43, 44, 47 Resistance (smoothing circuit)
45, 46 Capacitor (smoothing circuit)
59 Capacitor 110 Backlight (Load)
120 microcomputer

Claims (6)

A power supply unit; a drive unit that drives a load by PWM-controlling an output of the power supply unit according to a PWM signal; and a control unit that controls the output of the power supply unit in response to an output fluctuation of the power supply unit. In power supply,
A power supply apparatus comprising: an adjustment unit that adjusts a response speed of the control unit in accordance with a duty ratio of the PWM signal. The power supply unit is
A transformer connected to the drive unit on the secondary side;
A switching element connected to a primary side of the transformer, and repeatedly switching on and off at a frequency higher than the frequency of the PWM signal,
The controller is
The ON period of the switching element is controlled according to the output fluctuation of the power supply unit,
The adjustment unit is
2. The power supply device according to claim 1, wherein a change rate of an ON period of the switching element controlled by the control unit is adjusted according to a duty ratio of the PWM signal. The adjustment unit is
3. The power supply device according to claim 2, wherein a change speed of an on period of the switching element from an on / off time point of the PWM signal is adjusted according to a duty ratio of the PWM signal. The adjustment unit is
When the duty ratio is larger than the first threshold value (<50%) and smaller than the second threshold value (> 50%), the rate of change of the ON period from the ON / OFF time point of the PWM signal of the switching element is expressed as the duty ratio. The ratio is adjusted so as to be slower than the rate of change of the ON period from the ON / OFF time point of the PWM signal when the ratio is smaller than the first threshold value or larger than the second threshold value. Item 4. The power supply device according to Item 3. A smoothing circuit for rectifying and smoothing the PWM signal;
The adjustment unit is
Change rate of ON period from ON / OFF time of PWM signal of switching element when voltage smoothed by smoothing circuit is larger than first threshold voltage and smaller than second threshold voltage (> first threshold voltage) Is adjusted to be slower than the rate of change of the ON period from the ON / OFF time point of the PWM signal when the voltage is lower than the first threshold voltage or higher than the second threshold voltage. The power supply device according to claim 3.   A display device comprising: the power supply device according to any one of claims 1 to 5; and a backlight driven by a drive unit of the power supply device.

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