JP2007325459A - Power supply unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent damage to a system by making a protective function act constantly from immediately after the actuation of a power supply unit such as a DC-DC converter. <P>SOLUTION: The power supply unit comprises a power supply circuit for comparing an output voltage Vo1 to a load with a reference voltage at a comparator CP1 and controlling the turn-on/off of a switching element Q1 so that the output voltage Vo1 becomes constant based on the comparison result, a soft start circuit for elevating the reference voltage gradually up to a predetermined level when the power supply circuit is actuated, and a protective circuit for turning the switching element Q1 off when the output voltage Vo1 drops below an abnormality detection voltage Vref1 wherein the abnormality detection voltage Vref1 to the protective circuit is also varied according to the reference voltage outputted from the comparator CP 3 to the comparator CP 1 of a soft start circuit. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electromagnetic conversion type power supply device using a switching element such as a DC-DC converter.

  In a system using a built-in power source such as a digital still camera or a digital video camera, a power supply device using an electromagnetic conversion method such as a DC-DC converter is used. This power supply device outputs a pulsating flow obtained by driving a switching element such as an FET (Field Effect Transistor) by using a coil or a capacitor, so that a necessary output voltage corresponding to the system can be obtained. It has become.

In recent years, in such a power supply device, there is a possibility that the system may be destroyed when a circuit short circuit occurs. Therefore, as a countermeasure, an overcurrent protection function and an overvoltage protection function are provided, and a soft start function is also provided. It has also been proposed to protect against overcurrent during soft-start activation (see, for example, Patent Document 1), and the voltage generated in the capacitor for setting the soft-start time of the soft-start circuit is used as an error amplifier for output control. It has also been proposed to supply a control to a comparator of an overcurrent detection circuit (for example, see Patent Document 2).
JP-A-9-117132 (paragraph numbers [0027] to [0029], FIG. 2) Japanese Patent Laying-Open No. 2005-323413 (paragraph numbers [0050] to [0051], FIG. 1)

  However, in the conventional power supply apparatus as described above, the protection function is effective only when the system using the power supply apparatus is operating normally, and it is difficult to activate the protection function immediately before the system is started. For this reason, when the system is turned on despite the occurrence of a short circuit, an overcurrent or overvoltage is instantaneously applied to the system, causing damage.

  The present invention has been made in view of these points, and an object of the present invention is to provide a power supply device that can always prevent a system from being damaged by always operating a protection function immediately after the power supply device is started.

  In the present invention, in order to solve the above-described problem, in an electromagnetic conversion type power supply device that rectifies a pulsating current generated by turning on and off a switching element and outputs the rectified current to a load, the output voltage for the load is compared with a reference voltage. A power supply circuit for controlling on / off of the switching element so that the output voltage becomes constant based on the comparison result, and a soft start circuit for gradually increasing the reference voltage to a predetermined voltage when the power supply circuit is activated And a protection circuit that turns off the switching element when the output voltage with respect to the load falls below a set voltage, and the setting of the protection circuit according to the reference voltage output from the soft start circuit There is provided a power supply device characterized by changing a voltage.

  According to such a power supply device, the transient current at the start-up of the power supply device is suppressed by the soft start circuit. Further, since the set voltage of the protection circuit that operates when the output is short-circuited is changed according to the reference voltage of the power supply circuit output from the soft start circuit, the output voltage abnormality is detected in the protection circuit even during the soft start period.

  According to the power supply device of the present invention, the transient current at the time of starting the power supply device can be suppressed by the soft start circuit, and the protection circuit that operates when the output is short-circuited according to the reference voltage of the power supply circuit output from the soft start circuit is set. Since the voltage is changed, it is possible to detect an output voltage abnormality in the protection circuit even during the soft start period, and it is possible to prevent damage to the system by always operating the protection function immediately after startup.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of a system using a power supply device according to an embodiment of the present invention.

  This system is configured as a digital still camera or a digital video camera, and has a control unit having an input unit 1 having power switches 11 and 12, a sub-microcomputer (sub-microcomputer) 21, and a main microcomputer (main microcomputer) 22. Power supply including a system unit 3 having a display unit 32, an image recording / reproducing unit 31, and a display unit 32 using an LCD (Liquid Crystal Display), and an electromagnetic conversion type DC-DC converter 41 driven by a battery (battery) Part 4.

  An ASIC (Application Specific Integrated Circuit) can also be used for the sub-microcomputer 21 of the system control unit 2. Further, here, an example in which an image recording / reproducing unit 31 and a display unit 32 are provided as the system unit 3 is shown. The image recording / reproducing unit 31 captures an image, records image data obtained by the imaging on a recording medium, generates an image being captured or display data on the display unit 32 when the recorded image is reproduced, and the like. It is a block that performs the process.

  In this system, when a battery is inserted into the power supply unit 4, the DC-DC converter 41 is powered on, and power is supplied to the sub-microcomputer 21 of the system control unit 2. Thereby, the power switches 11 and 12 of the input unit 1 can be received. When the user presses the power switches 11 and 12, input signals from the power switches 11 and 12 are transmitted to the sub-microcomputer 21. The power switches 11 and 12 serve as triggers for starting the power supply, and one to a plurality of power switches are provided depending on the system. Here, two cases are shown.

  The sub-microcomputer 21 receiving the input signals from the power switches 11 and 12 sends a signal to the DC-DC converter 41 so as to supply power to each part of the system. When receiving this signal, the DC-DC converter 41 starts supplying power to the entire system. Then, when power is distributed throughout the system, each device becomes operable, and signals are transmitted from the main microcomputer 22 to the image recording / reproducing unit 31 and the display unit 32 between the sub-microcomputer 21 and the main microcomputer 22. be able to.

FIG. 2 is a circuit diagram showing a configuration example of a general electromagnetic conversion type power supply device used in the above system.
This power supply device is configured as a current feedback type step-down converter (DC-DC converter), and a switching element Q1 of a MOSFET (Metal Oxide Semiconductor-FET) is controlled by PWM (Pulse Width Modulation), and a pulsating flow obtained thereby. Is smoothed by a coil L1, a capacitor C1, and a resistor R1, and a constant output voltage Vo is supplied to a load.

  In the block 101 of FIG. 2, the output voltage Vo is divided by the resistors R11 and R12, and the divided voltage is compared with the reference voltage Vref by the comparator CP1 as an output detection voltage. In the block 102, the current flowing through the switching element Q1 to which the input voltage Vin is applied is fed back and converted into a voltage signal by the resistor R13, and the voltage signal is output from the comparator CP1 to the VFB (Voltage Feed Back) signal and the comparator. The comparison is performed by CP2, and a PWM signal is output from the comparator CP2. In block 103, the PWM signal from the comparator CP2 is input to the RS flip-flop FF1, and the switching element Q1 is PWM-controlled through the RS flip-flop FF1. At this time, the RS flip-flop FF1 outputs the PWM signal from the comparator CP2 input to the S terminal only when the ON signal is input to the R terminal.

  With the above control, the voltage required for the load is output as a constant output voltage Vo by electromagnetic conversion. There are various types of switching drive systems for the current feedback type step-down converter. Here, a system using a PWM signal is given as an example. Other methods include a method using a PFM (Pulse Frequency Modulation) signal.

  Next, the protection function of the DC-DC converter 41 configured as described above will be described. FIG. 3 is a diagram showing a configuration example of a conventional protection circuit and soft start circuit mounted on the power supply apparatus.

  Here, a plurality of switching elements Q1, Q2,... Are provided in the power supply circuit, and a plurality of constant output voltages Vo1, Vo2,... Smoothed by the coils L1, L2,..., Capacitors C1, C2,. The case where it supplies is shown. The gate drive signals for the switching elements Q1, Q2,... Are supplied via AND gates 51, 52, respectively, and the other input terminals of the AND gates 51, 52,. A detection signal is input in common.

  The protection circuit is for preventing an overcurrent due to an output short-circuit, and the output voltages Vo1, Vo2,... Are divided by resistors R21a, R22a, resistors R21b, R22b, respectively, and the divided voltages are used as detection voltages. Are compared by comparators CP4a, CP4b,.... That is, it compares with each abnormality detection voltage Vref1, Vref2, ... according to output voltage Vo1, Vo2, .... When the difference between these voltages (Vo1 and Vref1, Vo2 and Vref2,...) Increases, an abnormality detection (short circuit detection) signal is output through the OR gate 71. The abnormality detection signal is commonly input to the AND gates 51, 52,... Via a short-circuit detection delay circuit described later, and the switching elements Q1, Q2,. That is, if an output short circuit is detected even in one of the power supply circuits that output the output voltages Vo1, Vo2,..., The entire power supply system is stopped.

  On the other hand, the soft start circuit is provided for each of the power supply circuits that output the output voltages Vo1, Vo2,... To prevent excessive excessive current at the time of startup, and gradually increases the output voltage. This limits excessive current transients. Before the power switch PS (corresponding to the operation of the power switches 11 and 12 of the input unit 1 shown in FIG. 1) is pressed, the control element Q11 is turned on (ON), the capacitor C11 is discharged, and charge is accumulated. It has not been. For this reason, the soft start voltage from the comparator CP3 for comparison with the detected voltage of the output voltage Vo1 in the comparator CP1 of the power supply circuit is 0V. When the power switch PS is pressed, the control element Q11 is turned off, the electric charge flowing from the reference voltage Vref with a constant current is accumulated in the capacitor C11, and the capacitor C11 is charged. The soft start voltage input to the comparator CP1 rises in proportion to the charging time constant of the capacitor C11, and the output voltage rises.

  FIG. 4 is a diagram showing a rising waveform of the output voltage during the soft start. As shown in FIG. 4, the output voltage gradually increases during the soft start period, and thereafter, the output voltage becomes constant.

  In this soft start circuit, if the soft start voltage corresponding to the reference voltage Vref is used as the target of the comparison voltage in the comparator CP1, the abnormal voltage with respect to the target output voltage is increased when the soft start voltage reaches the output voltage Vo1. The threshold is not reached. This is because the reference voltage Vref is set based on a highly accurate reference voltage of BGR (Band Gap Reference), and the reference voltage Vref is higher than BGR. Therefore, the reference voltage Vref corresponds to the reference voltage Vref. The soft start voltage cannot be used as it is as the reference voltage in the comparator CP1. Therefore, in the comparator CP3, the reference voltage output is switched to BGR when the soft start voltage exceeds a certain threshold value. Thereby, when the soft start is completed and the target voltage is reached, the threshold value of BGR can be set for the comparator CP1.

  Here, the power supply of the system is turned on by the activation factor by the power switches 11 and 12 or the sub-microcomputer 21 shown in FIG. 1, but the power supply at the time of activation cannot instantaneously reach the specified voltage from 0V. The causes include delays in the start-up due to the impedance of the circuit, instantaneous charge build-up in the discharged capacitors in the circuit and a rush current flowing when the power is turned on instantaneously, and various other reasons. This is because the current needs to be limited. Therefore, in order to prevent such an overcurrent from flowing at the time of startup, the above-described soft start circuit is necessary.

  In the above protection circuit, the comparators CP4a, CP4b,... Compare the difference between the output voltage and the abnormality detection voltage. During the soft start period, the output voltage gradually approaches the set voltage, and the output voltages Vo1, Vo2 during that time. Are lower than the abnormality detection voltages Vref1, Vref2,..., So that the protection circuit may detect an output short circuit and malfunction. Therefore, conventionally, a short circuit detection delay circuit, which is a timer circuit, is provided at the output stage of the protection circuit. After detecting a short circuit due to a decrease in the output voltage, a temporary delay is provided until the actual output is stopped. Erroneous detection due to typical output fluctuations was prevented. This short circuit detection delay circuit includes a control element Q12 and a capacitor C12 to which a constant current from the reference voltage Vref is supplied, and an output thereof is input to the RS flip-flop FF2. The RS flip-flop FF2 validates the signal from the RS flip-flop FF2 by the output signal of UVLO (Undervoltage lockout) 61. With this configuration, the short circuit detection signal output from the OR gate 71 of the protection circuit is input to the AND gates 51, 52... After being delayed according to the charging time constant by the capacitor C12.

  As described above, conventionally, a short circuit detection delay circuit is used during soft start, and the delay time is extended from the soft start time, thereby avoiding erroneous detection. Or, during the soft start, the protection circuit is not operated to avoid erroneous detection. However, in such a configuration, there is a problem that an output short circuit cannot be detected during the soft start period.

FIG. 5 is a diagram showing an example of the rising waveform of the output voltage at the time of abnormality, and here shows how the power supply rises when an output short circuit occurs.
If the delay time of the short-circuit detection signal by the short-circuit detection delay circuit is substantially the same as, for example, the soft start period, short circuit detection by the protection circuit is impossible during the soft start period, and the detection period starts when the delay time ends. Therefore, an overcurrent may flow during the non-detectable period. However, in the conventional configuration, the soft start circuit and the short circuit detection delay circuit are completely separate, and there is no correlation between the soft start time and the delay time. Even in such a configuration, conventionally, an individual discrete element is often used as an output element (switching element). In such a case, the withstand capability such as overcurrent is large, so the delay time is extended from the soft start time. However, there was little practical problem because the device was hardly damaged. However, when the output element is incorporated in an integrated circuit, the tolerance of overcurrent is small, and the risk of element destruction increases.

  That is, in the circuit configuration shown in FIG. 3, a short-circuit detection delay circuit is required to provide both a soft start circuit that prevents excessive excessive current and a protection circuit that prevents overcurrent due to a short circuit. The detection delay circuit prevents the protection circuit from functioning within the delay time from the start. Therefore, in the present embodiment, the protection circuit works even during soft start. Specifically, the output voltage of the soft start circuit is supplied as a reference voltage for comparison with the output voltage in the power supply circuit, and also supplied as a comparison reference voltage for the protection circuit, and the reference voltage of the protection circuit is also soft started. I am letting.

FIG. 6 is a diagram illustrating a configuration example of the power supply device according to the embodiment of the present invention.
The protection circuit of this power supply device is a combination of the two functions of the soft start circuit and the abnormal voltage comparison circuit shown in FIG. The power supply circuit has the same configuration as that of the current feedback type DC-DC converter shown in FIG. 2, and turns on and off the switching elements Q1, Q2,... To which the input voltage Vin is supplied, and the generated pulsating current is coiled. Are smoothed by L1, L2,..., Capacitors C1, C2,..., Resistors R1, R2, and so on, and a plurality of constant output voltages Vo1, Vo2,.

  The gate drive signal output circuits to the switching elements Q1, Q2,... Have the same configuration for each element, and the output voltages Vo1, Vo2,. Control to keep. Further, the gate drive signals for the switching elements Q1, Q2,... Are output via the AND gates 51, 52, respectively, and the other input terminals of the AND gates 51, 52,. Are input in common.

  The protection circuit includes a soft start circuit having the same configuration as the soft start circuit shown in FIG. 3 for each power supply circuit corresponding to the output voltages Vo1, Vo2,. FIG. 6 shows only the soft start circuit (and abnormal voltage comparison circuit) for the power supply circuit corresponding to the output voltage Vo1 as an example. In this soft start circuit, the soft start voltage from the comparator CP3 is supplied as a reference voltage to be compared with the detected voltage of the output voltage Vo1 by the comparator CP1 which is an error amplifier of the power supply circuit, and the output voltage of the soft start circuit is supplied to the resistor R31. The voltage divided by the resistor R32 is supplied as the abnormality detection voltage Vref1 of the comparator CP4 that compares the output voltage Vo1 with the detection voltage divided by the resistors R21 and R22. Thereby, the short circuit detection signal is output from the comparator CP4. A short circuit detection signal corresponding to each output voltage Vo1, Vo2,... Is commonly input to the AND gates 51, 52.

  As described above, in the conventional circuit, the output voltage Vo1 for the comparator CP4 of the abnormal voltage comparison circuit is lower than the abnormality detection voltage Vref1 until the target output voltage at the time of soft start is reached, and the abnormal voltage is erroneously detected. There was a fear. On the other hand, in the circuit of FIG. 6, the abnormality detection voltage Vref1 is output to the target voltage as a voltage corresponding to the output of the soft start circuit so that it can be output even during the soft start period immediately after the system is started. An abnormal voltage (short circuit) can be detected. That is, a transient current at the time of startup can be suppressed by the soft start circuit, and a short-circuit prevention function can always be operated from the time of startup, thereby preventing damage to the system due to overcurrent.

  In FIG. 6, the protection circuit is provided with a conventional short-circuit detection delay circuit. Here, the delay time is used to accurately determine whether the output has actually short-circuited and the voltage has dropped. is doing. For example, the rush current increases when the battery is nearly discharged, because the current value increases due to the voltage drop. In such a state, the overcurrent protection function is erroneously activated by the rush current. However, since it is desired to operate the system for as long as possible using the remaining amount of the battery, it is necessary to distinguish between a voltage drop due to a rush current and a voltage drop due to a short circuit. For this reason, when the voltage drops for a certain period of time using a short-circuit detection delay circuit, it is judged that it is dangerous due to a short-circuit, and conversely, if a voltage drop is very short, it is judged that a rush current appears. The system is not stopped. Because of such a function, the delay time by the capacitor C12 of the short circuit detection delay circuit can be made shorter than the conventional time (the time corresponding to the soft start time), and the short circuit can be detected even during the soft start period. Become.

  In FIG. 6, a soft start circuit is individually provided for each power supply circuit corresponding to the output voltages Vo1, Vo2,..., But only one common soft start circuit may be provided for each power supply circuit. Good. In this case, for example, the output voltage of the soft start circuit is appropriately divided by a resistor, and a reference voltage (for example, a reference voltage to the comparator CP1) for the output circuit of the gate drive signal to the switching element, or a reference voltage for short circuit detection (For example, the abnormality detection voltage Vref1 to the comparator CP4) may be generated.

It is a block diagram which shows the structure of the system which uses the power supply device which concerns on embodiment of this invention. It is a circuit diagram which shows the structural example of the power supply apparatus of a general electromagnetic conversion system. It is a figure which shows the structural example of the conventional protection circuit and soft start circuit which are mounted in a power supply device. It is a figure which shows the rising waveform of the output voltage at the time of the soft start in a conventional circuit. It is a figure which shows an example of the rising waveform of the output voltage at the time of abnormality in a conventional circuit. It is a figure which shows the structural example of the power supply device concerning embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Input part, 2 ... System control part, 3 ... System part, 4 ... Power supply part, 11, 12 ... Power switch, 21 ... Sub microcomputer (sub microcomputer), 22 ... Main Microcomputer (main microcomputer) 31... Image recording / playback unit 32... Display unit 41... DC-DC converter 51 and 52. C2, C11, C12 ... Capacitor, FF1, FF2 ... RS flip-flop, CP1, CP2, CP3, CP4 ... Comparator, L1, L2 ... Coil, Q1, Q2 ... Switching element, Q11, Q12 ... Control Element, R1, R2, R11, R12, R13, R21, R22, R31, R32 ... resistance

Claims (6)

In the electromagnetic conversion type power supply device that rectifies the pulsating flow generated by turning on and off the switching element and outputs it to the load,
A power supply circuit that controls the on / off of the switching element so that the output voltage for the load is compared with a reference voltage and the output voltage becomes constant based on the comparison result;
A soft start circuit that gradually increases the reference voltage to a predetermined voltage when the power supply circuit is activated;
A protection circuit that turns off the switching element when an output voltage with respect to the load falls below a set voltage;
Have
A power supply apparatus, wherein the set voltage of the protection circuit is changed according to the reference voltage output from the soft start circuit.   The power supply circuit controls on / off of the switching element based on a comparison result between the output voltage and the reference voltage and an input current flowing in a drain terminal of the switching element. The power supply described.   The power supply circuit generates a gate drive signal for the switching element based on a comparison result between a comparison result between the output voltage and the reference voltage and a voltage according to an input current flowing through the drain terminal of the switching element. 3. The power supply apparatus according to claim 2, wherein pulse width modulation control is performed. A plurality of the power supply circuits that individually have the switching elements and output different voltages, and
A plurality of the protection circuits provided corresponding to the power supply circuits, respectively, for comparing the output voltage and the set voltage for each of the power supply circuits;
A plurality of the soft start circuits that are provided corresponding to the power supply circuits and supply voltages to the power supply circuits and the protection circuits corresponding to the power supply circuits;
The power supply device according to claim 1, further comprising:   5. The power supply device according to claim 4, wherein when the output voltage of the corresponding power supply circuit in each of the protection circuits becomes equal to or lower than the set voltage, the switching elements of all the power supply circuits are turned off in common. . A plurality of the power supply circuits that individually have the switching elements and output different voltages, and
A plurality of the protection circuits provided corresponding to the power supply circuits, respectively, for comparing the output voltage and the set voltage for each of the power supply circuits;
With
2. The power supply apparatus according to claim 1, wherein the reference voltage to each power supply circuit and the set voltage to each protection circuit are voltages according to an output from one soft start circuit.

Images