JPH05161349A - Switching power unit - Google Patents

Abstract

PURPOSE:To enable five load protection at operation of an overcurrent protective circuit and enable the margin in the heat design and the safe operation area of a switching transistor to be taken enough, in a pulse width control system of switching power unit which has a automatic recovery type overcurrent protective circuit. CONSTITUTION:A switch circuit 14 for conversion of a time constant is connected to the section to decide the time constant in discharge of the fundamental wave signal of a fundamental wave oscillating circuit 12, and when a overcurrent protective circuit 13 operates, the switch circuit 14 for time constant conversion operates by the control signal of the overcurrent protective circuit 13, and can remarkably elongate the time constant in discharge of the fundamental wave signal of the fundamental wave oscillating circuit 12, comparing it with stationary operation state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device, and more particularly, it is useful for load protection during operation of an overcurrent protection circuit of a pulse width control type switching power supply device having an automatic recovery type overcurrent protection circuit. is there.

[0002]

2. Description of the Related Art An example of a conventional pulse width control type switching power supply device is shown in FIG. Further, FIG. 5 shows signal waveforms of respective parts of the conventional pulse width control type switching power supply device shown in FIG. As shown in FIG. 4, a conventional pulse width control type switching power supply device includes a transformer 2 connecting an input power supply 1 to a primary side terminal, a switching transistor 3 connected to another primary side terminal of the transformer 2, This conventional pulse width control type switching power supply device is connected to both terminals of a rectifying diode 4 having an anode connected to the secondary side terminal of the transformer 2, a smoothing capacitor 5 connected to the cathode of the rectifying diode 4, and both terminals of the smoothing capacitor 5. Load 8
The output voltage terminal (+) 6 and the output voltage terminal (-) 7, which are the connection terminals of the control resistor 17, the control resistor 17 connected to the output voltage terminal (+) 6, and the input portion connected to the control resistor 17 A photocoupler 9 whose input current is controlled by a resistor 17, an AND circuit 10 whose output terminal is connected to the gate of the switching transistor 3, a pulse width control comparison circuit 11 whose output terminal is connected to the input terminal of the AND circuit 10,
The AND circuit 10 detects the overcurrent of the load 8 by detecting the current flowing through the switching transistor 3 and outputs the overcurrent signal.
To the overcurrent protection circuit 13, the pulse width control comparison circuit 11 to output a fundamental wave signal to the fundamental wave oscillation circuit 12, the control circuit power supply 16 to be the power supply of each control circuit of this switching power supply device, the control circuit It is composed of a circuit operation switch 15 for performing ON / OFF operation of the power supply 16.

The fundamental wave oscillator circuit 12 includes a first comparator 21.
, A second comparator 22, a first reference voltage power supply 23 connected to the inverting input terminal terminal of the first comparator 21, and a non-inverting input terminal terminal of the second comparator 22. A second reference voltage power supply 24, a first flip-flop 25 that uses the output signal of the first comparator 21 as a set signal and the output signal of the second comparator 22 as a reset signal, and a frequency setting capacitor 29. , A first constant current source 26 for supplying a charging current to the frequency setting capacitor 29, and a second constant current source 27 for taking in a discharging current of the frequency setting capacitor 29.
And a first switch for driving the first constant current source 26 and the second constant current source 27 when the non-inverted output of the first flip-flop 25 is at a high level, it becomes a closed circuit, and when it is at a low level, it becomes an open circuit. And a circuit 30. In this fundamental wave oscillation circuit 12, the fundamental wave signal is generated by the frequency setting capacitor 2
It is generated from one end of 9. The overcurrent protection circuit 13 inputs the fundamental wave signal generated in the fundamental wave oscillation circuit 12 to the inverting input terminal and inputs the third reference power source 32 to the non-inverting input terminal, and the switching transistor 3 Detecting resistor 36 for converting the source current of the above into a voltage, and a fourth comparator for inputting the detection voltage generated in the current detecting resistor 36 to the non-inverting input terminal and inputting the fourth reference power source 35 to the inverting input terminal. 34 and a second flip-flop 33 that uses the output signal of the fourth comparator 34 as a set signal and the output signal of the third comparator 31 as a reset signal.

The operation of the conventional pulse width control type switching power supply device will be described with reference to FIGS. 4 and 5. By closing the circuit operation switch 15,
When the voltage of the power supply 16 of the control circuit is applied to the photocoupler 9, AND circuit 10, pulse width control comparator 11, fundamental wave oscillation circuit 12, and overcurrent protection circuit 13, the photocoupler 9 outputs the output voltage terminal 6 and A potential difference with the output voltage terminal 7 is detected, and a voltage proportional to the potential difference is output to the inverting input terminal of the pulse width control comparator 11, and the non-inverting input terminal of the pulse width control comparator 11 is basically The fundamental wave signal of the wave oscillation circuit 12 is input. Here, the operation of the fundamental wave oscillation circuit 12 will be described. The non-inverting input terminal of the first comparator 21 and the inverting input terminal of the second comparator 22 are both connected to the frequency setting capacitor 29, and the first inverting input terminal of the first comparator 21 is connected. Reference voltage VH of the reference power supply 23
A second comparator connected to the non-inverting input terminal of the second comparator 22.
Is set to a potential higher than the reference voltage VL of the reference power supply 24.
If the output voltage of the non-inverting output terminal of the first flip-flop 25 is in the low state and the first switch circuit 30 is in the open circuit state, the second constant current source 27 is in the open circuit state. When the constant current I1 from the first constant current source 26 flows into the frequency setting capacitor 29 as a charging current and the potential of the frequency setting capacitor 29 rises and reaches the reference voltage VH of the first reference power source 23, the first Comparator 21
Since the output voltage of the first flip-flop 25 is in the high state, the set signal is input to the first flip-flop 25, and the non-inverted output terminal of the first flip-flop 25 is in the high state.
The switch circuit 30 of becomes a closed circuit state. The constant current of the second constant current source 27 is set to I2, and I2 is set so that I2> I1.
When the value is set, a current having a value of I2-I1 is discharged from the frequency setting capacitor 29 as a discharge current, and the potential of the frequency setting capacitor 29 continues to drop with the discharge.
When the voltage reaches the reference voltage VL of the reference power source 24, the output voltage of the second comparator 22 becomes a high state, the reset signal is input to the first flip-flop 25, and the non-inverted output terminal of the first flip-flop 25. Goes low and first
The switch circuit 30 becomes an open circuit state, and the above operation is repeated thereafter. Therefore, the frequency setting capacitor 2
9 or the output of the fundamental wave oscillation circuit 12, the amplitude is VH-V
A triangular fundamental wave signal having L, a charging time constant (VH-VL) Ct / I1, and a discharging time constant (VH-VL) Ct / (I2-I1) is generated. The Ct is the capacitance value of the frequency setting capacitor 29.

In the pulse width control comparator 11, the fundamental wave signal of the fundamental wave oscillation circuit 12 is compared with the output signal of the photocoupler 9, and if the voltage of the fundamental wave signal is smaller than the voltage of the output signal of the photocoupler 9. A low signal is output, and if the voltage of the fundamental wave signal is higher than the voltage of the output signal of the photocoupler 9, a high signal is output. The output signals of the pulse width control comparator 11 are output to one input terminal of the AND circuit 10.

The overcurrent protection circuit 13 detects the source current of the switching transistor 3 as a voltage by the current detecting resistor 36, and inputs the detected voltage to the non-inverting input terminal of the fourth comparator 34. If the detected voltage is lower than the reference voltage of the fourth reference power supply 35, the output voltage of the fourth comparator 34 is in the low state, and the set signal input terminal of the second flip-flop 33 is also in the low state. The inverting output terminal of the second flip-flop 33 is in the high state, and the high signal is input to the other input terminal of the AND circuit 10. On the other hand, if the source current of the switching transistor 3 increases and the detection voltage of the current detection resistor 36 becomes higher than the reference voltage of the fourth reference voltage 35, the output voltage of the fourth comparator 34 becomes the high state. , The set signal is input to the second flip-flop 33.
The inverting output terminal of the flip-flop 33 becomes low, and the low signal is input to the other input terminal of the AND circuit 10. Here, the reset signal of the second flip-flop 33 is obtained as follows. Fundamental wave oscillation circuit 1
The second fundamental wave signal is input to the inverting input terminal of the third comparator 31, and the third reference voltage 32 is connected to its non-inverting input terminal. When the reference voltage VL 'of the third reference voltage 32 is set to a potential slightly higher than the reference voltage VL of the second reference voltage 24, the third comparator 31 is surely provided in the vicinity of the lowest level state of the fundamental wave signal. The output voltage of the above becomes high, and the reset signal is input to the second flip-flop 33.
That is, the reset signal of the second flip-flop 33 is
The fundamental wave signal of the fundamental wave oscillation circuit 12 is always generated once in one cycle.

Therefore, as described above, in the steady operation, the output voltage of the photocoupler 9 and the fundamental wave oscillation circuit 1
When the voltage of the fundamental wave signal is higher than the output voltage of the photocoupler 9, the other input terminal of the AND circuit 10 is always in the high state and the switching transistor When the voltage of the fundamental wave signal is smaller than the output voltage of the photocoupler 9, the other input terminal of the AND circuit 10 is always in the low state, so that the drain and the source of the switching transistor 3 are in the low state. During that time, it will be shut off. As described above, the conduction time of the switching transistor 3 is controlled according to the output voltage between the output voltage terminals 6 and 7, and a stable output voltage is generated between the output voltage terminals 6 and 7. In the abnormal operation state in which the source current of the switching transistor 3 becomes excessive, as described above, the inverting output terminal of the second flip-flop 33 is in the low state, and the low signal is input to the other input terminal of the AND circuit 10. Therefore, the drain and source of the switching transistor 3 are cut off to protect the switching transistor 3 from damage due to overcurrent. As described above, the reset terminal of the second flip-flop 33 is
Since the reset signal is input in each cycle, the overcurrent protection circuit 13 is reset in each cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12, and the overcurrent protection circuit 13 operates in each cycle. Have.

[0008]

In the conventional pulse width control type switching power supply device, the overcurrent protection circuit 1 is used.
Since the abnormality of the source current of the switching transistor 3 is detected for each cycle of the fundamental wave signal of the fundamental oscillation circuit 12 by 3, the overcurrent protection circuit 13 operates due to the excessive source current of the switching transistor 3 and the switching transistor Although 3 is in the cutoff state, when the excessively large source current decreases and returns to the steady state, the switching transistor 3 becomes conductive again and repeats the cutoff of the switching transistor 3 by the steady operation. That is, it has an automatic restoration type characteristic.

However, when the overcurrent protection circuit 13 operates and the switching transistor 3 is cut off,
A delay time always exists in the overcurrent protection circuit 13, the AND circuit 10, the switching transistor 3, and the like. If the period of the fundamental wave signal of the fundamental wave oscillation circuit 12 is sufficiently long as compared with the delay time, the relationship between the output voltage between the output voltage terminals 6 and 7 and the output current of the load 8 is drooping characteristic or "F". Since the energy stored in the secondary side of the transformer 2 is completely discharged, the load current of the load 8 will not increase when the output voltage between the output voltage terminals 6 and 7 decreases. However, if the cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12 is shorter than the delay time and the delay time becomes a value that cannot be ignored, the energy stored in the secondary side of the transformer 2 Is not completely discharged and is accumulated, so that even if the output voltage between the output voltage terminals 6 and 7 decreases, the load current of the load 8 increases due to the influence of the accumulated energy. That is, the relationship between the output voltage and the load current becomes an "L" shape, and the load may be destroyed due to an increase in the load current.

In particular, the cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12 tends to be shortened in order to reduce the size and increase the efficiency of the switching power supply device in recent years, and the overcurrent protection circuit 13 and the AND circuit 10 are used. The increase in the load current of the load 8 when the output voltage between the output voltage terminals 6 and 7 decreases due to the delay time of the switching transistor 3 and the like causes a problem that the load 8 of the switching power supply device is destroyed.

Further, the overcurrent protection circuit 13 and the AND circuit 1
0, the delay time of the switching transistor 3 and the like always exists for a fixed time, that is, the switching transistor 3 cannot be shut off for a fixed time when the cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12 becomes short. There is also a problem that the thermal design of the switching transistor 3 and the margin of the safe operation area are lost.

The present invention has been made in view of the above problems, and in a pulse width control type switching power supply device having an automatic recovery type overcurrent protection circuit, load protection during operation of the overcurrent protection circuit is achieved. It is an object of the present invention to provide a switching power supply device which is capable of providing a sufficient margin of thermal design and safe operation area of a switching transistor.

[0013]

A switching power supply device according to the present invention has a pulse having an automatic recovery type overcurrent protection circuit which operates when an overcurrent is detected and stops operating with a fundamental wave signal of a fundamental wave oscillation circuit. In the width control type switching power supply device, when the overcurrent protection circuit detects an overcurrent, it has a time constant conversion switch circuit for changing the time constant of the discharge of the fundamental wave signal of the fundamental wave oscillation circuit. To do.

[0014]

In the switching power supply device according to the present invention, by providing the switch circuit 14 for time constant conversion, the source current of the switching transistor 3 increases and the overcurrent protection circuit 13 operates in an abnormal operating state in which a fundamental wave is generated. Since the time constant of the discharge of the fundamental wave signal of the oscillator circuit 12 can be set to be significantly longer than that in the steady operation state, a constant delay time exists in the overcurrent protection circuit 13, the AND circuit 10, the switching transistor 3, and the like. However, even if the delay time is a value that cannot be ignored with respect to the cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12, the cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12 in the abnormal operation state is compared with the delay time. Therefore, the energy stored on the secondary side of the transformer 2 can be completely discharged. Therefore,
When the output voltage between the output voltage terminal 6 and the output voltage terminal 7 decreases, the load current of the load 8 does not increase,
The relationship between the load current and the output voltage has a drooping characteristic or a fold-back characteristic, and it is possible to prevent the load 8 of the switching power supply device from being destroyed.

Further, even if the above delay time exists, the cycle of the basic signal of the fundamental wave oscillation circuit 12 becomes sufficiently long in the abnormal operation state, and the duty ratio of the delay time with respect to the cycle of the basic signal can be set sufficiently small. A sufficient margin can be secured for the thermal design and safe operation area of the switching transistor 3.

[0016]

Embodiments of the present invention will now be described with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a switching power supply device according to an embodiment of the present invention. FIG. 2 is a signal waveform diagram in each part of the switching power supply device according to the embodiment of the present invention shown in FIG. In the figure, parts common to those of the conventional switching power supply device shown in FIG. 4 are designated by the same reference numerals, and detailed description thereof will be omitted.

As shown in FIG. 1, a pulse width control type switching power supply device according to an embodiment of the present invention is a transformer 2 for connecting an input power supply 1 to a primary side terminal, and another primary side terminal of the transformer 2. Is connected to the switching transistor 3, the secondary side terminal of the transformer 2 is connected to the anode of the rectifying diode 4, the cathode of the rectifying diode 4 is connected to the smoothing capacitor 5, and the smoothing capacitor 5 is connected to both terminals. An output voltage terminal (+) 6 and an output voltage terminal (-) 7 which are connection terminals of a load 8 of a pulse width control type switching power supply device, a control resistor 17 connected to the output voltage terminal (+) 6, and a control resistor The output terminal is connected to the gate of the photocoupler 9 and the switching transistor 3 whose input portion is connected to the resistor 17 and whose input current is controlled by the controlling resistor 17. AND circuit 10 that, AND circuit 1
The output current is connected to the pulse width control comparison circuit 11 in which the output terminal is connected to the input terminal of 0, and the overcurrent of the load 8 is detected by detecting the current flowing through the switching transistor 3.
An overcurrent protection circuit 13 that outputs to the ND circuit 10, a fundamental wave oscillation circuit 12 that outputs a fundamental wave signal to the pulse width control comparison circuit 11, a power supply 16 of a control circuit that is a power supply of each control circuit of this switching power supply device, O of the control circuit power supply 16
A circuit operation switch 15 for performing N / OFF operation and a time constant conversion switch circuit 14 are included.

The fundamental wave oscillator circuit 12 includes a first comparator 21.
, A second comparator 22, a first reference voltage power supply 23 connected to the inverting input terminal terminal of the first comparator 21, and a non-inverting input terminal terminal of the second comparator 22. A second reference voltage power supply 24, a first flip-flop 25 that uses the output signal of the first comparator 21 as a set signal and the output signal of the second comparator 22 as a reset signal, and a frequency setting capacitor 29. , A first constant current source 26 for supplying a charging current to the frequency setting capacitor 29, and a second constant current source 27 for taking in a discharging current of the frequency setting capacitor 29.
And a third constant current source 27 provided in parallel with the second constant current source 27.
When the non-inverted output of the constant current source 28 and the first flip-flop 25 is at a high level, it becomes a closed circuit, and when it is at a low level, it becomes an open circuit, and the first constant current source 26 and the second constant current source 2
7 and a first switch circuit 30 for driving the third constant current source 28. In this fundamental wave oscillation circuit 12, a fundamental wave signal is generated from one end of the frequency setting capacitor 29. The overcurrent protection circuit 13 inputs the fundamental wave signal generated by the fundamental wave oscillation circuit 12 to the inverting input terminal and makes the third reference power source 32 a non-inverting input terminal, and the switching transistor 3 and the third comparator 31. The current detection resistor 36 for converting the source current into a voltage, and the detection voltage generated in the current detection resistor 36 are input to the non-inverting input terminal, and the fourth reference power source 35 is supplied.
To the inverting input terminal and the third comparator 3 as the output signal set signal of the fourth comparator 34.
It has a second flip-flop 33 which uses the output signal of 1 as a reset signal. Switch circuit for time constant conversion 1
4 is a second switch circuit 37 and a third switch circuit 3
Have eight. The first switch circuit 30, the second switch circuit 37, and the third switch circuit 38 are closed circuits when the control signal is in the high state, and open circuits when the control signal is in the low state.

During steady operation, the inverting output terminal of the second flip-flop 33 is in the high state, as described in the conventional switching power supply device shown in FIG.
Since the non-inverting output terminal of the flip-flop 33 is in the low state, the second switch circuit 37 is a closed circuit and the third switch circuit 38 is an open circuit. Therefore, since the third constant current source 28 is in an open circuit state, the fundamental wave signal of the fundamental wave oscillation circuit 12 has an amplitude VH-VL, a charging time constant (VH-VL) Ct / I1, and a discharging time constant. Is (VH-VL) Ct
It becomes a triangular wave of / (I2-I1).

In an abnormal operating state in which the source current of the switching transistor 3 increases and the detection voltage of the current detection resistor 36 becomes higher than the reference voltage of the fourth reference power source 35, the output voltage of the fourth comparator 34 Is set to a high state, the set signal is input to the second flip-flop 33. Therefore, since the inverting output terminal of the second flip-flop 33 is in the low state and the non-inverting output terminal is in the high state, the low signal is input to one input terminal of the AND circuit 10 and the output of the AND circuit 10 is The low state prevents the switching transistor 3 from being cut off and the source current from becoming excessive.

In this state, the inverting output terminal of the second flip-flop 33 in the overcurrent protection circuit 13 is in the low state and the non-inverting output terminal is in the high state. The switch circuit 37 is an open circuit and the third switch circuit 38 is a closed circuit. Therefore,
Since the non-inverted output of the first flip-flop 25 is in the low state and the switch circuit 30 is an open circuit, the charging current of the frequency setting capacitor 29 of the fundamental wave oscillation circuit 12 is constant in the first constant current source 26. It becomes the current I1. Before the overcurrent protection circuit 13 operates, that is, in the steady operation state, the second
Since the inverting output of the flip-flop 33 is in the high state and the non-inverting output is in the low state, the second switch circuit 37 of the time constant conversion switch circuit 14 is a closed circuit and the third switch circuit 38 is an open circuit. .. Therefore, the discharge current of the frequency setting capacitor 29 becomes I2-I1 as in the conventional switching power supply device shown in FIG.

However, in the abnormal operation state in which the overcurrent protection circuit 13 operates, as described above, the second switch circuit 37 of the time constant conversion switch circuit 14 is an open circuit and the third switch circuit 38 is a closed circuit. , Third constant current source 28
, The second constant current source 27 is in an open circuit state, so that the charging current of the frequency setting capacitor 29 is I3-I1. Here, the third constant current source 28
Is set to be sufficiently smaller than the constant current I2 of the second constant current source 27 and very slightly larger than the constant current I1 of the first constant current source 26, the frequency during abnormal operation Since the discharge current of the setting capacitor 29 can be set to be extremely smaller than that in the steady operation, the discharge time constant of the frequency setting capacitor 29 can be set to be sufficiently long in the abnormal operation state as compared with the steady operation state. it can.
That is, when the source current of the switching transistor 3 increases and the overcurrent protection circuit 13 operates, the cycle of the fundamental wave signal of the fundamental wave oscillation circuit 12 at the time of discharging can be made extremely longer than in the steady state. Becomes

FIG. 3 is a block diagram showing a switching power supply device according to another embodiment of the present invention. As shown in FIG. 3, the third constant current source 28 is connected to the frequency setting capacitor 2
9 and the power supply 16 of the control circuit, and as described above, when the source current of the switching transistor 3 increases and the overcurrent protection circuit 13 operates, the non-inverted output of the second flip-flop 33 becomes high. Therefore, the time constant conversion switch circuit 14 is a closed circuit. Therefore, the first
The switch circuit 30 becomes an open circuit, and the charging current of the frequency setting capacitor 29 becomes the first constant current source I1.
Before the overcurrent protection circuit 13 operates, that is, in the steady operation state, the time constant conversion switch circuit 14 is an open circuit, so that the discharge current of the frequency setting capacitor 29 becomes I2-I1 as described above. However, in the abnormal operation state in which the overcurrent protection circuit 13 operates, the time constant conversion switch circuit 1
Since 4 and the first switch circuit 30 are closed circuits, if the constant currents of the first constant current source 26, the second constant current source 27, and the third constant current source 28 are I1, I2, and I3, respectively. The discharge current of the frequency setting capacitor 29 is I2-I1-I3. Here, the first constant current source 26 and the third constant current source 28
Of the third constant current source 28 so that the sum I1 + I3 of the respective constant currents of 1 and 2 is very slightly smaller than the constant current I2 of the second constant current source 27.
When the constant current I3 is set, the discharge current of the frequency setting capacitor 29 at the time of abnormal operation can be made extremely smaller than that at the time of steady operation as in the switching power supply device according to the embodiment of the present invention shown in FIG. In the operating state,
The discharge time constant of the frequency setting capacitor 29 can be set to be sufficiently long as compared with that during steady operation.

[0025]

As described above, according to the switching power supply device of the present invention, by providing the switch circuit for time constant conversion, the fundamental wave oscillation circuit operates in the abnormal operation state in which the overcurrent protection circuit operates. Since the time constant of the discharge of the wave signal can be set to be significantly longer than that in the steady operation state, there is a certain delay time in the overcurrent protection circuit, the AND circuit, the switching transistor, etc. Even if the value is not negligible with respect to the period of the fundamental wave signal of the oscillator circuit, the energy accumulated on the secondary side of the transformer can be completely discharged. Therefore, when the output voltage to the load of the switching power supply according to the present invention decreases, the load current does not increase, and the relationship between the load current and the output voltage has a drooping characteristic or a fold-back characteristic. It is possible to prevent the load from being destroyed.

Further, even if the delay time is present, the cycle of the basic signal of the fundamental wave oscillation circuit becomes sufficiently long in the abnormal operation state, and the duty ratio of the delay time with respect to the cycle of the basic signal can be set to be sufficiently small. A sufficient margin is provided for the thermal design and safe operation area of the transistor, and the degree of freedom in designing the switching power supply device according to the present invention is increased.

[Brief description of drawings]

FIG. 1 is a block diagram showing a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a signal waveform diagram in each part of the switching power supply device according to the embodiment of the present invention shown in FIG.

FIG. 3 is a block diagram showing a switching power supply device according to another embodiment of the present invention.

FIG. 4 is a block diagram showing an example of a conventional switching power supply device.

5 is a signal waveform diagram in each part of the conventional switching power supply device shown in FIG.

[Explanation of symbols]

 1; Input power supply 2; Transformer 3; Switching transistor 12; Fundamental wave oscillation circuit 13; Overcurrent protection circuit 14; Time constant conversion switch circuit

Claims (1)

[Claims] 1. A switching power supply device of a pulse width control system having an automatic recovery type overcurrent protection circuit which operates when an overcurrent is detected and stops its operation by a fundamental wave signal of a fundamental wave oscillation circuit. A switching power supply device comprising a time constant conversion switch circuit for changing a time constant of discharge of a fundamental wave signal of the fundamental wave oscillation circuit when the circuit detects an overcurrent.