JPH05236744A - Multioutput swtching power source unit - Google Patents

Abstract

PURPOSE:To prevent the occurrence of an accident, such as the damage of the load of a second output, catching fire, etc., even when a fault occurs in a step-down stabilization circuit which obtains the low-voltage second output from a first output. CONSTITUTION:A chopper circuit 7 obtains a 5-V second output for control from a 24-V first output for driving, the output voltage of which is fed back to a PW control circuit 3 for controlling the driving pulse width of an FETQ 1. When an abnormal voltage is generated in the second output, a Zener diode ZD7 on a second output terminal side clips the abnormal voltage and, at the same time, the abnormal voltage is transmitted to the circuit 3 by means of a photocoupler 6 to stop the FETQ 1 and to a transistor Q3 by means of a photocoupler 9. Since the transistor Q3 is turned on, the electric charges of the first output accumulated in a capacitor C4 are discharged through a resistor R14 and the voltage of the first output abruptly drops to about zero.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-output switching power supply device used in office automation equipment such as copying machines and laser printers.

[0002]

2. Description of the Related Art Generally, a commercially available DC power supply device that can obtain stabilized DC power by connecting to an AC power supply such as a commercial power supply,
Alternatively, there are various DC power supply devices built in the main device. Recently, as these DC power supply devices, switching power supply devices using a high frequency switching type DC-DC converter have been widely used because of their small size, light weight, low cost and excellent power efficiency.

In OA equipment such as copiers and laser printers, at least a control power supply (generally + 5V) for a circuit for controlling the OA equipment and a drive power supply (generally + 24V) for driving a photosensitive member and transferring a transfer sheet. Two types of power sources are required. With the increase in speed and multi-functionality of the main body equipment, the precision of power supplies for control and drive is becoming more and more demanding.

In order to construct such a power supply using an AC power supply as an input, the input AC power is rectified and smoothed once to
A first DC output (for example, 24V) having a required voltage is obtained by converting the primary DC power to a secondary DC power and inputting the primary DC power to a first DC-DC converter including a transformer having primary and secondary insulation. To get Further, the first DC output is input to a non-insulating second DC-DC converter (for example, a step-down chopper) to output a second voltage having a required lower voltage.
The DC output (for example, 5 V) is obtained.

In order to stabilize the direct current output of the first DC-DC converter, the primary winding of the transformer and the series circuit of the switching elements are connected to the input direct current power,
The voltage of the first DC output is detected, and the switching element is driven by the drive pulse whose pulse width is modulated according to the detected voltage. That is, by adjusting the duty of the switching element, load fluctuation, input fluctuation, etc. are compensated.

In order to stabilize the DC output of the second DC-DC converter, the output of the first DC-DC converter is a step-down type composed of a second switching element, a choke coil, a commutation diode, etc. Type in the chopper,
The voltage of the second DC output is detected, the second switching element is driven by the drive pulse whose pulse width is modulated according to the detected voltage, and the second DC-DC converter is driven in the same manner as the first DC-DC converter. Compensates for load fluctuations and input fluctuations of the DC converter.

In such a multi-output switching power supply device, if a failure occurs in the control circuit or the switching element of the second DC-DC converter, the first DC-D
There is a risk of abnormal voltage such as the output voltage of the C converter appearing as it is in the output of the second DC-DC converter. That is, the control circuit with a 5V rating has a drive power supply of 24
Since V is applied, the damage is so great that not only the equipment and circuit elements that use this control power supply are damaged, but also serious accidents such as smoking and ignition may occur in some cases.

Therefore, generally, for example, JP-A-64-3 is used.
As disclosed in Japanese Patent No. 4175, there has been a proposal to stop driving of the switching element on the primary side when an overvoltage occurs in the load on the secondary side of the transformer. However, although the above proposal is effective for the voltage abnormality of the first DC output, there is a problem that it does not operate even if the voltage abnormality of the second DC output occurs. Therefore, for example, the actual Kaihei 1-13
As disclosed in Japanese Patent No. 1291, there has been a proposal to stop driving of the switching element on the primary side even if a voltage abnormality occurs in the second DC output.

[0009]

However, when the second DC-DC converter fails in the short circuit state,
As shown in FIG. 6, the output voltage of the second DC output rises to the output voltage of the first DC output (24V), and
Even if the driving of the switching element of the DC converter is stopped and the electric power is not supplied, the first and second DC-Ds
The capacitance of the smoothing capacitor of the C converter, and the first and second
Since it decays relatively slowly with a time constant determined by the impedance of the load, there is still a risk that the control circuit will burn out and cause smoke and fire.

The present invention has been made in view of the above points, and even if a step-down type stabilizing circuit for obtaining the second DC output from the first DC output fails, the second DC It is an object of the present invention to prevent the occurrence of a serious accident such as smoke or fire resulting from damage to each circuit or element connected to the second DC output and the resulting damage without causing an abnormal voltage at the output.

[0011]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention relates to a transformer provided with a secondary winding of primary DC power obtained by rectifying and smoothing AC power input from an AC power supply. The first direct current output is obtained by rectifying and smoothing the electric power induced in the secondary winding of the transformer by inputting it to the series circuit of the primary winding and the switching element and turning the switching element on and off. The first DC output is stabilized by driving the switching element with the drive pulse whose pulse width is controlled according to the result obtained by comparing the DC output of In a multi-output switching power supply device that receives a second DC output of a voltage lower than the first DC output by inputting to a step-down type stabilizing circuit,

A first aspect of the present invention comprises an overvoltage protection circuit for stopping on / off of a switching element when a second direct current output exceeds a predetermined value, and an overvoltage protection circuit in parallel with an output terminal of the second direct current output. A constant voltage element that operates at a value higher than the operating voltage is provided.

A second invention is an overvoltage protection circuit for stopping on / off of a switching element when the second DC output exceeds a predetermined value, and a smoothing of the first DC output when the overvoltage protection circuit operates. And a discharge circuit for rapidly discharging the electric charge of the capacitor.

In the second aspect of the present invention, the discharge circuit is provided with a trigger means composed of a light receiving element operated by a light emitting element of another photocoupler connected in series with the light emitting element of the photocoupler forming the overvoltage protection circuit. Good. Alternatively, the discharge circuit may be provided with a trigger unit that operates according to a current flowing through the light emitting element of the photocoupler that constitutes the overvoltage protection circuit.

[0015]

In the multi-output switching power supply device configured as described above, when a failure occurs in the step-down type stabilizing circuit and the second DC output exceeds a predetermined value, the overvoltage protection circuit first operates to switch the switching element. Stop the power supply by stopping the on / off of. Next, according to the first aspect of the invention, the constant voltage element operates to suppress the voltage of the second DC output to a voltage slightly higher than the operation start voltage of the overvoltage protection circuit. To prevent damage.

According to the second invention, at the same time as the operation of the overvoltage protection circuit, the discharging circuit rapidly discharges the electric charge accumulated in the smoothing capacitor of the first DC output.
The DC output voltage of does not rise abnormally and prevents damage to the connected circuits and elements.

Trigger means for operating the discharge circuit,
A photocoupler that operates the overvoltage protection circuit and a light emitting element are configured with other photocouplers connected in series with each other, or configured to operate according to the current flowing in the light emitting element of the photocoupler that operates the overvoltage protection circuit. Therefore, the smoothing capacitor for the first DC output can be surely discharged at the same time as the operation of the overvoltage protection circuit.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a circuit diagram showing a two-output switching power supply device which is a first embodiment of a multi-output switching power supply device according to the first invention.

The AC power input from the AC power supply 1 is rectified by the rectifier 2 composed of a diode bridge and charges the primary smoothing capacitor C1. The primary DC power charged in the capacitor C1 is an FET (field effect transistor) which is a switching element that is turned on / off by a drive pulse output from the output terminal DP of the pulse width (hereinafter referred to as “PW”) control circuit 3. The primary current that is intermittently generated by Q1 flows through a series circuit composed of the primary winding N1 of the transformer 4, the FET Q1, and the resistor R4 that is the primary current detection resistor, and is used for external output and internal output of the transformer 4, respectively. Of 2
Electric power is induced in the secondary windings N2 and N3.

The secondary winding N3 for internal output is a diode D
PW through a rectifying / smoothing circuit composed of a capacitor C3
Power is supplied to the power input terminal Vcc of the control circuit 3 and is not output to the outside. The voltage dividing point of the voltage divider composed of the resistors R1 and R2 connected between both terminals of the capacitor C1 is also connected to the terminal Vcc through the diode D1, which is the PW control circuit 3 at the start of the power supply device. When a steady state is reached and power is supplied from the secondary winding N3, the diode D1 acts to prevent backflow and becomes irrelevant to the PW control circuit 3.

A series-parallel circuit of a resistor R3, a capacitor C2 and a diode D2 connected in parallel with the primary winding N1 of the transformer 4 is a snubber circuit, and a series circuit 1 flowing through the primary winding N1
The FET absorbs the transient voltage generated when the next current is turned on and off, especially the flyback voltage generated when the current is turned off.
Protect Q1.

2 for external output by turning on / off the primary current
The power induced in the secondary winding N2 becomes a current passing through the diode D4 and the choke coil L1 when the primary current is turned on to charge the capacitor C4, while the core of the choke coil L1 is smoothed to smooth the secondary current. The electric power stored as magnetic energy in the capacitor is reconverted into a current when the primary current is off and passes through the commutation diode D5 to the capacitor C4.
To further charge.

Thus rectified and smoothed, the capacitor C4
The electric power charged to the first DC output (24V for driving)
Is output as. The output voltage V1 is the resistance R8, R
The voltage is divided by 9 to form a series circuit with the resistor R7 and the light emitting element 5a of the photocoupler 5 and output to the shunt regulator ZD1 connected between the output terminals.

The shunt regulator ZD1 is provided with a photocoupler 5 according to the difference between the input divided voltage signal and the internal reference voltage.
The light emitting element 5a of the
Is fed back to the PW control circuit 3 via. PW
The control circuit 3 controls the pulse width of the drive pulse according to the signal fed back, and controls the output voltage V1 of the first DC output to be a preset value (24V). Therefore, the output voltage V1 of the first DC output is stabilized against input fluctuations and load fluctuations.

When an excessive current flows through the primary side of the transformer 4 due to a fault such as a short circuit between the output terminals of the first or second DC output, the excessive current flows through the resistor R4 for detecting the primary current. Is detected by the PW control circuit 3 to limit the operation of the FET Q1 and prevent its damage. Further, when an abnormal voltage is generated in the first DC output due to a failure of the photocoupler 5 or the like, a current flows through the Zener diode ZD3 connected in parallel between the output terminals of the first DC output, and the abnormal voltage is generated. Clamp with voltage.

Further, the Zener diode ZD2 and the resistor R
10. A current flows through the series circuit including the light emitting element 6a of the photocoupler 6, and the light emitting element 6a emits light. The optical signal is fed back to the PW control circuit 3 via the light receiving element 6b connected in series with the resistors R5 and R6. The PW control circuit 3 controls the FET according to the signal input from the light receiving element 6b.
The operation of Q1 is stopped.

A part of the first DC output charges the capacitor C5 for blocking switching noise and inputs it to the step-down chopper circuit 7 which is a step-down stabilizing circuit. The chopper circuit 7 includes a capacitor C5, a transistor Q2 for turning on / off an input current, and a commutation diode D.
6, a smoothing circuit including a choke coil L2, a capacitor C6 and a dummy resistor R11, and a resistor R1 for detecting an output current
2 and a PW control circuit 8 that outputs a drive pulse to the transistor Q2 to control the ON / OFF of the transistor Q2.

When the transistor Q2 is on, the current from the capacitor C5 passes through the choke coil L2 and charges the capacitor C6, while the electric power stored as magnetic energy in the core of the choke coil L2 is transferred to the transistor Q2.
When 2 is off, it is converted back to current and commutation diode D6
By further charging capacitor C6 through
The voltage across the terminals of the capacitor C6 is smoothed and the resistance R12
And is output as a second DC output (5V for control).

The output side terminal voltage of the output current detecting resistor R12 is fed back to the PW control circuit 8 as an output voltage signal of the second output voltage V2, and the voltage between both terminals is fed back to the PW control circuit 8. The PW control circuit 8 outputs a drive pulse, the pulse width of which is controlled according to the fed back output voltage signal, to the base of the transistor Q2, and controls the on / off of the drive pulse to control the output voltage V2 to the input fluctuation or load Stabilizes to 5V against fluctuations.

If an overcurrent flows due to an abnormality of the load of the second DC output, the PW control circuit 8 detects the abnormality by the output current signal generated between the terminals of the resistor R12 and limits the operation of the transistor Q2. Since the overcurrent is prevented by this, the transistor Q2 is not damaged.

Further, due to some reason, a short circuit occurs between the emitter and collector of the transistor Q2 of the chopper circuit 7,
The drive pulse output from the PW control circuit 8 is low (Q2 on)
If the first output voltage V1 (24V) appears in the second DC output as it is due to a hot line short circuit such as not returning to high as it is, the second constant is generated by the time constant of the choke coil L2 and the capacitor C6. The output voltage begins to rise.

The second output voltage is the original output voltage V2
When the voltage exceeds a predetermined value that is set slightly higher than (5V), the Zener diode ZD4 and the resistor R that constitute the overvoltage protection circuit are formed.
13. A current flows through the series circuit including the light emitting element 6a of the photocoupler 6, and the light emitting element 6a emits light. The optical signal is fed back to the PW control circuit 3 via the light receiving element 6b. The PW control circuit 3 stops the operation of the FET Q1 according to the signal input from the light receiving element 6b so that no more power is supplied to the secondary side of the transformer 4.

Even if the electric power supplied to the secondary side of the transformer 4 is stopped, the electric charges of the capacitors C4 and C5 charged to the first output voltage V1 flow into the capacitor C6. However, the current is caused to flow through the Zener diode ZD5 which is a constant voltage element which is connected to the output terminal of the second DC output in parallel and which operates at a slightly larger value by the operating voltage of the overvoltage protection circuit. Become.

Therefore, as shown in FIG. 4, the second output voltage is clamped by the operating voltage of the Zener diode ZD5, which is slightly higher than the original output voltage V2 (5V), and does not rise further. When the voltage across the terminals of the capacitor C4 drops due to the load connected to the first DC output, and the voltage across the terminals of the capacitors C5 and C6 falls below the operating voltage of the Zener diode ZD5 accordingly, the voltage across the terminals of the capacitor C6. Is exponentially reduced to zero by the load connected to the second DC output.

A second DC output overvoltage protection circuit composed of a Zener diode ZD4, a resistor R13, a photocoupler 6, and a PW control circuit 3, and a Zener diode ZD5.
The constant voltage element consisting of the Zener diode ZD2, the resistor R10, the photocoupler 6 and the PW described above.
The configuration and operation are almost the same as those of the first DC output overvoltage protection circuit including the control circuit 3 and the Zener diode ZD3.

However, although there is an example in which a similar overvoltage protection circuit and a constant voltage element are provided for the first DC output, an overvoltage protection circuit and a constant voltage element are provided for the second DC output. There was no multi-output switching power supply circuit. However, the switching element (FE
Even if TQ1) runs away and continues to oscillate, the voltage rise of the first DC output due to it will be 1.4 times to 1.
It is about 5 times (36 V), and even if the driving equipment connected to it is slightly overheated, there is almost no risk of serious damage such as breakage, smoking, or ignition.

On the other hand, when an abnormality occurs in the chopper circuit 7, the second output voltage is 5V to 24V, which is approximately 5V.
Since the abnormal voltage is doubled, not only the control circuits and elements connected to it become irreversibly damaged, but also the drive unit that becomes uncontrollable runs out of control and becomes secondary. There is a possibility that a temporary destruction will occur. Therefore, the second
The effect of providing the overvoltage protection circuit and the constant voltage element to the DC output of is extremely large.

However, in the Zener diode ZD5 (also ZD3) used in this way, a considerable current flows when an accident occurs until the voltage across the terminals of the capacitor C6 drops below the operating voltage of the Zener diode ZD5. ,
It is necessary to use a relatively large capacity Zener diode.

FIG. 2 is a circuit diagram showing a second embodiment of the multi-output switching power supply device according to the second invention. The same parts as those of the first embodiment shown in FIG. Is omitted. The second embodiment differs from the first embodiment in that the light emitting element 6a of the photocoupler 6 is newly connected in series with the light emitting element 9a of the photocoupler 9 and its light receiving element 9b to trigger the first DC output. And a zener diode Z provided with a discharge circuit for discharging the capacitor C4.
That is, D3 and ZD5 are replaced with Zener diodes ZD6 and ZD7 each having a much smaller capacity.

The discharge circuit is provided in parallel with the output terminal of the first DC output (that is, both terminals of the capacitor C4), and is constituted by a series circuit of a discharge resistor R14 having a large capacity and a low resistance and a transistor Q3 which is its switching element. Has been done.
The base of the transistor Q3 is connected via a resistor R15 to a trigger circuit composed of a series circuit of a resistor R16, a light receiving element 9b, and a resistor R17.

When an abnormal voltage occurs in either the first or second DC output, the Zener diode ZD2 and the resistor R1
A current flows to the light emitting elements 6a and 9a of the photocouplers 6 and 9 through 0 or the Zener diode ZD4 and the resistor R13, and the light emitting elements 6a and 9a simultaneously emit light. The light emitted from the light emitting element 6a of the photocoupler 6 is transmitted to the PW control circuit 3 by the light receiving element 6b, and the PW control circuit 3 uses the FETQ.
It has already been explained that the operation of No. 1 is stopped and the supply of electric power thereafter is stopped.

At the same time, the light emission of the light emitting element 9a of the photocoupler 9 causes a current to flow through the light receiving element 9b, and the positive voltage generated between the terminals of the resistor R17 turns on the transistor Q3 via the resistor R15. , Capacitor C4
The charge that has been discharged through the discharge resistor R14,
The voltage of the first DC output drops to near zero in a very short time.

When an abnormal voltage occurs only in the first DC output, the chopper circuit 7 is operating normally, so that the second DC output is not affected and the voltage of the first DC output drops. Even if the electric power is not supplied, the main body device is controlled for a short time by the electric power stored in the capacitor C6, and the runaway of the drive system is prevented.

When an abnormal voltage is generated in the second DC output due to a hot line short circuit, a rapid voltage drop of the first DC output and the choke coil L2 and the capacitor C6 are generated as shown by the broken line in FIG. Due to the action, a slight voltage rise, which is much smaller than the original output voltage V1 of the first DC output, appears for an extremely short time, but this voltage rise is also caused by the Zener diode ZD7 as shown by the solid line V2. Since the voltage is clamped at a voltage slightly higher than (5V), the control system circuit and elements connected to the second DC output are not damaged.

As described above, the discharge circuit according to the second aspect of the present invention not only can suppress the abnormal voltage generated in the second DC output to protect the connected equipment, but also can increase the first DC output. It has the same effect on the abnormal voltage generated. Further, by using together with the Zener diode ZD7 which is the constant voltage element according to the first aspect of the present invention, the connected equipment can be protected more reliably.

Further, by using the first and second inventions in combination, the Zener diode ZD that clamps an abnormal voltage generated in either the first or the second DC output is generated.
6 or ZD7, a current flows only for a very short time, so that the Zener diodes ZD6 and ZD7 are respectively the first
As compared with the Zener diodes ZD3 and ZD5 shown in the embodiment, it is possible to use a device having a much smaller capacity and hence a lower cost.

FIG. 3 is a circuit diagram showing a third embodiment of a multi-output switching power supply device according to the second invention, in which the same parts as those in the first and second embodiments are designated by the same reference numerals. Is omitted. The third embodiment is also provided with a discharge circuit, but the difference from the second embodiment is that the current flowing through the light emitting element 6a of the photocoupler 6 is used to trigger the discharge circuit. That is, the photocoupler 9 and the resistor R
Instead of the trigger circuit consisting of 15, R16 and R17, a voltage divider consisting of resistors R18 and R19 is used.

When a current flows through a series circuit composed of the Zener diode ZD4 for detecting the abnormal voltage of the second DC output, the resistor R13, and the light emitting element 6a of the photocoupler 6, it is generated at both ends of the resistor R13 and the light emitting element 6a. The voltage is the resistance R1
The voltage is divided by a voltage divider composed of 8 and R19, and the voltage dividing point and the base of the transistor Q3 are connected to trigger the discharge circuit composed of the discharge resistor R14 and the transistor Q3.

When a current flows through a series circuit composed of the Zener diode ZD2 for detecting an abnormal voltage of the first DC output, the resistor R10, and the light emitting element 6a, the voltage generated across the light emitting element 6a is applied to the resistors R13, R18 ,. Since the voltage is divided by the voltage divider composed of R19, by changing the constant of each element, it is possible to select whether or not to activate the discharge circuit when an abnormal voltage occurs in the first DC output.

When the discharge circuit is activated regardless of which DC output has an abnormal voltage, the + terminal of the light emitting element 6a of the photocoupler 6 may be connected to the base of the transistor Q3 via a limiter resistor.

In the third embodiment, the photocoupler 6 of the second embodiment is eliminated and the trigger circuit is composed of only resistance elements, so that the same effect can be obtained at low cost. However, in the case of the second embodiment, even if there is some leakage current in the Zener diodes ZD2 and ZD4, there is no problem as long as the light emitting element 9a does not emit light, but in the third embodiment, malfunction of the discharge circuit due to leakage current occurs. Zener diode Z
It is necessary to use elements with a small leak current for D2 and ZD4 or pay attention to the distribution of each constant of the resistance group.

Although the case of the two-output switching power supply device has been described as the embodiment, the present invention can be applied to a switching power supply device having two or more DC outputs obtained by stepping down the first DC output. Needless to say.

[0053]

As described above, the multi-output switching power supply device according to the present invention has the first DC output to the second DC output.
Even if a failure occurs in the step-down type stabilizing circuit for obtaining the DC output, the abnormal voltage does not occur in the second DC output, and each circuit or element connected to the second DC output It is possible to prevent the occurrence of serious accidents such as breakage and smoke and ignition resulting from the damage.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a multi-output switching power supply device according to the first invention.

FIG. 2 is a circuit diagram showing a second embodiment of the multi-output switching power supply device according to the second invention.

FIG. 3 is a circuit diagram showing a third embodiment of a multi-output switching power supply device according to the second invention.

4 is a waveform chart showing an example of a change in output voltage when a second DC output abnormality occurs in the first embodiment shown in FIG.

FIG. 5 is a waveform diagram showing an example of changes in output voltage when a second DC output abnormality occurs in the second and third embodiments shown in FIGS. 2 and 3.

FIG. 6 is a waveform diagram showing an example of a change in output voltage when a second DC output abnormality occurs in the conventional multi-output switching power supply device.

[Explanation of symbols]

1 AC power source 3,8 PW (pulse width) control circuit 4 Transformer 6 Photocoupler constituting an overvoltage protection circuit 6a, 9a Light emitting element 6b, 9b Light receiving element 7 Chopper circuit (step-down stabilizing circuit) 9 Constituting trigger means Photocoupler N1 Primary winding N2 Secondary winding Q1 FET (field effect transistor: switching element) ZD5, ZD7 Zener diode (constant voltage element)

Claims (4)

[Claims] 1. A primary DC power obtained by rectifying and smoothing AC power input from an AC power supply is input to a series circuit of a primary winding of a transformer having a secondary winding and a switching element, and The result obtained by rectifying and smoothing the power induced in the secondary winding of the transformer by turning on / off the switching element to obtain the first DC output, and comparing the first DC output with the reference voltage. The first DC output is stabilized by driving the switching element with a drive pulse whose pulse width is controlled in accordance with the first pulse, and the first DC output is input to a step-down type stabilizing circuit. In a multi-output switching power supply device that obtains a second DC output having a lower voltage than the DC output, an overvoltage protection circuit that stops ON / OFF of the switching element when the second DC output exceeds a predetermined value, Multiple-output switching power supply device according to claim to the output terminal of the second DC output to the provision of the constant voltage element that operates at a value greater than the operating voltage of the overvoltage protection circuit in parallel. 2. A primary DC power obtained by rectifying and smoothing AC power input from an AC power supply is input to a series circuit of a primary winding of a transformer having a secondary winding and a switching element, The result obtained by rectifying and smoothing the power induced in the secondary winding of the transformer by turning on / off the switching element to obtain the first DC output, and comparing the first DC output with the reference voltage. The first DC output is stabilized by driving the switching element with a drive pulse whose pulse width is controlled in accordance with the first pulse, and the first DC output is input to a step-down type stabilizing circuit. A multi-output switching power supply device for obtaining a second DC output having a lower voltage than a DC output, comprising: an overvoltage protection circuit for stopping ON / OFF of the switching element when the second DC output exceeds a predetermined value; Multiple-output switching power supply apparatus characterized by pressure protection circuit provided a discharge circuit to rapidly discharge electric charge of the first smoothing capacitor of the DC output of when operating. 3. The multi-output switching power supply device according to claim 2, wherein the discharge circuit is operated by a light emitting element of another photocoupler connected in series with a light emitting element of a photocoupler forming the overvoltage protection circuit. A multi-output switching power supply device characterized in that trigger means comprising a light-receiving element is provided. 4. The multi-output switching power supply device according to claim 2, wherein the discharge circuit is provided with a trigger unit that operates according to a current flowing through a light emitting element of a photocoupler that constitutes the overvoltage protection circuit. Multi output switching power supply.