JPH0819251A - Switching power supply apparatus - Google Patents

Abstract

PURPOSE:To provide a switching power supply apparatus which has an output voltage protective circuit and which has the high reliability and has a very small occupying area. CONSTITUTION:A power supply 51, a transformer 54 which isolates a primary side transmission line and a secondary side transmission line electrically from each other, a main switch 53 provided in the primary side transmission line, a synchronous rectifier 60 which rectifies a current supplied by the power supply synchronously and a smoothing circuit 70 which smooths a voltage outputted by the synchronous rectifier 60 are provided. Further, a comparator 20 which compares the pulse duration time of a signal which controls the main switch 53 with the predetermined range of the duration time and which outputs a comparison result and a stop signal transmitting circuit 30 which supplies a stop signal to the switching controller in accordance with the comparison result are provided.

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 using a synchronous rectification circuit, and more particularly to a switching power supply device which detects an abnormal output voltage such as an overvoltage or a low voltage and stops the power supply to a load. is there.

[0002]

2. Description of the Related Art A conventional switching power supply device using a synchronous rectification circuit will be described with reference to FIG. Main switch 53
Is connected to the primary side of the transformer 54 in series with the input voltage source 51. The main switch 53 is turned on / off by a high frequency control signal output from the main switch control circuit 52, and a high frequency voltage is added to the primary side of the transformer 54. The synchronous rectification circuit 60 is connected to the secondary side of the transformer 54 and uses a field effect transistor or the like to connect the transformer 5
The high-frequency current induced on the secondary side of 4 is synchronously rectified. The output of the synchronous rectifier circuit 60 is smoothed by a smoothing circuit 70 including a choke coil and a capacitor. Further, the voltage smoothed by the smoothing circuit 70 is input to the output voltage protection circuit 100 and further supplied to the load 80.

The output voltage protection circuit 100 is provided on the secondary side of the switching power supply device 102, and has a low voltage detection circuit (not shown), an overvoltage detection circuit (not shown), and a stop signal transmission circuit (not shown). Composed of. When the output voltage of the switching power supply device 102 deviates from the proper fluctuation range of the rated voltage and drops, the low voltage detection circuit sends an abnormal voltage detection signal to the stop signal sending circuit. Upon receiving this abnormal voltage detection signal, the stop signal sending circuit sends a stop signal to the main switch control circuit 52 via the photocoupler 101 to stop the voltage supply to the load 80.

When the output voltage of the switching power supply device 102 deviates from the proper fluctuation range of the rated voltage and rises, the overvoltage detection circuit sends an abnormal voltage detection signal to the stop signal sending circuit. Upon receiving this abnormal voltage detection signal, the stop signal sending circuit sends a stop signal to the main switch control circuit 52 via the photocoupler 101 to stop the voltage supply to the load 80.

[0005]

The switching power supply device 102 normally insulates the primary side and the secondary side via a transformer 54. Therefore, the output voltage protection circuit 100 connected to the secondary side outputs a stop signal to the primary side by using the photocoupler 101 between the primary side and the secondary side in order to stop the voltage supply to the load 80. Send to the circuit. However, since the photocoupler 101 is one of the least reliable components in the power supply device, the photocoupler 1 has the conventional configuration.
However, there is a problem in that the reliability of the entire power supply device is reduced by using 01. Also, since the connection part on the primary side and the connection part on the secondary side are connected to both ends of the coupler, without processing the ready-made coupler to an appropriate length, the wiring becomes long, which makes it difficult to route the wiring. There is also a problem that the occupied area including the wiring is very large.

[0006]

A switching power supply device according to the present invention is provided with a transformer, a wire connected to the primary side of the transformer, and a wire, and a voltage can be applied to the primary side of the transformer. , A switching circuit that makes it impossible,
And a synchronous rectification circuit connected to the secondary side of the transformer. Furthermore, a first control signal that enables the application of a voltage to the primary side of the transformer and a second control signal that disables the application of a voltage to the primary side of the transformer are sent to the switching circuit. A switching control circuit for sending out and a protection circuit for comparing whether the duration of the first control signal is within a preset duration range and sending out a stop signal to the switching control circuit based on the comparison result. Have and.

[0007]

Function: The transformer causes mutual induction on the primary side and the secondary side. The wiring is connected to the primary side of the transformer and allows current to flow to the primary side of the transformer. The switching circuit is provided in the wiring and can apply a voltage to the primary side of the transformer.
Make it impossible. The synchronous rectification circuit is connected to the secondary side of the transformer and performs synchronous rectification. The switching control circuit switches between a first control signal that enables application of a voltage to the primary side of the transformer and a second control signal that disables application of a voltage to the primary side of the transformer. Send to the circuit.
The protection circuit compares whether the duration of the first control signal is within a preset duration range, and sends a stop signal to the switching control circuit based on the comparison result.

[0008]

1 is a block diagram showing a switching power supply device according to a first embodiment of the present invention. In Figure 1,
Besides the switching power supply 1, a load 80 connected to it is also shown. First, the configuration of the switching power supply device 1 will be described. The main switch 53
The primary side of the transformer 54 is connected in series with the input voltage source 51. The main switch 53 is a main switch control circuit 52.
A high-frequency drive signal (hereinafter, referred to as a main switch control signal) output from the switch is turned on / off. As a result, a part of the current transmission path that connects the power source 51 and the transformer 54 becomes conductive or non-conductive, and high frequency waves flow to the primary side of the transformer 54. A synchronous rectification circuit 60 and a smoothing circuit 70 are connected to the secondary side of the transformer 54 as shown in the figure. The synchronous rectifier circuit 60 is a rectifier circuit that synchronously rectifies the high frequency wave induced on the secondary side of the transformer 54 by using a field effect transistor or the like. Synchronous rectification means that when the main switch 53 is in the conducting state, the rectifying field effect transistor is in the conducting state, and the commutation electrolytic transistor is in the non-conducting state, and when the main switch is in the non-conducting state, the rectifying field effect transistor is in the conducting state. Is a non-conducting state, and the commutation electrolytic transistor is in a conducting state to rectify. The output of the synchronous rectification circuit 60 is a smoothing circuit 7 including a choke coil and a capacitor.
It is connected to 0 and smoothed. The smoothed voltage is supplied to the load 80 connected to the output side of the smoothing circuit 70.

When the synchronous rectifier circuit is used as the rectifier circuit of the switching power supply device, the following characteristics occur. That is, when a constant voltage is supplied to the load, the main switch control signal output from the main switch control circuit 52 does not need to be changed even if the size of the load changes, and may remain almost constant. Therefore, when the main switch control signal changes, it can be determined that the voltage supplied to the load 80 is in an abnormal state such as an overvoltage or a low voltage. Therefore, the output voltage protection circuit 10 can determine whether or not the voltage value supplied to the load is abnormal only by monitoring the main switch control signal. When a change occurs in the main switch control signal, the output voltage protection circuit 10 considers the supply voltage value to the load 80 of the switching power supply device 1 to be abnormal, and sends a stop signal to the main switch 53. The voltage supply to the load 80 is stopped. Next, the output voltage protection circuit 10, which is a main part of the present invention, will be described. The output voltage protection circuit 10 includes a comparison circuit 20, a reference signal generation circuit 40, and a stop signal transmission circuit 30, and uses the main switch control signal as an input signal. The comparison circuit 20 uses the main switch control signal as one of the input signals, and further uses the reference signal generated by the reference signal generation circuit 40 as the other input signal. The output signal of the comparison circuit 20 is input to the stop signal transmission circuit 30, and the output signal of the stop signal transmission circuit 30 is input to the main switch control circuit 52. As described above, the output voltage protection circuit 10 of the present invention monitors the main switch control signal, and thus the switching power supply device 1
Can be connected to the primary side of the.

Next, a concrete circuit of the first embodiment is shown in FIG. In FIG. 2, only the main switch control circuit 52, the main switch control terminal 52a, and the output voltage protection circuit 10 in the configuration of the first embodiment shown in FIG. 1 are shown, and other configurations are omitted because they are not shown. . First, the configuration of the reference signal generation circuit 40 that constitutes the output voltage protection circuit 10 will be described. The reference signal generation circuit 40 is composed of a monostable multivibrator 41 and a monostable multivibrator 42, and the monostable multivibrators 41 and 42 use the main switch control signal as a trigger signal. The output signal of the monostable multivibrator 41 is sent to the NOR gate 22 of the comparison circuit 20, and the output signal of the monostable multivibrator 42 is sent to the AND gate 23 of the comparison circuit 20.

Next, the structure of the comparison circuit 20 will be described.
The comparison circuit 20 includes an inverter 21, a NOR gate 22,
It is composed of AND gates 23 and 26 and monostable multivibrators 24 and 25. The inverter 21 is
The main switch control signal is used as an input signal and the inverted signal is input as N
It outputs to the OR gate 22 and the AND gate 23, respectively. The output signal of the NOR gate 22 is input to the monostable multivibrator 24 as a trigger signal. Also, A
The output signal of the ND gate 23 is input to the monostable multivibrator 24 as a trigger signal. The output signals of the monostable multivibrators 24 and 25 are input to the AND gate 26, and the output signal of the AND gate 26 is input to the stop signal sending circuit 30. Stop signal sending circuit 30
Sends an operation stop signal to the main switch control circuit 52 based on the output signal of the AND gate 26.

FIG. 3 is a structural example of a monostable multivibrator. One end of the capacitor C1 is connected to the trigger terminal, and the other end of the capacitor C1 is connected to one end of the resistor R1. The other end of the resistor R1 has a voltage source + VCC
(Not shown in FIGS. 1 and 2). The connection point between the capacitor C1 and the resistor R1 is NAND9
1 is connected to the input terminal 1. The output terminal of the NAND 91 is connected to one end of the capacitor C2, and the other end of the capacitor C2 is connected to one end of the resistor R2. The other end of the resistor R2 is grounded (hereinafter referred to as GND).
Have been. Further, the other end of the capacitor C2 and the resistor R2
Is connected to the input terminals 1 and 2 of the NAND 92, and the output terminal of the NAND 92 is
It is connected to the input terminal 2 of 91. The output terminal of the NAND 91 becomes the output terminal of this monostable multivibrator. With the above configuration, a pulse having a time width determined by the time constant composed of the capacitor C2 and the resistor R2 is obtained from the output terminal.

Next, the operation of the output voltage protection circuit 10, which is the essential part of the first embodiment of the present invention, will be described in detail with reference to the timing chart shown in FIG. In the normal state, the main switch control signal output from the main switch control circuit 52 is a signal formed of pulses having an appropriate time width, as shown in FIG. However, the main switch control signal having the pulse of the proper time width is the signal from the first pulse to the third pulse. Here, the pulse is
It is a signal from the time when the signal level rises from low to high to the time when the signal level falls from high to low. That is, the main switch control signal indicates a signal that brings the main switch 53 into the conductive state.

First, consider the case where the main switch control signal is a signal having a pulse of a proper time width in a normal state. When the main switch control circuit 52 outputs a pulse signal having a substantially constant and proper time width, the monostable multivibrator 41 uses the main switch control signal as a trigger, and thus the main switch control signal synchronized with the main switch control signal is used. A pulse having a time width slightly shorter than the pulse width of the switch control signal is output. This is because the monostable multivibrator 41
This is because the duration of the pulse to be output is set in advance to be slightly shorter than the pulse duration of the main switch control signal during normal operation. Here, the time width of the pulse and the duration of the pulse have the same meaning. The output signal of the monostable multivibrator 41 is shown in FIG. 4 as the output signal of the monostable multivibrator 41. Similarly, since the monostable multivibrator 42 uses the main switch control signal as a trigger, it outputs a pulse synchronized with the main switch control signal and having a time width slightly longer than the pulse. This is because the monostable multivibrator 41 presets the duration of the pulse to be output to be slightly longer than the pulse duration of the main switch control signal during normal operation. This output signal is shown in FIG. 4 as the monostable multivibrator 42 output signal. By the above,
It can be seen that the pulse falling timing of the main switch control signal is between the pulse falling timing of the monostable multivibrator 41 output signal and the pulse falling edge of the monostable multivibrator 42 output signal. .

Since the NOR gate 22 receives the output signal of the inverter 21 and the output signal of the monostable multivibrator 41 as input signals, the NOR gate 22 outputs a pulse signal having a constant pulse interval as shown as an output signal of the NOR gate 22. Is output. Here, the pulse interval is a time interval from the fall timing of one pulse to the rise timing of the next pulse. Since the AND gate 23 uses the output signal of the inverter 21 and the output signal of the monostable multivibrator 42 as input signals, it outputs a pulse signal having a constant pulse interval as shown as an output signal of the AND gate 23 in FIG. .

The monostable multivibrator 24 is a NOR
Since the output signal of the gate 22 is used as a trigger signal, NOR
If the output signal of the gate 22 is a pulse signal having an appropriate pulse interval, a signal having a high signal level is always output. This is because the monostable multivibrator 24 presets the duration of the pulse to be output to be slightly longer than the cycle of the main switch control signal during normal operation. Here, the proper pulse interval is a pulse interval approximately equal to the cycle of the main switch control signal. In FIG. 4, as the output signal of the monostable multivibrator 24,
The signal output by the monostable multivibrator 24 is shown.
Similarly, since the monostable multivibrator 25 also uses the output signal of the AND gate 23 as a trigger signal, if the output signal of the AND gate 23 has a proper pulse interval as described above, the signal level is always high. Is outputting the signal. This is also similar to the monostable multivibrator 2
5 is set in advance so that the duration of the pulse to be output is slightly longer than the period of the main switch control signal during normal operation. FIG. 4 shows the monostable multivibrator 25 output signal as the monostable multivibrator 2 output signal.
5 shows a signal output from the device. Since the AND gate 26 uses the output signal of the monostable multivibrator 24 and the output signal of the monostable multivibrator 25 as input signals, when the main switch control signal is a signal having a pulse with an appropriate time width, the signal level is Is outputting a high signal.

The stop signal transmission circuit 30 includes an AND gate 2
The signal input from 6 is inverted, and the main switch control circuit 5
Send to 2. In the stop signal transmission circuit 30, an inverted signal is shown in FIG. 4 as an output signal of the stop signal transmission circuit. When the signal level of the stop signal sending circuit output signal is high, the main switch control circuit 52 stops the operation of the main switch and does not supply power to the load.

Next, consider a case where an abnormality occurs in the switching power supply device 1 and, as a result, the time width of the pulse of the main switch control signal output from the main switch control circuit 52 becomes longer than a predetermined time width. In this case, the main switch 53
Since the conduction time is longer than the predetermined time, the voltage value supplied to the load 80 becomes higher than the predetermined voltage value. Therefore, when the conduction time of the main switch 53 is longer than the predetermined time, the output voltage protection circuit 10 must detect this and stop driving the main switch 53. less than,
The operation of the output voltage protection circuit 10 when the time width of the pulse of the main switch control signal becomes longer than the predetermined time width will be described with reference to FIG. The fourth pulse of the main switch control signal shown in FIG. 4 is a pulse longer than a predetermined time width. However, it is assumed that the duration of the fourth pulse is longer than the duration of the pulse preset by the monostable multivibrator 42. In this case, the AND gate 23 output signal does not pulse at a predetermined time corresponding to the fourth pulse of the main switch control signal. This is A
This is because the ND gate 23 uses the inverter 21 output signal, which is an inverted signal of the main switch control signal, and the monostable multivibrator 42 output signal as input signals. on the other hand,
The output signal of the NOR gate 22 generates a pulse longer than a normal time width at a predetermined time corresponding to the fourth pulse of the main switch control signal. This is because the NOR gate 22 uses the inverter 21 output signal, which is an inverted signal of the main switch control signal, and the monostable multivibrator 41 output signal as input signals.

The monostable multivibrator 24 is a NOR
The gate 22 output signal is set as a trigger signal so that a pulse having a duration slightly longer than the cycle of the main switch control signal in the normal state is output. Therefore, when the main switch control signal is a pulse having an appropriate time width, the monostable multivibrator 24 generates a signal whose continuous signal level is high. However, if the pulse of the output signal of the AND gate 23 does not occur at a predetermined time, the output signal of the monostable multivibrator 25 becomes a signal with a low signal level. This is shown in the monostable multivibrator 25 output signal of FIG. Since the AND gate 26 uses the output signals of the monostable multivibrators 24 and 25 as input signals, when the output signal of the monostable multivibrator 25 becomes a signal of low signal level, the output signal of the AND gate 26 also becomes a signal of low signal level. Become. As a result, the stop signal sending circuit 30 sends a signal for stopping the operation of the main switch control circuit, and along with this, the main switch 5
The operation of 3 stops.

As described above, the case where the time width of the pulse of the main switch control signal is longer than the predetermined time width is considered as the case where the abnormality occurs in the switching power supply device 1. However, even when the time width of the output pulse of the main switch control circuit becomes shorter than the predetermined time width, the output voltage protection circuit 10 similarly instructs the main switch control circuit 52 to stop the main switch 53. Send a stop signal. However, in this case, the NOR gate 22
No pulse is generated at a predetermined time, and as a result, the stop signal sending circuit 30 sends a stop signal to the main switch control circuit 52.

In the first embodiment described in detail above, when the pulse duration of the main switch control signal becomes longer than the first predetermined pulse duration or the second predetermined pulse duration. When it becomes shorter, the stop signal is sent to the main switch control circuit 52 so as to stop the power supply to the load. The first predetermined pulse duration, which is the critical point of whether or not the stop signal is transmitted, is determined by the output pulse duration of the monostable multivibrator 42. Further, the second predetermined pulse duration, which is the critical point of whether or not to send the stop signal, is
It is determined by the output pulse duration of the monostable multivibrator 41. As described above, by appropriately setting the output pulse durations of the monostable multivibrators 41 and 42, the allowable range of the pulse duration of the main switch control signal, in other words, the magnitude of the power supplied to the load, can be set. Allowable range can be set. Further, in the first embodiment, only the duration of the pulse of the main switch control signal is monitored, but the value of the voltage supplied to the load 80 is not limited to the duration of the above-mentioned pulse, but also the pulse interval. Also depends on the length of. Therefore, if more accurate monitoring is required, the pulse interval time must be monitored as well. In this case, the output voltage protection circuit 1 of the first embodiment
The pulse interval time can be monitored by inputting a signal obtained by inverting the main switch control signal to a circuit having the same configuration as 0. The output voltage protection circuit 10 of the first embodiment described in detail above monitors each pulse of the main switch control signal, and immediately stops even if one pulse has an inappropriate time width. The signal is sent to the main switch control circuit. Therefore, it can be said that the output voltage protection circuit 10 of the first embodiment is an output voltage protection circuit with a very high detection sensitivity.

Next, a second embodiment of the present invention will be shown.
Although the second embodiment has the block configuration shown in FIG. 1, the specific circuit forming the block is different from that of the first embodiment. A concrete circuit of the second embodiment is shown in FIG. 5, only the main switch control circuit 52 and the output voltage protection circuit 10 are shown in the configuration of the first embodiment shown in FIG.
The other structure is similar to that of the first embodiment and is omitted in the drawing. First, the configurations of the reference voltage generation circuit 40 and the comparison circuit 20 will be described. Reference voltage generation circuit 4
0 is constituted by DC voltage sources V1 and V2. The negative terminals of the DC voltage sources V1 and V2 are connected to GND, and the positive terminals of the DC voltage sources V1 and V2 are connected to the comparison circuit 20. The comparison circuit 20 includes an inductor L
1, a capacitor C3, an AND gate 27, and comparators 28 and 29. One end of the inductor L1 is connected to the main switch control terminal 52a of the main switch control circuit 52, and the other end is connected to the plus input terminal of the comparator 28 and the minus input terminal of the comparator 29. One end of the capacitor C3 has an inductor L
1 and the positive input terminal of the comparator 28, and the other end is connected to GND. The minus input terminal of the comparator 28 is connected to the plus terminal of the DC voltage source V1 of the reference voltage generating circuit 40. The plus input terminal of the comparator 29 is connected to the plus terminal of the DC voltage source V2 of the reference voltage generating circuit 40. Also,
The output terminals of the comparators 28 and 29 are connected to the input terminal of the AND gate 27, and the output terminal of the AND gate 27 is connected to the input terminal of the stop signal sending circuit 30.

The operation of the second embodiment of the present invention will be described below with reference to the time charts shown in FIGS. 6 and 7.
First, with reference to FIG. 6, consider a case where the main switch control signal is a pulse signal having an appropriate time width. However, the pulse signal having the proper time width is from the first pulse to the third pulse of the main switch control signal in the figure. The main switch control signal output from the main switch control circuit 52 is smoothed by the inductor L1 and the capacitor C3 and input to the comparators 28 and 29. This comparator 28
The signals input to and 29 are shown in FIG. 6 as comparator input signals. The comparator input signal is compared with the voltage value of the DC voltage source V1 by the comparator 28. Further, the comparator input signal is compared with the voltage value of the DC voltage source V2 by the comparator 29.
Here, the DC voltage source V1 generates a voltage V1 slightly higher than the voltage value of the appropriate comparator input signal, and the DC voltage source V2 generates a voltage V2 slightly lower than the voltage value of the appropriate comparator input signal. It has occurred. The values of V1 and V2 are the smoothed value of the main switch control signal corresponding to the low voltage and the proper voltage threshold value and the overvoltage and the proper voltage threshold value, respectively, at the load voltage. Therefore,
V1 is called the low voltage detection threshold voltage, and V2
Will be referred to as an overvoltage detection threshold voltage.

When the main switch control circuit 52 outputs a substantially predetermined main switch control signal, the voltage value of the comparator input signal is between the low voltage detection threshold voltage value and the overvoltage detection threshold voltage value. is there. Therefore, both the comparators 28 and 29 output a signal whose signal level is high. Comparator 28
The output signals of the comparators 28 and 29 are shown in FIG. 6 as the output signal and the output signal of the comparator 29. When the output signals of the comparators 28 and 29 are both high in signal level, the stop signal sending circuit 30 receives the high signal level and the stop signal sending circuit 30 does not send the stop signal. The stop signal sent by the stop signal sending circuit 30 is shown in FIG. 6 as a stop signal.

Next, consider a case where an abnormality occurs in the main switch control circuit 52, the time width of the pulse of the main switch control signal becomes longer than a predetermined time width, and an overvoltage is supplied to the load 80. Fourth in the main switch control signal in FIG.
A pulse indicates this condition. When the pulse width of the main switch control signal becomes longer than the predetermined time width, the voltage value of the comparator input signal exceeds the value of the overvoltage detection threshold voltage. As a result, the comparator 29
The output signal has a low signal level. Therefore, the AND gate 27 sends the signal whose signal level is low to the stop signal sending circuit 30. As a result, the stop signal sending circuit 30 sends a signal whose signal level is high for stopping the operation of the main switch control circuit 52, and the operation of the main switch 53 is stopped accordingly.

Further, consider a case where an abnormality occurs in the main switch control circuit 52, the pulse width of the main switch control signal becomes shorter than a predetermined time width, and an insufficient voltage is supplied to the load. FIG. 7 shows the waveform of each signal when the pulse width of the main switch control signal becomes shorter than the predetermined time width.
However, the main switch control signal in the figure is a normal pulse from the first pulse to the third pulse, and the fourth pulse is a pulse shorter than a predetermined time width. When the time width of the pulse of the main switch control signal becomes shorter than the predetermined time width, the voltage value of the comparator input signal falls below the value of the low voltage detection threshold voltage. As a result, the comparator 28
The output signal has a low level. Therefore,
The AND gate 27 sends a signal whose signal level is low to the stop signal sending circuit 30. As a result, the stop signal sending circuit 30 sends a signal whose signal level is high for stopping the operation of the main switch control circuit 52, and the operation of the main switch 53 is stopped accordingly. In the second embodiment of the present invention, the main switch control signal is smoothed in the comparison circuit 20 instead of monitoring each individual pulse of the main switch control signal. Therefore, the characteristic close to the voltage actually supplied to the load 80 is obtained. You are monitoring a signal with.

In the output voltage protection circuit 10 according to the first and second embodiments described above in detail, the voltage value supplied from the switching power supply device 1 to the load is lower than the first predetermined voltage value. If it is larger or smaller than the second predetermined voltage value, the switching power supply 1 to the load 8
It operates so as to stop the power supply to 0. Here, the output voltage protection circuit 10 is configured to send a stop signal to the main switch control circuit 52 in order to stop the power supply to the load. However, the output voltage protection circuit 1
For 0, it is not always necessary to send the stop signal to the main switch control circuit 52, and it is sufficient to send the stop signal to any means capable of stopping the power supply to the load in the swinging power supply device 1.

[0028]

As described above, according to the present invention, when the switching power supply device uses the synchronous rectification circuit, the voltage value supplied to the load is indirectly monitored by monitoring the main switch control signal. Paying attention to the fact that this can be done, the signal targeted by the voltage protection circuit is used as the main switch control signal. As a result, the voltage protection circuit can be arranged on the primary side of the switching power supply device, and it is not necessary to use a photocoupler for sending a stop signal from the voltage protection circuit, which has been conventionally required. For this reason, the reliability of the entire power supply device is improved, wiring is facilitated, and
The occupied area including the wiring can be made very small.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment.

FIG. 2 is a circuit diagram showing a specific circuit of the first embodiment.

FIG. 3 is a circuit diagram showing a configuration example of a monostable multivibrator.

FIG. 4 is a time chart showing operation waveforms of the first embodiment.

FIG. 5 is a circuit diagram showing a specific circuit of the second embodiment.

FIG. 6 is a first time chart showing operation waveforms of the second embodiment.

FIG. 7 is a second time chart showing operation waveforms of the second embodiment.

FIG. 8 is a block diagram of a conventional output voltage protection circuit.

[Explanation of symbols]

 1, 102 Switching power supply device 10, 100 Output voltage protection circuit 20 Comparison circuit 3 21 Inverter 22 NOR gate 23, 26, 27 AND gate 24, 25, 41, 42 Monostable multivibrator 28, 29 Comparator 30 Stop signal sending circuit 40 Reference signal generation circuit 51 Input voltage source 52 Main switch control circuit 52a Main switch control terminal 53 Main switch 54 Transformer 60 Synchronous rectifier circuit 70 Smoothing circuit 80 Load 91, 92 NAND 101 Photocoupler C1, C2, C3 Capacitor R1, R2 Resistor L1 Inductor + VCC Voltage source V1, V2 DC voltage source

Claims (2)

[Claims] 1. A transformer, a wiring connected to a primary side of the transformer, a switching circuit provided on the wiring, which enables or disables application of a voltage to the primary side of the transformer, A synchronous rectification circuit connected to the secondary side of the transformer, and a first enabling the application of voltage to the primary side of the transformer
And a switching control circuit for sending to the switching circuit a second control signal that makes it impossible to apply a voltage to the primary side of the transformer, and a duration of the first control signal. A switching power supply device comprising: a protection circuit that compares whether or not the duration is within a preset duration and sends a stop signal to the switching control circuit based on the comparison result. 2. The switching power supply device according to claim 1, wherein the protection circuit synchronizes with the first control signal, and
And a second reference signal generating a second pulse having a duration longer than the duration of the first pulse, in synchronization with the first control signal. A signal generation circuit, comparing the duration of the first control signal with the duration of the first and second pulses, and comparing the duration of the first control signal with the first pulse. The switching power supply device is characterized in that the stop signal is sent when the duration of the first control signal is shorter than the duration of the second pulse or when the duration of the first control signal is longer than the duration of the second pulse.