JPH08237945A - Switching power supply - Google Patents

Abstract

PURPOSE: To realize both automatic voltage switching and open protection through simple circuitry while stabilizing the output voltage. CONSTITUTION: A series circuit of the primary winding L1 of a transformer T, a switching transistor Q1 and an emitter resistor R1 is connected across a rectifier circuit REC. The switching transistor Q1 is self-excited through a feedback circuit having base winding L3 to induce a voltage in the output winding L2. Detection winding L4 inducing a voltage proportional to the voltage induced in the output winding L2 is provided on the primary and charged with a smoothing capacitor C2. During an interval when the charging voltage exceeds the Zener voltage of a Zener diode ZD1, a transistor Q3 is turned ON to sustain turn OFF of the switching transistor Q1. When the input voltage is high or a light or no load is applied to the output side, OFF time of the switching transistor Q1 is prolonged after increase of induced voltage thus stabilizing the output voltage and providing open protection.

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 circuit for switching a current flowing through a primary winding by turning a switching element on and off to output a secondary current from an output winding on a secondary side, and more particularly to a switching power supply circuit. , A switching power supply circuit capable of coping with changes in primary side input voltage and changes in secondary side load state.

[0002]

2. Description of the Related Art Conventionally, by connecting a switching element in series with a primary winding and turning this switching element on and off, the current flowing into the primary winding is switched and the output winding on the secondary side is switched. A switching power supply circuit is known that induces a voltage and supplies a load current to a load such as a secondary battery or a motor connected to the output side.

In this type of power supply circuit, a power supply circuit provided with an automatic voltage switching circuit for stabilizing the output voltage even when the input voltage changes is proposed (Japanese Patent Laid-Open No. 5-917).
57, JP-A-6-46573). That is, since the current flowing through the primary winding changes according to the input voltage, a voltage proportional to the input voltage is generated, and this voltage shortens the ON time or the OFF time of the switching element to increase the primary winding. The output is stabilized by suppressing the energy stored in. In addition, Japanese Unexamined Patent Publication
No. 159975, Japanese Patent Laid-Open No. 3-86072,
In Japanese Patent Laid-Open No. 6-38528, a voltage generated on the secondary side is detected and a signal corresponding to this voltage level is fed back to the primary side via a feedback circuit to control the on-time and the oscillation frequency of the switching element. It is disclosed that the output voltage is stabilized in this way.

Particularly, in the power supply circuit disclosed in Japanese Patent Laid-Open No. 6-38528, the oscillation cycle of the switching element is adjusted with respect to the fluctuation of the input voltage, and the irregular intermittent oscillation is likely to occur at light load or no load. A protective function is also provided so that the output voltage can be stabilized by preventing this.

[0005]

However, each of the power supply circuits described above is provided with an automatic voltage switching circuit, or a circuit for feeding back to stabilize the output voltage on the secondary side or open protection at the time of no load is provided. Either prepare. In addition, even if the insulated power supply circuit is provided with both of the above circuits, the number of parts required for each circuit increases, and the circuit configuration becomes complicated, large, and space saving and cost reduction are met. I can't.

The present invention has been made in view of the above,
It is an object of the present invention to provide a switching power supply circuit that realizes both automatic voltage switching and open protection with a simple circuit configuration and that can stabilize the output voltage.

[0007]

The present invention provides a switching power supply circuit for switching a current flowing through a primary winding by turning on and off a switching element to output a secondary current from an output winding on a secondary side, A detection winding that generates a voltage proportional to the induced voltage in the output winding, and an off control circuit that keeps the switching element off while the voltage level induced in the detection winding exceeds a first threshold value. It is provided (Claim 1).

The off control circuit is adapted to compare the induced voltage in the detection winding with the first threshold value via a time constant circuit (claim 2).

Further, according to the present invention, while the first dummy resistor connected in parallel to the detection winding via the first switch and the voltage level induced in the detection winding exceeds the second threshold value,
And a first dummy control circuit for bringing the first switch into conduction (claim 3).

The first dummy control circuit is a first Zener diode connected to the detection winding, and the first switch is a first transistor which is turned on by turning on the first Zener diode. 4).

According to the present invention, a second dummy resistor connected in parallel to the detection winding via a second switch and a voltage level induced in the detection winding exceed a first threshold value.
A second dummy control circuit for conducting the second switch, wherein the second dummy control circuit is a second Zener diode connected to the detection winding, and the second switch is for turning on the second Zener diode. Is a second transistor that is turned on by the
The switching element is turned off by turning on the Zener diode (Claim 5).

According to the present invention, a constant voltage circuit is connected to the output side of the output winding (claim 6).

Further, according to the present invention, a dummy circuit is connected to the output side of the output winding, and the dummy circuit includes a third dummy resistor connected in parallel between the output windings via a third switch. And a third switch for conducting the third switch while the voltage level induced in the output winding exceeds a third threshold value.
And a dummy control circuit (claim 7).

According to the present invention, a fourth dummy resistor connected in parallel to the output side of the output winding via a third transistor between the output windings and a voltage level induced in the output winding. A third zener diode that is turned on to semi-conduct the third transistor while the voltage exceeds a third threshold, and the constant voltage circuit includes a fourth transistor connected in series to the output winding. The transistor is made semi-conductive by turning on the third Zener diode (claim 8).

Further, it is preferable that the fourth dummy resistor is provided on the output side of the fourth transistor (claim 9).

[0016]

According to the first aspect of the present invention, the current flowing through the primary winding is switched by turning on and off by the self-excited oscillation of the switching element, so that the secondary current is output from the output winding on the secondary side. To be done. A voltage proportional to the induced voltage in the output winding is generated in the detection winding, and the switching element is kept off while the voltage level exceeds the first threshold value. Therefore, when a high voltage exceeding the first threshold value is induced in the detection winding when the input voltage is high or the output side is lightly loaded or unloaded, during that period, the off time of the switching element is extended or The oscillation is interrupted intermittently for a plurality of times of the excitation oscillation cycle, and the supply amount of the magnetic energy to the secondary side is suppressed for each cycle or totally.

According to the second aspect of the invention, in the off control circuit, the off time of the switching element is extended or the oscillation is interrupted intermittently for a plurality of times of the self-excited oscillation cycle depending on the time constant of the time constant circuit. The amount of magnetic energy supplied to the secondary side is suppressed in each cycle or totally.

According to the third aspect of the present invention, while the voltage level induced in the detection winding exceeds the second threshold value, the first
Since the magnetic energy stored in the detection winding due to the switch being turned on is consumed by the first dummy resistor, the output voltage is clamped.

According to the fourth aspect of the present invention, while the voltage level induced in the detection winding exceeds the Zener voltage of the first Zener diode, the first Zener diode is turned on to turn on the first transistor and the switching element. Is kept off.

According to the invention of claim 5, while the voltage level induced in the detection winding exceeds the first threshold value which is the Zener voltage of the second Zener diode, the second Zener diode is turned on and the detection winding is turned on. The magnetic energy stored in the line is consumed by the second dummy resistor and the switching element is turned off.

According to the sixth aspect of the invention, since the constant voltage circuit is connected to the output side of the output winding, the output voltage is further stabilized.

According to the seventh aspect of the invention, since the third switch is turned on while the voltage level induced in the output winding exceeds the third threshold value, the magnetic energy stored in the output winding is reduced. The part is consumed up to the required level by the third dummy resistor.

According to the eighth aspect of the present invention, while the voltage level induced in the output winding exceeds the third threshold value which is the Zener voltage of the third Zener diode, the third Zener diode is on (semiconductive). Part of the magnetic energy stored in the output winding is consumed by the fourth dummy resistor to a required level and the fourth dummy resistor connected in series to the output winding.
The transistor is made semi-conductive to perform a constant voltage operation.

According to the invention of claim 9, since the fourth dummy resistor is provided on the output side of the fourth transistor, the loss due to the current consumption in the fourth dummy resistor is shared with the loss in the fourth transistor. Will be done.

[0025]

1 is a circuit diagram showing a first embodiment of a switching power supply circuit according to the present invention. REC is a rectifier circuit such as a diode bridge, and is an AC power source, for example, a commercial power source E
This is to convert the AC input input from to DC.
To the output terminal side of the rectifier circuit REC, a series circuit including a primary winding L1 of the transformer T, a switching transistor Q1 and an emitter resistor R1 is connected. The transformer T includes a primary winding L1, a secondary-side output winding L2, a base winding L3, and a detection winding L4. Of these windings, the primary winding L1 and the base winding L3 have the same polarity,
The winding direction of the coil is set so that the output winding L2 and the detection winding L4 have the same polarity. Therefore, the base winding L3 is proportional to the voltage induced in the primary winding L1 (L
1, a voltage level depending on the turns ratio of L3 is induced, while a voltage level proportional to the voltage induced on the output winding L2 (dependent on the turns ratio of L2 and L4) is induced in the detection winding L4. Is induced.

A series circuit consisting of a base winding L3, a resistor R2 and a capacitor C1 is connected between the base and emitter of the switching transistor Q1 and constitutes a feedback circuit for self-excited oscillation together with the primary winding L1. The starting resistor R3 is connected between the positive side of the rectifying circuit REC and the base of the switching transistor Q1, and the direct current from the rectifying circuit REC is charged in the capacitor C1 via the starting resistor R3 to raise the base potential, thereby switching. The transistor Q1 is activated, that is, turned on.

The transistor Q2 is connected between the base of the switching transistor Q1 and the negative line, and its base is connected to the connection point between the emitter of the switching transistor Q1 and the emitter resistor R1. The transistor Q2 is turned on when the voltage generated across the emitter resistor R1 by the collector current flowing through the primary winding L1 exceeds the base-emitter voltage of the transistor Q2, and the base potential of the switching transistor Q1 is lowered to turn the switching transistor Q1 on. I try to turn it off. By controlling the on-time by the detection resistor R1 and the transistor Q2, the set magnetic energy is accumulated in the primary winding L1, that is, the required output current is obtained at the output side.

Another transistor Q3 is connected between the base of the switching transistor Q1 and the negative line, and an off control circuit is connected between the base and the detection winding L4. This OFF control circuit is provided with the detection winding L
4, a series circuit composed of a diode D1, a resistor R4 and a smoothing capacitor C2 connected to both ends, and a Zener diode ZD1 and a resistor R5 connected between the connection point of the resistor R4 and the smoothing capacitor C2 and the base of the transistor Q3. It is composed of a series circuit.

A rectifying diode Do is connected to one side of the output winding L2, a smoothing capacitor Co is connected to the output side thereof, and an output end for connection with a load is formed on the output side thereof. Then, a load such as a battery pack having a chargeable / dischargeable secondary battery or a battery application device having a built-in secondary battery is detachably connected to these terminals. The load current is output to the load side by the voltage induced across the output winding L2.

Next, the basic operation of this power supply circuit will be described. When the commercial power source E is turned on and a direct current starts to flow from the rectifier circuit REC, the capacitor C1 is charged through the starting resistor R3 and the base potential of the switching transistor Q1 rises. The rise of the base potential causes the switching transistor Q1 to start turning on. Therefore, a current flows into the primary winding L1 and at the same time, the base winding L1.
A voltage is induced in 3. The voltage induced in the base winding L3 causes a base current to be generated due to the time constant of the resistor R2 and the capacitor C1 which form part of the feedback circuit for self-excited oscillation, and thus the base potential of the switching transistor Q1 is increased to perform switching. The transistor Q1 turns on rapidly. When the switching transistor Q1 is turned on, the current flowing through the primary winding L1 rises, and the voltage across the detection resistor R1 starts to rise. Then, when the voltage across the detection resistor R1 exceeds the base-emitter voltage of the transistor Q2, the transistor Q2 is turned on and the base potential of the switching transistor Q1 is lowered, so that the switching transistor Q1 starts to turn off, and at the same time, the base winding L3. The switching transistor Q1 is suddenly turned off due to the action of the induced voltage of the opposite polarity, and as a result, the inflow of the power supply current to the primary winding L1 is stopped.

When the inflow of the power supply current to the primary winding L1 is stopped, the magnetic energy accumulated in the primary winding L1 is released to the output winding L2 side, and the output winding L2 receives this magnetic energy. To induce a voltage, and generate and supply a rectified and smoothed load current to the connected load via the diode Do and the smoothing capacitor Co.

On the other hand, after the flow of the power supply current to the primary winding L1 is stopped, the capacitor C1 starts to be charged from the rectifier circuit REC via the starting resistor R3, so that the switching transistor Q1 is started again, that is, turned on. . By repeating the above operation, the switching transistor Q
The ON and OFF operations of 1 are repeated, and the switching of the primary winding L1 is continued.

Next, the off control operation will be described.
The voltage level induced in the output winding L2 fluctuates or changes when the commercial power supply E is changed to a power supply of a different level, when the load state on the secondary side is lightly loaded, or when it is unloaded. To do. Therefore, by inducing a voltage proportional to the voltage induced in the output winding L2 in the detection winding L4,
The state of the induced voltage of the output winding L2 is detected. The voltage induced in the detection winding L4 passes through the forward diode D1 and supplies the charging current to the resistor R4 and the smoothing capacitor C2 forming the time constant circuit. Smoothing capacitor C
The voltage of 2 gradually rises due to the charging current, and when this voltage exceeds the Zener voltage of the Zener diode ZD1,
The Zener diode ZD1 is turned on only for the period exceeding that, and the base current is supplied to the transistor Q3, whereby the transistor Q3 is turned on. When the transistor Q3 is turned on, the base potential of the switching transistor Q1 is lowered, so that the switching transistor Q1
Is kept off.

The characteristics of the OFF control of the switching transistor Q1 are slightly different depending on the time constants of the resistor R4 and the smoothing capacitor C2. That is, when the time constant is close to the frequency of the self-excited oscillation, for example, at the level of 2 times or 3 times the frequency, it acts to extend the off time of the switching transistor Q1, thereby accumulating in the primary winding L1. The generated magnetic energy is suppressed for each oscillation cycle to stabilize the output voltage. On the other hand, when the time constant is relatively large as compared with the frequency of the self-excited oscillation, the switching transistor Q1 continues to be in the off state for a plurality of self-excited oscillation cycles, that is, acts as intermittent oscillation. As a result, for example, when no load is applied, the magnetic energy accumulated in the primary winding L1 is totally suppressed to achieve open protection.

FIG. 2 is a circuit diagram showing a second embodiment of the switching power supply circuit according to the present invention. In the second embodiment, a dummy and its control circuit are added, and those having the same numbers as in FIG. 1 perform the same operation.

The dummy and its control circuit include a dummy resistor R
_A series circuit composed of _L and a transistor Q4 as a switch is connected to both ends of the detection winding L4 via a diode D1, and a resistor R6 and a zener diode ZD2 are provided between the positive electrode of the smoothing capacitor C2 and the base of the transistor Q4. And a series circuit composed of is connected.

Next, the operation of this circuit will be described. This circuit works effectively when the input voltage is high or the output side is lightly loaded or unloaded.
The detection winding L4 is proportional to the rise of the induced voltage in the output winding L2.
When the induced voltage rises and the charging voltage of the smoothing capacitor C2 exceeds the Zener voltage of the Zener diode ZD2,
This Zener diode ZD2 turns on, turning on the transistor Q4. When the transistor Q4 turns on,
The dummy resistor R _L is connected to the detection winding L4 via the diode D1.
Since the magnetic energy released to the detection winding L4 is consumed by the dummy resistor R _L , as a result, the rebound of the output voltage generated when the switching transistor Q1 is turned off reaches a required upper limit level. Can be clamped. In particular, when the output side is unloaded, magnetic flux due to the secondary current does not occur, so the generated magnetic flux on the secondary side remains and the induced voltage increases,
For this reason, the off time of the switching transistor Q1 may become long and a high input voltage may be directly applied.
As described above, by consuming a part of the energy in the dummy resistor R _L , the off time can be shortened by that amount and the open protection of the switching transistor Q1 can be surely achieved.

FIG. 3 is a circuit diagram showing a third embodiment of the switching power supply circuit according to the present invention. The third embodiment is the Zener diodes ZD1 and ZD1 shown in the second embodiment (FIG. 2).
D2 is shared by the Zener diode ZD1 to simplify the circuit. Note that, in the figure, those denoted by the same reference numerals as those in FIG. 2 perform the same operation.

In FIG. 3, the other end of the resistor R6 is connected to the connection point between the Zener diode ZD1 and the resistor R5. By adopting such a configuration, the induced voltage of the detection winding L4 rises and smooths. When the charging voltage of the capacitor Co exceeds the Zener voltage, both the transistors Q3 and Q4 are turned on at the same time, and the switching transistor Q1 is turned off and the dummy resistor R _L partially consumes the magnetic energy accumulated in the primary winding. (Clamp control) can be performed in parallel with a simple configuration.

FIG. 4 is a circuit diagram showing a fourth embodiment of the switching power supply circuit according to the present invention. In the fourth embodiment, the primary side has the same circuit configuration as the first embodiment (FIG. 1), and the secondary side is provided with a dummy, its control circuit and a constant voltage circuit. Incidentally, in the figure, those denoted by the same reference numerals as those in FIG. 1 perform the same operation.

In FIG. 4, the dummy control circuit comprises a Zener diode ZD3, a resistor R7 and a transistor Q5 as a switch, and short-circuits the smoothing capacitor Co via the dummy resistor R _L. That is, a series circuit including a dummy resistor R _L and a transistor Q5 is connected to both ends of the smoothing capacitor Co, and a series circuit including a Zener diode ZD3 and a resistor R7 is connected between the positive electrode side of the smoothing capacitor Co and the base of the transistor Q5. Has been done.

The constant voltage circuit comprises a transistor Q6, a Zener diode ZD4 and a resistor R8. The transistor Q6 is connected between the positive electrode of the smoothing capacitor Co and the output terminal, and the resistor R8 is connected between the base and collector of the transistor Q6. The Zener diode ZD4 is connected between the base of the transistor Q6 and the negative electrode of the smoothing capacitor Co.

Next, the operation of these circuits will be described. When the input voltage is high, in a light load state, or when there is no load, the voltage across the smoothing capacitor Co rises. When the voltage exceeds the Zener voltage of the Zener diode ZD3, the Zener diode ZD3 turns on and the transistor Q5 is turned on. Turn it on. Therefore, both ends of the smoothing capacitor Co are short-circuited via the dummy resistor R _L , so that the charge accumulated in the smoothing capacitor Co is consumed by the dummy resistor R _L, which results in the switching transistor Q1.
The output voltage bounce that occurs at the off time of can be clamped to the desired upper level. In particular, when the output side is unloaded, current does not flow to the output side, and the constant voltage circuit described later does not function. Therefore, the off time of the switching transistor Q1 becomes long and a high input voltage is directly applied to the switching transistor Q1. Although it may be applied, the dummy resistor R _L consumes energy up to a required level in this way, whereby the open protection of the switching transistor Q1 can be surely achieved.

In the constant voltage circuit, when the voltage on the output side is within the normal range, the Zener diode ZD4 remains off. Therefore, the base current flows through the resistor R8, the transistor Q6 turns on, and the transistor on the output side turns on. Supply load current to the load. On the other hand, if the voltage across the smoothing capacitor Co exceeds the Zener voltage of the Zener diode ZD4 due to high input voltage or light load, this Zener diode is turned on (including the semi-conducting state) and the base potential is lowered. Since the transistor Q6 is controlled in an unsaturated region or the like, the load current is limited and the output voltage is stabilized.
Since the Zener voltages of the Zener diodes ZD3 and ZD4 determine which of the clamp control and the constant voltage control operates first, they may be equal or higher.

FIG. 5 is a circuit diagram showing a fifth embodiment of the switching power supply circuit according to the present invention. The fifth embodiment is a Zener diode ZD3, ZD3 shown in the fourth embodiment (FIG. 4).
The circuit is simplified by using D4 as one Zener diode ZD5. Note that, in the figure, those denoted by the same reference numerals as those in FIG. 4 perform the same operation.

In FIG. 5, a resistor R9 is connected between the base and collector of the transistor Q6, and the transistor Q6 is connected.
A Zener diode ZD5 is connected between the base of the transistor Q5 and the base of the transistor Q5.

When the input voltage is high, in a light load state, or when there is no load, the voltage across the smoothing capacitor Co rises, and when the voltage reaches the Zener voltage level of the Zener diode ZD5, the Zener diode ZD5 is turned on. When it starts to turn on, the transistor Q6 becomes semi-conductive and the current is limited. At the same time, the transistor Q5 also becomes semi-conductive, so that part of the current passing through the transistor Q6 is consumed by the dummy resistor R _L. Therefore, the constant voltage control and the clamp control can be performed in parallel with a simple configuration. In particular, when the output side is unloaded, the off time of the switching transistor Q1 may be long and a high input voltage may be directly applied. However, as described above, the dummy resistor R _L supplies energy up to a required level. Switching transistor Q by consuming
It is possible to achieve reliable open protection of 1. Further, since the dummy resistor R _L is provided on the output terminal side of the transistor Q6, the loss at the dummy resistor R _L is reduced by sharing it with the loss at the transistor Q6. It is possible to suppress the loss variation in the transistor Q6 depending on time, and to stabilize the output voltage more.

[0048]

According to the first aspect of the present invention, the detection winding that generates a voltage proportional to the induced voltage of the output winding and the voltage level induced in the detection winding exceed the first threshold value. Since the switching element is provided with an OFF control circuit for keeping the switching element OFF, the amount of magnetic energy supplied to the secondary side is changed at every cycle, when the input voltage is high or the output side is lightly loaded or unloaded. It can be totally suppressed. Further, since both the automatic voltage switching and the open protection can be performed by the simple circuit configuration of the detection winding and the off control circuit, the number of parts can be reduced, and the space and cost can be reduced.

According to the second aspect of the invention, the induced voltage in the detection winding of the off control circuit is passed through the time constant circuit to the first control circuit.
Since it is configured to be compared with the threshold value, it is possible to appropriately set the suppression for each cycle of the supply amount of the magnetic energy to the secondary side or the total suppression by selecting the time constant.

According to the third aspect of the present invention, while the voltage level induced in the detection winding exceeds the second threshold value, the detection winding is shorted to the first dummy resistor, so that the output voltage has a certain level. Since it is clamped by and the bounce at the time of the off change is suppressed, the off time when the input voltage is directly applied to the switching element can be shortened, and reliable open protection can be achieved.

According to the fourth aspect of the present invention, the use of the first Zener diode facilitates setting of the second threshold value, and the use of the first transistor as the first switch facilitates control.

According to the fifth aspect of the present invention, the magnitude comparison operation with the threshold value is shared by the second Zener diodes, so that the circuit configuration can be simplified accordingly.

According to the invention described in claim 6, since the constant voltage circuit is connected to the output side of the output winding, the output voltage can be more stabilized.

According to the invention described in claim 7, since the current is consumed by the third dummy resistor while the voltage level induced in the output winding exceeds the third threshold value, the output voltage has a certain level. Since it is clamped by and the rebound at the time of off change is suppressed, the off time when the input voltage is directly applied to the switching element can be shortened, and reliable open protection can be achieved.

According to the eighth aspect of the invention, the magnitude comparison operation with the threshold value is shared by the third Zener diode, so that the circuit configuration can be simplified accordingly.

According to the invention described in claim 9, since the fourth dummy resistor is provided on the output side of the fourth transistor, the loss in the fourth dummy resistor is distributed with the loss in the fourth transistor. Therefore, it is possible to reduce the loss variation in the fourth transistor under light load and no load, and it is possible to further stabilize the output voltage.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a switching power supply circuit according to the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the switching power supply circuit according to the present invention.

FIG. 3 is a circuit diagram showing a third embodiment of the switching power supply circuit according to the present invention.

FIG. 4 is a circuit diagram showing a fourth embodiment of the switching power supply circuit according to the present invention.

FIG. 5 is a circuit diagram showing a fifth embodiment of a switching power supply circuit according to the present invention.

[Explanation of symbols]

 E Commercial power supply REC Rectifier circuit T Transformer L1 Primary winding L2 Output winding L3 Base winding L4 Detection winding Q1 Switching transistor Q2-Q6 Transistors R1-R9 Resistance Co-C2 Capacitor Do-D1 Diode ZD1-ZD5 Zener diode

Claims (9)

[Claims] 1. A switching power supply circuit in which a current flowing through a primary winding is switched by turning on and off a switching element to output a secondary current from an output winding on a secondary side. A switching comprising: a detection winding that generates a proportional voltage; and an off control circuit that keeps the switching element off while the voltage level induced in the detection winding exceeds a first threshold value. Power supply circuit. 2. The switching power supply circuit according to claim 1, wherein the OFF control circuit is configured to compare the induced voltage of the detection winding with the first threshold value via a time constant circuit. . 3. The switching power supply circuit according to claim 1, wherein a first dummy resistor connected in parallel to the detection winding via a first switch and a voltage level induced in the detection winding are of a first level. A switching power supply circuit comprising: a first dummy control circuit that keeps the first switch conductive while exceeding two thresholds. 4. The first dummy control circuit is a first Zener diode connected to the detection winding, and the first dummy control circuit includes:
4. The switching power supply circuit according to claim 3, wherein the switch is a first transistor which is turned on by turning on the first Zener diode. 5. The switching power supply circuit according to claim 1, wherein a second dummy resistor connected in parallel to the detection winding via a second switch and a voltage level induced in the detection winding have a first threshold value. A second zener diode connected to the detection winding, wherein the second switch is the second zener diode connected to the detection winding. A switching power supply circuit which is a second transistor which is turned on by turning on a Zener diode, and wherein the off control circuit turns off the switching element by turning on the second Zener diode. 6. The switching power supply circuit according to claim 1, wherein a constant voltage circuit is connected to the output side of the output winding. 7. The switching power supply circuit according to claim 6, wherein a dummy circuit is connected to the output side of the output winding, and the dummy circuit is connected in parallel between the output windings via a third switch. And a third dummy control circuit that conducts the third switch while the voltage level induced in the output winding exceeds a third threshold value. . 8. The switching power supply circuit according to claim 6, wherein a fourth dummy resistor connected in parallel to the output side of the output winding via a third transistor, and the output winding. A third zener diode that is turned on to semi-conduct the third transistor while the voltage level induced in the output voltage exceeds a third threshold value, and the constant voltage circuit is connected in series with the output winding. The switching power supply circuit is characterized in that the fourth transistor is semi-conductive when the third Zener diode is turned on. 9. The switching power supply circuit according to claim 8, wherein the fourth dummy resistor is provided on the output side of the fourth transistor.