JP2002078328A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply, in which an active clamp system is combined with a synchronous rectifying circuit and which can protect a MOS-FET from damages caused by the application of a voltage exceeding a gate dielectric strength. SOLUTION: This switching power supply has a switching transistor, by which the gate voltage of a 2nd field effect transistor connected in parallel with the secondary winding of a transformer is drawn into a ground potential, and a Zener diode which supplied an input current for turn-on of the switching transistor, to draw the gate voltage of the 2nd field effect transistor into a ground potential, when the gate voltage of the 2nd field effect transistor exceeds the dielectric strength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a switching power supply, and more particularly, to a switching power supply combining an active clamp system and a synchronous rectifier circuit.

[0002]

2. Description of the Related Art FIG. 9 is a circuit diagram showing a configuration of a conventional switching power supply in which an active clamp system and a synchronous rectification circuit are combined.

As shown in FIG. 9, a conventional switching power supply includes a main transformer T1 and a MOSFET (MOS type field effect) for switching power supplied from a DC power supply E1 to a primary winding of the main transformer T1 at a predetermined cycle. Transistor) Q1 and a MOSFET Q for rectifying AC power output from the secondary winding of the main transformer T1.
3, Q4 and a choke coil L1 which is a smoothing element for smoothing the rectified output by the MOSFETs Q3, Q4.
And the smoothing capacitor C2 and the MOSFET Q1
At the time of F, clamp capacitors C1 and M for limiting the voltage applied to the primary winding of the main transformer T1.
An OSFET Q2, and the load is a smoothing capacitor C2.
And connected in parallel.

Here, the source (S) of the MOSFET Q1
The drain (D) is connected in series with the primary winding of the main transformer T1; The clamp capacitor C1 and the source (S) -drain (D) of the MOSFET Q2 are connected in series, and the clamp capacitor C connected in series is connected.
1 and a MOSFET Q2 are connected in parallel with the primary winding of the main transformer T1.

On the other hand, the source (S) of MOSFET Q3
The drain (D) is connected in series with the secondary winding of the main transformer T1, and the source (S) -drain (D) of the MOSFET Q4 is connected in parallel with the secondary winding of the main transformer T1. The gate (G) of the MOSFET Q3 is connected to one end of a secondary winding of the main transformer T1 via a resistor R2, and the gate (G) of the MOSFET Q4 is connected to a resistor R1 at the other end of the secondary winding of the main transformer T1. Connected through. Note that the MOSFETs Q1 and Q2 may be any switching elements provided with control input terminals, and may be replaced with, for example, bipolar transistors.

In such a configuration, the MOSFET Q
Reference numeral 1 denotes ON / OFF controlled by a gate command supplied from a control circuit (not shown). When the MOSFET Q1 is ON, the MOSFET Q3 is turned ON and the MOSFET Q4 is turned OFF. The voltage Vo is supplied.

On the other hand, when MOSFET Q1 is OFF,
MOSFET Q3 turns off, MOSFET Q4 turns on
Therefore, the load current is returned through the MOSFET Q4, and the rectified voltage Vo is supplied to the load. MOSF
The ETQ2 is controlled by a gate command supplied from a control circuit (not shown) to be turned on during a predetermined period when the MOSFET Q1 is turned off. This allows MO
When the SFET Q1 is OFF, the voltage applied to the primary winding of the main transformer T1 is limited to the voltage VC1 across the clamp capacitor C1.

Here, assuming that the winding ratio of the main transformer T1 is N and the output voltage of the DC power supply E1 is Vi, MOSF
As for the gate voltage Vg3 of the ETQ3, the MOSFET Q1 is ON.
Is the voltage generated in the secondary winding of the main transformer T1, and the gate voltage Vg4 of the MOSFET Q4 is
Since the voltage is generated in the secondary winding of the main transformer T1 when the FET Q1 is OFF, Vg3 = Vi / N (1) Vg4 = VC1 / N (2)

The voltage VC1 across the clamp capacitor C1
Depends on the output voltage Vi of the DC power supply E1 and the ON / OFF duty ratio of the MOSFET Q1.
1, the ON period is Ton, the OFF period is Toff, and the cycle is T.
If s Holds.

[0010]

As described above, in the conventional switching power supply, the voltage VC1 across the clamp capacitor C1 is equal to the output voltage Vi of the DC power supply E1 and the MOSFET.
Since it depends on the ON / OFF duty ratio of TQ1,
The load current increases and the ON period Ton of the MOSFET Q1
Becomes longer and the off period Toff becomes shorter, the equation (3)
, The voltage VC across the clamp capacitor C1
1 rises.

Therefore, as shown in equation (2), MO
Since the gate voltage Vg4 of the SFET Q4 increases, a voltage exceeding the gate insulation withstand voltage is supplied, and the MOSFET Q4 may be damaged.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and has been made in consideration of the following problems.
An object of the present invention is to provide a switching power supply that combines an active clamp method and a synchronous rectification circuit that can prevent damage to ET.

[0013]

To achieve the above object, a switching power supply according to the present invention comprises a transformer and a first switching element for switching DC power supplied to a primary winding of the transformer at a predetermined cycle. A clamp capacitor and a second switching element for limiting an applied voltage to a primary winding of the transformer when the first switching element is off, and an AC power output from a secondary winding of the transformer. A first field-effect transistor connected in series with a secondary winding of the transformer to rectify the secondary winding of the transformer; and a secondary winding of the transformer to rectify AC power output from the secondary winding of the transformer. A second field effect transistor connected in parallel with the line,
And a smoothing element for smoothing a rectified output by the second field-effect transistor and a second field-effect transistor, wherein the gate voltage of the second field-effect transistor is pulled to a ground potential. When the gate voltage of the switching transistor and the gate voltage of the second field-effect transistor exceed the withstand voltage,
And a Zener diode that supplies an input current for turning on the switching transistor in order to pull the gate voltage of the second field effect transistor to the ground potential.

At this time, the transformer has a first secondary winding, a second secondary winding, and a third secondary winding, and the second field-effect transistor is provided with the first secondary winding. Connected in parallel with a secondary winding, a gate of the first field-effect transistor is connected to the second secondary winding, and a gate of the second field-effect transistor is connected to the third secondary winding. And a diode connected in series with the Zener diode to prevent a negative voltage from being applied to the input terminal of the switching transistor.

Further, another configuration of the switching power supply of the present invention includes a transformer, a first switching element for switching DC power supplied to a primary winding of the transformer at a predetermined cycle, and the first switching element. When the element is off, a clamp capacitor and a second switching element for limiting the voltage applied to the primary winding of the transformer, and the second switching element for rectifying AC power output from the secondary winding of the transformer. A first field-effect transistor connected in series with a secondary winding of the transformer; and a first field-effect transistor connected in parallel with the secondary winding of the transformer to rectify AC power output from the secondary winding of the transformer. A second field effect transistor, and a smoothing device for smoothing a rectified output by the first field effect transistor and the second field effect transistor. A switching power supply comprising: a third field effect transistor for limiting a gate voltage of the second field effect transistor to a predetermined voltage equal to or lower than a dielectric strength voltage; When the gate voltage exceeds the withstand voltage, a Zener diode for limiting the gate voltage of the third field-effect transistor is used to set the gate voltage of the second field-effect transistor to the predetermined voltage. .

At this time, the transformer includes a first secondary winding, a second secondary winding, and a third secondary winding, and the second field-effect transistor is provided with the first secondary winding. Connected in parallel with a secondary winding, a gate of the first field-effect transistor is connected to the second secondary winding, and a gate of the second field-effect transistor is connected to the third secondary winding. And a diode connected in series with the Zener diode to prevent a negative voltage from being applied to the gate of the third field-effect transistor.

Further, the semiconductor device may further include a flywheel diode connected in parallel with the second field-effect transistor for circulating a load current.

In the switching power supply configured as described above, the switching transistor for drawing the gate voltage of the second field-effect transistor to the ground potential and the voltage of the gate voltage of the second field-effect transistor exceeding the withstand voltage. In order to draw the gate voltage of the second field-effect transistor to the ground potential, a structure having a Zener diode for supplying an input current for turning on the switching transistor, or insulating the gate voltage of the second field-effect transistor A third field-effect transistor for limiting the voltage to a predetermined voltage lower than the breakdown voltage; and a gate voltage of the second field-effect transistor when the gate voltage of the second field-effect transistor exceeds the withstand voltage. To set the gate voltage of the third field-effect transistor In the structure having a zener diode for limiting to, a gate voltage of the second field effect transistor is limited to less than the withstand voltage.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

(First Embodiment) FIG. 1 is a circuit diagram showing a configuration of a switching power supply according to a first embodiment of the present invention.

As shown in FIG. 1, the switching power supply of the present embodiment has a switching transistor for drawing the gate voltage Vg4 of the MOSFET Q4 to the ground potential (0 V) in addition to the configuration of the conventional switching power supply shown in FIG. The configuration includes a transistor Q5, a Zener diode D1 and a resistor R4 inserted in series between the base (B) of the switching transistor Q5 and the other end (node B) of the secondary winding of the main transformer T1. The collector (C) of the switching transistor Q5 is connected to the gate of the MOSFET Q4, and the emitter (E) is connected to the ground potential.

The Zener voltage of the Zener diode D1 is set to a value such that a Zener current flows when a voltage exceeding the gate withstand voltage is generated in the secondary winding of the main transformer T1. The other configuration is the same as the conventional configuration, and the description thereof will be omitted.

Next, the operation of the switching power supply shown in FIG. 1 will be described with reference to FIG.

FIG. 2 is a timing chart showing the operation of the switching power supply shown in FIG.

As shown in FIG. 2, when the load current is steady, the period when the MOSFET Q1 is ON (time t1 to time t1)
In 2), the voltage at one end (node A) of the secondary winding of the main transformer T1 becomes Vi / N, and the gate voltage Vg3 of the MOSFET Q3 also becomes Vi / N.
3 turns ON. Further, the voltage Vb (= Vg4) at the other end (node B) of the secondary winding of the main transformer T1 becomes 0V, and the gate voltage Vg4 of the MOSFET Q4 also becomes 0V, so that the MOSFET Q4 is turned off. Therefore, the choke coil L1 and the smoothing capacitor C
2, the rectified voltage Vo is supplied to the load.

On the other hand, during the period when the MOSFET Q1 is OFF (time t2 to t3), the voltage at one end (node A) of the secondary winding of the main transformer T1 becomes 0 V, and the MOSFET
Since the gate voltage Vg3 of TQ3 also becomes 0 V, the MOSF
ETQ3 turns off. Further, the voltage Vb at the other end (node B) of the secondary winding of the main transformer T1 becomes VC1 / N, and the gate voltage Vg4 of the MOSFET Q4 also becomes VC1 / N.
Therefore, the MOSFET Q4 is turned ON. Therefore, the current flowing to the load flows back through the MOSFET Q4.

Here, when the load current is steady and the voltage Vb generated at the other end (node B) of the secondary winding of the main transformer T1 is lower than the gate insulation withstand voltage, the Zener current flows through the Zener diode D1. Since there is no switching transistor Q5, the switching transistor Q5 is maintained in the OFF state.

Next, if the load suddenly changes and the load current increases during the period from time t4 to t5, the on-period Ton of the MOSFET Q1 becomes longer and the off-period Toff becomes shorter, so that the equations (2) and (3) are used. As shown, the voltage Vb at the other end (node B) of the secondary winding of the main transformer T1 becomes higher than in the normal state, and the gate voltage V
g4 becomes higher than in the steady state.

The switching power supply of this embodiment is a MOS
While the FET Q1 is OFF, the main transformer T
Voltage Vb generated at the other end (node B) of the secondary winding 1
When (= VC1 '/ N) exceeds the Zener voltage of the Zener diode D1, a Zener current flows into the base of the switching transistor Q5 via the resistor R4 to turn on the switching transistor Q5.

As a result, the gate voltage Vg4 of the MOSFET Q4 is pulled down to the ground potential (0 V),
Q4 turns off. At this time, the load current is MOSF
It is returned via the parasitic diode Dp of ETQ4.

Therefore, the damage of the MOSFET Q4 can be prevented by selecting the value of the Zener voltage of the Zener diode D1 to be equal to or less than the gate insulation withstand voltage of the MOSFET Q4.

(Second Embodiment) FIG. 3 is a circuit diagram showing a configuration of a switching power supply according to a second embodiment of the present invention.

The switching power supply of this embodiment differs from the first embodiment in the configuration of the main transformer and the circuit configuration connected to the base of the switching transistor Q5. The other configuration is the same as that of the first embodiment, and the description is omitted.

As shown in FIG. 3, the main transformer T2
Is a configuration including three secondary windings T2a, T2b, and T2c. The secondary winding T2b of the main transformer T2 is connected to the gate of the MOSFET Q3 via the resistor R2,
The secondary winding T2c is connected to the MOSFET Q4 via the resistor R1.
Connected to the gate. Also, the base of the switching transistor Q5 and the secondary winding T of the main transformer T2.
2c, a Zener diode D connected in series
1, a resistor R4 and a diode D3 are inserted.

The Zener voltage of the Zener diode D1 is set to a value such that a Zener current flows when a voltage exceeding the gate withstand voltage is generated in the secondary winding T2c of the main transformer T2. The diode D2 is inserted to prevent a negative voltage from being applied to the base of the switching transistor Q5.

In such a configuration, the switching power supply of the present embodiment also has a MOSF like the first embodiment.
During the period when ETQ1 is OFF, the main transformer T1
When the voltage generated in the secondary winding T2c is higher than the Zener voltage of the Zener diode D1, the switching transistor Q5 is turned ON, and the gate voltage Vg4 of the MOSFET Q4 is pulled to the ground potential, so that the MOSFET Q
4 turns off. Therefore, the Zener diode D1
By setting the value of the Zener voltage to be equal to or less than the gate withstand voltage of the MOSFET Q4, the damage of the MOSFET Q4 can be prevented.

In the switching power supply of this embodiment, the gate voltage Vg of MOSFET Q3 is
3, and the voltage of the gate voltage Vg4 of the MOSFET Q4 can be expressed by the above equations (1) and (2). In the equation (1), "N" is the primary winding and the secondary winding of the main transformer T2. This is the winding ratio of T2b, and "N" in equation (2) is the winding ratio of the primary winding and the secondary winding T2c of the main transformer T2.

(Third Embodiment) FIG. 4 is a circuit diagram showing a configuration of a switching power supply according to a third embodiment of the present invention.

As shown in FIG. 4, the switching power supply of the present embodiment has a structure in which a diode D3 is connected in parallel with a MOSETQ4 as a flywheel diode. Other configurations and operations are the same as those of the first embodiment, and therefore, description thereof will be omitted.

In the switching power supply according to the present embodiment, similarly to the first embodiment, by setting the value of the Zener voltage of the Zener diode D1 to be equal to or lower than the gate insulation withstand voltage of the MOSFET Q4, the damage of the MOSFET Q4 can be prevented. . Further, since the flywheel diode D3 is provided in parallel with the MOSET Q4, the diode D3
The load current is circulated through 3. Therefore, it is possible to obtain a switching power supply capable of flowing a load current larger than that of the first embodiment.

(Fourth Embodiment) FIG. 5 is a circuit diagram showing a configuration of a switching power supply according to a fourth embodiment of the present invention.

As shown in FIG. 5, the switching power supply of this embodiment has a configuration in which the switching power supply of the second embodiment and the switching power supply of the third embodiment are combined. Therefore, the configuration and operation of the switching power supply of the present embodiment are the same as those of the second and third embodiments, and the description thereof will be omitted.

The switching power supply according to the present embodiment can obtain the same effects as those of the second and third embodiments.

(Fifth Embodiment) FIG. 6 is a circuit diagram showing a configuration of a switching power supply according to a fifth embodiment of the present invention.

As shown in FIG. 6, the switching power supply according to the present embodiment has a MOSFET Q6 for limiting the gate voltage Vg4 of the MOSFET Q4 and a Zener diode D in addition to the configuration of the conventional switching power supply shown in FIG.
3, a configuration including a diode D4 and a resistor R5.

The source (S) -drain (D) of MOSFET Q6 is connected in series with resistor R1 to
It is inserted between the gate of TQ4 and the other end (node B) of the secondary winding of the main transformer T1. Also, MOSFET
Zener diodes D4 and D5 connected in series are inserted between the gate of Q6 and the ground potential, and the gate (G) and drain (D) of MOSFET Q6 are further connected.
A resistor R5 is inserted between them.

The Zener voltage of the Zener diode D4 is set to a value such that a Zener current flows when a voltage exceeding the gate withstand voltage is generated on the secondary side of the main transformer T1. The other configuration is the same as the conventional configuration, and the description thereof will be omitted.

Next, the operation of the switching power supply shown in FIG. 6 will be described with reference to FIG.

FIG. 7 is a timing chart showing the operation of the switching power supply shown in FIG.

As shown in FIG. 7, when the load current is steady, the period during which the MOSFET Q1 is ON (time t1 to time t1)
In 2), the voltage at one end (node A) of the secondary winding of the main transformer T1 becomes Vi / N, and the gate voltage Vg3 of the MOSFET Q3 also becomes Vi / N.
3 turns ON. Further, the voltage Vb (= Vg4) at the other end (node B) of the secondary winding of the main transformer T1 becomes 0V, and the gate voltage Vg4 of the MOSFET Q4 becomes 0V, so that the MOSFET Q4 is turned off. Therefore, the choke coil L1 and the smoothing capacitor C
2, the rectified voltage Vo is supplied to the load.

On the other hand, during the period when the MOSFET Q1 is OFF (time t2 to t3), the voltage at one end (node A) of the secondary winding of the main transformer T1 becomes 0 V, and the MOSFET
Since the gate voltage Vg3 of TQ3 also becomes 0 V, the MOSF
ETQ3 turns off. Further, the voltage Vb at the other end (node B) of the secondary winding of the main transformer T1 becomes VC1 / N, and the gate voltage Vg4 of the MOSFET Q4 also becomes VC1 / N.
Therefore, the MOSFET Q4 is turned ON. Therefore, the current flowing to the load flows back through the MOSFET Q4.

Here, when the load current is steady and the voltage Vb generated at the other end (node B) of the secondary winding of the main transformer T1 is equal to or lower than the gate withstand voltage, the Zener current flows through the Zener diode D4. There is no MOSFE
TQ6 is maintained in the ON state.

Next, when the load suddenly changes and the load current increases during the period from time t4 to time t5, the ON period Ton of the MOSFET Q1 becomes longer and the OFF period Toff becomes shorter, so that the above equations (2) and (3) are used. As shown, the voltage Vb at the other end (node B) of the secondary winding of the main transformer T1 becomes higher than in the normal state, and the gate voltage V
g4 becomes higher than in the steady state.

The switching power supply of this embodiment is a MOS
When the FET Q1 is OFF, the main transformer T1
Vb (V) generated at the other end (node B) of the secondary winding of
C1 '/ N) exceeds the Zener voltage of the Zener diode D4, a Zener current flows through the Zener diode D4 via the resistor R5, and the MOSFET Q
A voltage value Vz + VF5 obtained by adding the Zener voltage Vz of the Zener diode D4 and the forward voltage VF5 of the diode D5 to the gate (G) of No. 6 is applied. Therefore, MOS
The source (S) of the FET Q6 is connected from Vz + VF5 to the MOS.
The voltage Vz + obtained by subtracting the cutoff voltage Vgs of the FET Q6
VF5-Vgs is output, and the gate voltage Vg4 of MOSFET Q4 is limited to Vz + VF5-Vgs.

Therefore, by selecting the value of the Zener voltage of the Zener diode D4 to be equal to or less than the gate insulation withstand voltage of the MOSFET Q4, the MOSFE is selected as in the first embodiment.
TQ4 can be prevented from being damaged.

Note that the switching power supply of this embodiment
As in the third embodiment, a flywheel diode D3 may be provided in parallel with the MOSFET Q4. In that case, the same effect as in the third embodiment can be obtained.

(Sixth Embodiment) FIG. 8 is a circuit diagram showing a configuration of a switching power supply according to a sixth embodiment of the present invention.

As shown in FIG. 8, the switching power supply of this embodiment has a configuration in which the switching power supply of the second embodiment and the switching power supply of the fifth embodiment are combined. Therefore, the configuration and operation are the same as in the second embodiment and the fifth embodiment, and a description thereof will be omitted.

The switching power supply of this embodiment can obtain the same effects as those of the second and fifth embodiments.

Further, as in the third embodiment, the MOS
Diode D3 for flywheel in parallel with FET Q4
Is provided, the same effect as in the third embodiment can be obtained.

[0061]

Since the present invention is configured as described above, the following effects can be obtained.

A switching transistor for drawing the gate voltage of the second field-effect transistor to the ground potential; and a gate voltage of the second field-effect transistor when the gate voltage of the second field-effect transistor exceeds the withstand voltage. And a Zener diode for supplying an input current for turning on the switching transistor in order to pull the gate voltage to the ground potential, or for limiting the gate voltage of the second field effect transistor to a predetermined voltage equal to or lower than the withstand voltage When the gate voltage of the third field-effect transistor and the gate voltage of the second field-effect transistor exceed the withstand voltage, the third field-effect transistor is set to set the gate voltage of the second field-effect transistor to a predetermined voltage. With a Zener diode to limit the gate voltage of the transistor With formed, the gate voltage of the second field effect transistor is limited to less than the withstand voltage.
Therefore, damage of the second field effect transistor can be prevented.

[Brief description of the drawings]

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

FIG. 2 is a timing chart showing the operation of the switching power supply shown in FIG.

FIG. 3 is a circuit diagram showing a configuration of a second embodiment of the switching power supply of the present invention.

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

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

FIG. 6 is a circuit diagram showing a configuration of a switching power supply according to a fifth embodiment of the present invention.

FIG. 7 is a timing chart showing the operation of the switching power supply shown in FIG.

FIG. 8 is a circuit diagram showing a configuration of a switching power supply according to a sixth embodiment of the present invention.

FIG. 9 is a circuit diagram showing a configuration of a conventional switching power supply in which an active clamp system and a synchronous rectification circuit are combined.

[Explanation of symbols]

 C1 Clamp capacitor C2 Smoothing capacitor D1, D4 Zener diode D2, D3, D5 Diode E1 DC power supply L1 Choke coil Q1-Q4, Q6 MOSFET Q5 Switching transistor R1, R2, R4, R5 Resistor T1, T2 Main transformer

Claims (7)

[Claims] 1. A transformer, a first switching element for switching DC power supplied to a primary winding of the transformer at a predetermined cycle, and a primary element of the transformer when the first switching element is off. A clamp capacitor and a second switching element for limiting an applied voltage to the winding; and a series connection with the secondary winding of the transformer for rectifying AC power output from the secondary winding of the transformer. A first field-effect transistor, a second field-effect transistor connected in parallel with a secondary winding of the transformer to rectify AC power output from a secondary winding of the transformer, A switching element for smoothing a rectified output by the first field-effect transistor and the second field-effect transistor. A switching transistor for pulling the gate voltage of the second field-effect transistor to the ground potential; and a gate of the second field-effect transistor when the gate voltage of the second field-effect transistor exceeds a withstand voltage. A Zener diode that supplies an input current for turning on the switching transistor to pull the voltage to the ground potential. 2. The transformer comprises a first secondary winding, a second
And a third secondary winding, wherein the second field-effect transistor is connected in parallel with the first secondary winding, and the gate of the first field-effect transistor is The switching power supply according to claim 1, wherein the switching power supply is connected to a second secondary winding, and a gate of the second field-effect transistor is connected to the third secondary winding. 3. The switching power supply according to claim 2, further comprising a diode connected in series with said Zener diode to prevent a negative voltage from being applied to an input terminal of said switching transistor. 4. A transformer, a first switching element for switching DC power supplied to a primary winding of the transformer at a predetermined cycle, and a primary element of the transformer when the first switching element is off. A clamp capacitor and a second switching element for limiting an applied voltage to the winding; and a series connection with the secondary winding of the transformer for rectifying AC power output from the secondary winding of the transformer. A first field-effect transistor, a second field-effect transistor connected in parallel with a secondary winding of the transformer to rectify AC power output from a secondary winding of the transformer, A switching element for smoothing a rectified output by the first field-effect transistor and the second field-effect transistor. A third field-effect transistor for limiting a gate voltage of the second field-effect transistor to a predetermined voltage equal to or lower than a withstand voltage; and a gate voltage of the second field-effect transistor exceeding a withstand voltage. A Zener diode for limiting the gate voltage of the third field effect transistor in order to set the gate voltage of the second field effect transistor to the predetermined voltage. 5. The transformer according to claim 1, wherein the transformer has a first secondary winding, a second
And a third secondary winding, wherein the second field-effect transistor is connected in parallel with the first secondary winding, and the gate of the first field-effect transistor is The switching power supply according to claim 4, wherein the switching power supply is connected to a second secondary winding, and a gate of the second field effect transistor is connected to the third secondary winding. 6. The switching power supply according to claim 4, further comprising a diode connected in series with the Zener diode to prevent a negative voltage from being applied to a gate of the third field effect transistor. 7. The switching power supply according to claim 1, further comprising a flywheel diode connected in parallel with said second field-effect transistor for circulating a load current.