JP3507779B2 - Switching power supply - Google Patents

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, and more particularly to a switching power supply combining an active clamp system and a synchronous rectification 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 electric 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 the AC power output from the secondary winding of the main transformer T1.
3, Q4 and choke coil L1 which is a smoothing element for smoothing the rectified output by MOSFETs Q3, Q4
And the smoothing capacitor C2 and the MOSFET Q1 are OF
When F, clamp capacitors C1 and M for limiting the voltage applied to the primary winding of the main transformer T1
The load has a smoothing capacitor C2 having an OSFET Q2.
And is 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.
1 and 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 the secondary winding of the main transformer T1 via a resistor R2, and the gate (G) of the MOSFET Q4 has a resistor R1 at the other end of the secondary winding of the main transformer T1. Connected through. The MOSFETs Q1 and Q2 may be switching elements having an input terminal for control, and may be replaced with bipolar transistors, for example.

In such a structure, the MOSFET Q
ON / OFF of 1 is controlled by a gate command supplied from a control circuit (not shown). When the MOSFET Q1 is ON, the MOSFET Q3 is ON and the MOSFET Q4 is OFF, so that the load is rectified through the choke coil L1 and the smoothing capacitor C2. The voltage Vo is supplied.

On the other hand, when the MOSFET Q1 is off,
MOSFET Q3 is off, MOSFET Q4 is on
Therefore, the load current is circulated through the MOSFET Q4, and the rectified voltage Vo is supplied to the load. In addition, MOSF
The ETQ2 is controlled by a gate command supplied from a control circuit (not shown) so as to turn on in a predetermined period when the MOSFET Q1 is off. Because of this, MO
The voltage applied to the primary winding of the main transformer T1 when the SFET Q1 is OFF 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 source E1 is Vi, MOSF
The gate voltage Vg3 of ETQ3 turns on the MOSFET Q1.
Is a voltage generated in the secondary winding of the main transformer T1, and the gate voltage Vg4 of the MOSFET Q4 is MOS.
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.
The ON period of 1 is Ton, the OFF period is Toff, and the cycle is T.
If s, The relationship is established.

[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 MOSFE.
Since it depends on the ON / OFF duty ratio of TQ1,
The load current increases and the ON period Ton of the MOSFET Q1 increases.
Becomes longer and the off period Toff becomes shorter, the equation (3)
As shown in, the voltage VC across the clamp capacitor C1
1 goes up.

Therefore, as shown in equation (2), MO
Since the gate voltage Vg4 of the SFET Q4 rises, there is a possibility that a voltage exceeding the gate withstand voltage is supplied and the MOSFET Q4 is damaged.

The present invention has been made in order to solve the problems of the above-mentioned conventional techniques, and MOSF is provided by applying a voltage exceeding the gate withstand voltage.
It is an object of the present invention to provide a switching power supply that combines an active clamp system 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 for limiting an applied voltage to the primary winding of the transformer and a second switching element when the first switching element is off, and AC power output from the secondary winding of the transformer A first field effect transistor connected in series with the secondary winding of the transformer for rectifying the current, and a secondary winding of the transformer for rectifying the 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 switching power supply for pulling a gate voltage of the second field effect transistor to a ground potential. When the gate voltage of the switching transistor and the second field effect transistor exceeds the withstand voltage,
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 includes a first secondary winding, a second secondary winding, and a third secondary winding, and the second field effect transistor is the first secondary winding. The gate of the first field-effect transistor is connected in parallel with a secondary winding, the gate of the first field-effect transistor is connected to the second secondary winding, and the gate of the second field-effect transistor is connected to the third secondary winding. It may be connected, and may have a diode connected in series with the Zener diode in order to prevent a negative voltage from being applied to the input terminal of the switching transistor.

Another configuration of the switching power supply of the present invention is a transformer, a first switching element for switching DC power supplied to the primary winding of the transformer in a predetermined cycle, and the first switching element. A clamp capacitor and a second switching element for limiting an applied voltage to the primary winding of the transformer when the element is off, and the clamp capacitor for rectifying the 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 connected in parallel with the secondary winding of the transformer for rectifying the AC power output from the secondary winding of the transformer. Smoothing for smoothing the rectified output by the second field effect transistor and the first field effect transistor and the second field effect transistor A switching power supply including: 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 the withstand voltage; and a second field effect transistor 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 when the gate voltage exceeds the withstand voltage. , A negative voltage on the gate of the third field effect transistor
The Zener die to prevent the
And a diode connected in series with the diode .

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 the first secondary winding. The gate of the first field-effect transistor is connected in parallel with a secondary winding, the gate of the first field-effect transistor is connected to the second secondary winding, and the gate of the second field-effect transistor is connected to the third secondary winding. It may be connected.

Further, a flywheel diode for circulating a load current, which is connected in parallel with the second field effect transistor, may be provided.

In the switching power supply configured as described above, the switching transistor for pulling the gate voltage of the second field effect transistor to the ground potential and the voltage for which the gate voltage of the second field effect transistor exceeds the withstand voltage. At this time, in order to pull the gate voltage of the second field effect transistor to the ground potential, a zener diode for supplying an input current for turning on the switching transistor is provided, or the gate voltage of the second field effect transistor is insulated. When the gate voltage of the third field effect transistor for limiting to a predetermined voltage equal to or lower than the withstand voltage and the gate voltage of the second field effect transistor exceeds the withstand voltage, the gate voltage of the second field effect transistor is set to the predetermined voltage. The gate voltage of the third field effect transistor to set 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 first embodiment of a switching power supply according to the present invention.

As shown in FIG. 1, in addition to the configuration of the conventional switching power supply shown in FIG. 9, the switching power supply of this embodiment is a switching transistor for pulling the gate voltage Vg4 of the MOSFET Q4 to the ground potential (0V). The configuration includes Q5, a Zener diode D1 and a resistor R4 that are 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.

Further, the Zener voltage of the Zener diode D1 is set to such a value that the Zener current flows when a voltage exceeding the gate withstand voltage is generated in the secondary winding of the main transformer T1. Since other configurations are the same as the conventional ones, 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 how the switching power supply shown in FIG. 1 operates.

As shown in FIG. 2, when the load current is steady, the MOSFET Q1 is ON (time t1 to t).
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, as in the conventional case, the choke coil L1 and the smoothing capacitor C are provided.
The rectified voltage Vo is supplied to the load through 2.

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 0V, and the MOSFE is turned on.
Since the gate voltage Vg3 of TQ3 also becomes 0V, 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 through the load circulates through MOSFET Q4.

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 D1. Since it is not present, the switching transistor Q5 is maintained in the OFF state.

Next, when the load changes abruptly 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 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 steady state, and the gate voltage V of the MOSFET Q4 becomes
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
The voltage Vb generated at the other end (node B) of the secondary winding of No. 1
When (= VC1 ′ / N) has a value exceeding 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 to the ground potential (0V),
Q4 turns off. At this time, the load current is MOSF.
It is returned through the parasitic diode Dp of ETQ4.

Therefore, the MOSFET Q4 can be prevented from being damaged by selecting the value of the Zener voltage of the Zener diode D1 to be equal to or lower than the gate withstand voltage of the MOSFET Q4.

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

The switching power supply of this embodiment differs from that of the first embodiment in the structure of the main transformer and the circuit structure connected to the base of the switching transistor Q5. Since other configurations are similar to those of the first embodiment, the description thereof will be omitted.

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

The Zener voltage of the Zener diode D1 is set to such a value 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 this embodiment is also a MOSF, as in the first embodiment.
Main transformer T1 while ETQ1 is OFF
When the voltage generated in the secondary winding T2c of the MOSFET Q4 exceeds the Zener voltage of the Zener diode D1, the switching transistor Q5 is turned on, the gate voltage Vg4 of the MOSFET Q4 is pulled to the ground potential, and the MOSFET Q4.
4 turns off. Therefore, the Zener diode D1
It is possible to prevent the MOSFET Q4 from being damaged by selecting the value of the Zener voltage of 1 or less as the gate withstand voltage of the MOSFET Q4 or less.

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

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

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

Similarly to the first embodiment, the switching power supply of the present embodiment can prevent damage to the MOSFET Q4 by selecting the value of the Zener voltage of the Zener diode D1 to be equal to or lower than the gate withstand voltage of the MOSFET Q4. . Further, since the flywheel diode D3 is provided in parallel with the MOSETQ4, the diode D3
The load current is circulated via 3. Therefore, it is possible to obtain a switching power supply capable of flowing a larger load current than that of the first embodiment.

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

As shown in FIG. 5, the switching power supply of this embodiment is a combination of the switching power supply of the second embodiment and the switching power supply of the third embodiment. Therefore, the configuration and operation of the switching power supply of this embodiment are the same as those of the second and third embodiments, and therefore their explanations are omitted.

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

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

As shown in FIG. 6, in addition to the configuration of the conventional switching power supply shown in FIG. 9, the switching power supply of this embodiment has a MOSFET Q6 for limiting the gate voltage Vg4 of the MOSFET Q4 and a Zener diode D.
3, the diode D4, and the resistor R5.

The source (S) -drain (D) of the MOSFET Q6 is connected in series with the resistor R1 and is connected to the MOSFE.
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
A Zener diode D4 and a diode D5 connected in series are inserted between the gate of Q6 and the ground potential, and the gate (G) and drain (D) of the MOSFET Q6 are further inserted.
A resistor R5 is inserted between them.

The Zener voltage of the Zener diode D4 is set to such a value that a Zener current flows when a voltage exceeding the gate withstand voltage occurs on the secondary side of the main transformer T1. Since other configurations are the same as the conventional ones, 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 how the switching power supply shown in FIG. 6 operates.

As shown in FIG. 7, when the load current is steady, the MOSFET Q1 is ON (time t1 to t).
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, since 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, the MOSFET Q4 is turned off. Therefore, as in the conventional case, the choke coil L1 and the smoothing capacitor C are provided.
The rectified voltage Vo is supplied to the load through 2.

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 0V, and the MOSFE is turned on.
Since the gate voltage Vg3 of TQ3 also becomes 0V, 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 through the load circulates through MOSFET Q4.

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. MOSFE because there is no
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 t5, the on period Ton of the MOSFET Q1 becomes longer and the off period Toff becomes shorter. Therefore, 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 steady state, and the gate voltage V of the MOSFET Q4 becomes
g4 becomes higher than in the steady state.

The switching power supply of this embodiment is a MOS
Main FET T1 when FET Q1 is off
Of the voltage Vb (V
When C1 ′ / N) is a value exceeding 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 is applied to the gate (G) of 6. Therefore, MOS
The source (S) of FET Q6 is from Vz + VF5 to 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 the 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 lower than the gate withstand voltage of the MOSFET Q4, the MOSFE can be obtained as in the first embodiment.
It is possible to prevent damage to TQ4.

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

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

As shown in FIG. 8, the switching power supply according to the present embodiment has a combination of the switching power supply according to the second embodiment and the switching power supply according to the fifth embodiment. Therefore, the configuration and the operation are similar to those of the second and fifth embodiments, and the 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
Flywheel diode D3 in parallel with FET Q4
By providing, it is possible to obtain the same effect as that of the third embodiment.

[0061]

Since the present invention is constructed as described above, it has the following effects.

When the switching transistor for pulling the gate voltage of the second field effect transistor to the ground potential and the gate voltage of the second field effect transistor exceed the withstand voltage, the gate voltage of the second field effect transistor. A zener diode that supplies an input current for turning on the switching transistor in order to pull the gate 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 voltages of the third field effect transistor and the second field effect transistor exceed the withstand voltage, the third field effect transistor is set in order to set the gate voltage of the second field effect transistor to a predetermined voltage. With a Zener diode for limiting 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 to the second field effect transistor can be prevented.

[Brief description of drawings]

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

FIG. 2 is a timing chart showing how the switching power supply shown in FIG. 1 operates.

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

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

FIG. 7 is a timing chart showing how the switching power supply shown in FIG. 6 operates.

FIG. 8 is a circuit diagram showing a configuration of a sixth embodiment of a switching power supply 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 diodes E1 DC power supply L1 choke coil Q1-Q4, Q6 MOSFET Q5 switching transistor R1, R2, R4, R5 resistors T1, T2 main transformer

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-11-69803 (JP, A) JP-A-11-332225 (JP, A) JP-A-6-343262 (JP, A) JP-A-7- 106565 (JP, A) JP 56-40272 (JP, A) JP 548474 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H02M 3/28 H02M 7 /twenty one

Claims (6)

(57) [Claims] 1. A transformer, a first switching element that switches DC power supplied to a primary winding of the transformer at a predetermined cycle, and a primary of the transformer when the first switching element is off. A clamp capacitor and a second switching element for limiting the voltage applied to the winding, and a series connection with the secondary winding of the transformer for rectifying the AC power output from the secondary winding of the transformer. A first field-effect transistor, a second field-effect transistor connected in parallel with the secondary winding of the transformer for rectifying the AC power output from the secondary winding of the transformer, And a smoothing element for smoothing the 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 voltage of the second field effect transistor when the gate voltage exceeds the withstand voltage. A zener diode that supplies an input current for turning on the switching transistor in order to pull the gate voltage to the ground potential. 2. The transformer comprises a first secondary winding and a second secondary winding.
Secondary winding, and a third secondary winding, the second field effect transistor is connected in parallel with the first secondary winding, 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 the 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 the Zener diode to prevent a negative voltage from being applied to the input terminal of the switching transistor. 4. A transformer, a first switching element that switches DC power supplied to a primary winding of the transformer at a predetermined cycle, and a primary of the transformer when the first switching element is off. A clamp capacitor and a second switching element for limiting the voltage applied to the winding, and a series connection with the secondary winding of the transformer for rectifying the AC power output from the secondary winding of the transformer. A first field-effect transistor, a second field-effect transistor connected in parallel with the secondary winding of the transformer for rectifying the AC power output from the secondary winding of the transformer, And a smoothing element for smoothing the rectified output by the first field effect transistor and the second field effect transistor. A third field effect transistor for limiting the gate voltage of the second field effect transistor to a predetermined voltage equal to or lower than the withstand voltage, and a voltage of which the gate voltage of the second field effect transistor exceeds the withstand voltage. At this time, in order to set the gate voltage of the second field effect transistor to the predetermined voltage, a Zener diode for limiting the gate voltage of the third field effect transistor, and a third field effect transistor Negative voltage on the gate
In order to prevent being added, the Zener diode
And a diode connected in series with the switching power supply. 5. The transformer comprises a first secondary winding and a second secondary winding.
Secondary winding, and a third secondary winding, the second field effect transistor is connected in parallel with the first secondary winding, 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 the gate of the second field effect transistor is connected to the third secondary winding. 6. A parallel with the second field effect transistor.
Flywheel for returning load current, connected to
Any one SL <br/> mounting of the switching power supply of claims 1 to 5 having Le diode.