JP2001095241A - Synchronous rectifying circuit for power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce exchange loss of power supply which results from commutating FET. SOLUTION: This synchronous rectifying circuit is provided with an FET for supplying the DC power source 1 to the coil P of a transformer 4, when a rectangular waveform is impressed to the gate and the source-to-drain is conductive, an FET7 in the rectifying side which becomes conductive with a forward bias voltage induced in a coil S of the transformer 4 and charges a capacitor 11 via a choke coil 10, an FET8 in the commutation side for making nonconductive the FET7 on the rectifying side with an inverse bias of the flyback voltage generated in the coil S of the transformer 4, when the FET3 is non conductive and charging the capacitor 11 with an reverse electromotive force from the choke coil 10, moreover, an FET9 for auxiliary switch for connecting the gate to the input side of the choke coil 10 and also connecting the drain and source between the gate and source of the FET8 for commutation, and a drive circuit 14 for supplying a DC voltage to the FET9 of the auxiliary switch.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply synchronous rectifier circuit, and more particularly to a power supply synchronous rectifier circuit for performing DC-DC conversion.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram showing an example of a conventional power supply synchronization circuit.

In a conventional example, a transformer 4 having a primary winding P and a secondary winding S, and a DC power supply 1 which is connected to the primary side of the transformer 4 when a rectangular wave is applied to the gate and the source and the drain conduct. FET of primary side main switch supplied to winding P
And a rectifying-side FET 7 for applying a forward bias voltage induced in the secondary winding S of the transformer 4 to the gate via the resistor 5 to conduct and charge the smoothing capacitor 11 via the choke coil 10. And the primary side main switch F
When the ET3 is non-conductive, the rectifying FET 7 becomes non-conductive due to the reverse bias of the flyback voltage generated in the secondary winding S of the transformer 4, and at this time, a forward bias voltage is applied to the gate via the resistor 15. The commutation-side F for passing back electromotive force from the choke coil 10 and charging the capacitor 11
ET16.

[0004]

In the above-mentioned conventional synchronous rectifier circuit of a power supply, the voltage applied to the gate of the FET on the commutation side is, as shown in FIG.
Utilizes the flyback voltage from

Since the voltage waveform shown in FIG. 4 is not a rectangular wave, an area TA having an insufficient drive voltage and an area TB having an excessive drive are required for a gate threshold voltage VG required in a section in which the FET on the commutation side is to be conducted. appear.

For this reason, the conduction section of the FET on the commutation side is shortened, and the conversion loss of the power supply is increased.

Further, the high peak value of the flyback voltage hinders the reduction of the breakdown voltage of the FET.

[0008]

A synchronous rectifier circuit of a power supply according to the present invention is used in a power supply device, and switches a DC voltage from a DC power supply with a switch element to supply the DC voltage to a primary winding of a transformer. A rectifying FET connected in series to a secondary winding of a transformer is driven in synchronization with a conduction section of the switch element, and a voltage induced in the secondary winding is passed through a choke coil for smoothing. Synchronous rectification that charges a capacitor, drives a commutation FET connected to the input side of the choke coil in synchronization with the non-conducting section of the switch element, and back electromotive force from the choke coil charges the capacitor. The circuit is characterized in that a voltage applied to a gate of the commutation FET is shaped into a rectangular wave.

Further, an auxiliary switch FET for shaping into the rectangular wave is connected between the gate and the source of the commutation FET.

Further, the synchronous rectifier circuit of the power supply of the present invention has the following features.
A transformer having a secondary winding and a secondary winding, an FET of a primary main switch for supplying a DC power to the primary winding when a rectangular wave is applied to a gate and conduction between a source and a drain is provided, A rectifying-side FET that conducts by a forward bias voltage induced in the secondary winding and charges a smoothing capacitor via a choke coil; and a secondary winding when the FET of the primary main switch is non-conductive. FET on the rectification side by reverse bias of flyback voltage generated in the line
A non-conducting, a commutation-side FET for passing back electromotive force from the choke coil and charging the capacitor, a gate connected to the input side of the choke coil, and a gate of the commutation FET. An auxiliary switch FET for connecting a drain and a source between sources is provided, and a drive circuit for supplying a DC voltage to the auxiliary switch FET is provided.

Further, a gate of the FET of the auxiliary switch of the drive circuit is connected to an input side of the choke coil via a resistor, and a connection point between a drain of the FET of the auxiliary switch and the gate of the commutation FET is provided. The DC voltage is supplied from the output side of the choke coil via a resistor, and the auxiliary switch FET and the commutation FET
Are connected to each other.

[0012] Further, the invention is characterized in that it has a voltage dividing resistor for adjusting a DC voltage supplied to the FET of the auxiliary switch of the drive circuit.

Further, a low breakdown voltage MOSFET is used as the commutation FET.

[0014]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 shows a circuit according to an embodiment of the present invention.

In this embodiment, a transformer 4 having a primary winding P and a secondary winding S, and a DC power supply 1 which is connected to a square wave when a rectangular wave is applied to a gate and a source and a drain are connected to each other.
Of the primary side main switch to be supplied to the primary side winding P
T3, a rectifying-side FET 7 which conducts by a forward bias voltage induced in the secondary winding S of the transformer 4 and charges the smoothing capacitor 11 via the choke coil 10;
When the FET 3 of the primary side main switch is non-conductive, the rectifying side FET 7 is made non-conductive by the reverse bias of the flyback voltage generated in the secondary winding S of the transformer 4 and the back electromotive force from the choke coil 10 is passed. A commutation-side FET 8 for charging the capacitor 11; a gate connected to the input side of the choke coil 10; an auxiliary switch FET 9 connecting a drain and a source between the gate and the source of the commutation FET 8; And a drive circuit 14 for supplying a DC voltage to the FET 9 of the switch.

The gate of the FET 9 of the auxiliary switch of the drive circuit 14 is connected via the resistor 6 to the choke coil 10.
Connected to the connection point between the drain of the FET 9 of the auxiliary switch and the gate of the FET 8 for commutation via the resistor 12 to the DC voltage on the output side of the choke coil 10,
The sources of the auxiliary switch FET 9 and the commutation FET 8 are connected.

Next, the operation of this embodiment will be described with reference to FIG. A smoothing capacitor 2 is connected in parallel to the switching DC power supply 1. In the FET 3, a rectangular wave is applied to the gate, and the DC power supply 1 is supplied via the temporary winding P of the transformer 4.

When the FET 3 of the primary side main switch is conducting (ON), the DC power supply 1 is applied to the primary winding P, and an AC voltage VS corresponding to the transformation ratio of the transformer 4 is generated in the secondary winding S of the transformer 4. I do.

The gate of the rectifying-side FET 7 is forward-biased by the AC voltage VS via the resistor R5, and is turned on (ON).
Then, an output current flows through the load 13 in which the choke coil 10 and the capacitor 11 are connected in parallel. At the same time, the gate of the auxiliary switch FET 9 is also forward biased and turned on.
Then, the gate and the source of the FET 8 on the commutation side are short-circuited to
The source and the drain of ET8 are made non-conductive (OFF).

Next, when the FET 3 of the primary side main switch is non-conductive (OFF), a flyback voltage is generated in the secondary winding S of the transformer 4, the gate of the rectifying side FET 7 is reverse-biased, and the source and drain are switched. Non-conduction (OFF)
Becomes

At the same time, the gate of the auxiliary switch FET 9 is also reverse-biased, so that the source and the drain are non-conductive (OFF), and the load 13
The DC voltage on the side is applied via the resistor 12, and the source and the drain of the FET 8 conduct (ON).

When the FET 8 is turned on, the choke coil 10 is turned on.
The back electromotive force (energy) stored in the power supply is regenerated as a charging current for the capacitor 11 through the FET 8 on the commutation side.

Such an operation is repeatedly performed for each period of the rectangular wave applied to the FET of the primary side main switch, and the output DC voltage is supplied to the load 13.

Next, another embodiment of the present invention will be described with reference to FIG. In the drive circuit 18 of the present embodiment, a voltage dividing resistor 17 is provided between the gate and the source of the FET 8 on the commutation side, and the gate voltage of the FET 8 can be arbitrarily set by the resistors 12 and 17. Can be optimized.

By doing so, the gate voltage of the FET 8 on the commutation side is shaped into a stable rectangular wave, and
The resistance at the time of conduction of T8 can be reduced. Further, application of an excessive drive voltage can be prevented, so that loss during conduction and drive loss can be reduced.

Since the gate drive voltage of the FET 8 can be set to an optimum value, a low breakdown voltage type MOSFET can be used.

[0027]

As described above, in the present invention, the first effect is that the conduction loss and the drive loss of the commutation side FET can be reduced.

The reason is that by setting the gate voltage to a stable rectangular wave, the ON resistance of the commutation side FET can be reduced over the entire region. Another reason is that drive loss can be reduced because application of an excessive drive voltage can be prevented.

The second effect is that the gate drive voltage of the FET on the commutation side can be easily set, so that the low breakdown voltage type MO is used.
An SFET can be used and the loss can be reduced.

The reason for this is that generally, it is necessary to lower the breakdown voltage of the FET in order to lower the ON resistance, but the gate threshold voltage also decreases. In the drive method using the conventional flyback voltage, since the peak voltage is high, F
This was a bottleneck in lowering the withstand voltage of ET.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention.

FIG. 2 is a circuit diagram of another embodiment of the present invention.

FIG. 3 is a circuit diagram of an example of a conventional synchronous rectifier circuit of a power supply.

FIG. 4 is a diagram for explaining the operation of the conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2,11 Capacitor 3,7,8,9,16 FET 4 Transformer 5,6,12,17 Resistance 10 Choke coil 13 Load 14,18 Drive circuit

Claims (6)

[Claims] A DC voltage from a DC power supply is switched by a switch element and used in a power supply device.
The rectifying FET, which is supplied to the secondary winding and is connected in series with the secondary winding of the transformer, is driven in synchronization with the conduction section of the switch element, and is induced in the secondary winding. The voltage charges the smoothing capacitor via the choke coil,
A synchronous rectifier circuit that drives a commutation FET connected to an input side of the choke coil in synchronization with a non-conducting section of the switch element, and back electromotive force from the choke coil charges the capacitor, A synchronous rectifier circuit for a power supply, wherein a voltage applied to the gate of the commutation FET is shaped into a rectangular wave. 2. The synchronous rectifier circuit of a power supply according to claim 1, wherein an auxiliary switch FET for shaping the rectangular wave is connected between the gate and the source of the commutation FET. 3. A transformer having a primary side winding and a secondary side winding, and a primary side supplying a DC power to the primary side winding when a rectangular wave is applied to a gate and conduction between a source and a drain occurs. A rectifying-side FET that conducts by a forward bias voltage induced in the secondary winding and charges a smoothing capacitor through a choke coil;
When the FET of the primary side main switch is off,
A commutation-side FET that makes the rectification-side FET non-conductive with a reverse bias of a flyback voltage generated in a secondary winding and passes back-electromotive force from a choke coil to charge the capacitor; An auxiliary switch FET that connects a gate to the input side of the choke coil and connects a drain and a source between the gate and the source of the commutation FET;
And a drive circuit for supplying a DC voltage to the FET of the auxiliary switch. 4. The drive circuit, wherein the gate of the FET of the auxiliary switch is connected via a resistor to the input side of the choke coil, and the connection point between the drain of the FET of the auxiliary switch and the gate of the FET for commutation. 4. The power supply synchronization according to claim 3, wherein the DC voltage is supplied from an output side of the choke coil via a resistor to a source of the FET of the auxiliary switch and a source of the FET for commutation. Rectifier circuit. 5. The synchronous rectifier circuit of a power supply according to claim 3, further comprising a voltage dividing resistor for adjusting a DC voltage supplied to the FET of the auxiliary switch. 6. A low breakdown voltage MO as the commutation FET.
The synchronous rectifier circuit of a power supply according to claim 3, wherein an SFET is used.