JPH10248248A - Drive circuit for synchronous rectification of mosfet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a drive circuit which reduces the conduction loss of a rectifying element. SOLUTION: A drive circuit is provided with a primary-side control circuit 5 which is connected to a main switch 4, with a capacitor 14 by which a signal synchronized with a driving signal is transmitted to the main switch 4 and which is continued to a light-emitting-side photocoupler 15-1, with a light- receiving-side photocoupler 12-2 which is connected to the input line of a detection signal, with a commutation-side MOSFET 6 which is connected to a negative-side output terminal, with a rectification-side MOSFET 7 which is connected to a transformer 3, with a light-receiving-side photocoupler 15-2 which receives a signal synchronized with the main switch 4 and with a light-emitting- side photocoupler 12-1 which transmits a detection signal from a detection circuit 11 which detects an output voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
It belongs to a drive circuit for OSFET synchronous rectification, and particularly to a MOSFET synchronous rectification drive circuit using a MOSFET having smaller conduction loss than a diode as a rectifier.

[0002]

2. Description of the Related Art As a conventional technique, an MO of a switching power supply is used.
FIG. 3 shows a first example of the SFET synchronous rectifier circuit. A conventional MOSFET synchronous rectifier circuit has a transformer 3, a main switch 4 connected to the primary side of the transformer 3, and a gate at one end which generates a positive voltage when the main switch 4 is turned on on the secondary side of the transformer 3. The other end is connected to the drain, the source is connected to the negative side of the output, and the commutation side MOSFE is turned on by the voltage generated on the secondary side when the main switch 4 is turned on.
T6, a rectifying side which is connected to one end on the secondary side of the transformer 3, the other end is connected to the gate, the source is connected to the negative side of the output, and the main switch 4 is turned on by the voltage generated on the secondary side when the main switch 4 is off A smoothing coil 8 connected between one end of the secondary side of the transformer 3 and the positive side of the output, and a secondary side smoothing capacitor 9 connected between the positive side and the negative side of the output. , A detection circuit 11 for detecting an output voltage, and a photocoupler 1 on the light emitting side for transmitting a detection signal from the detection circuit 11
2-1 is connected to a photocoupler 12-2 on the light receiving side that receives a detection signal from the photocoupler 12-1 on the light emitting side, and the detection signal is received by the photocoupler 12-2 on the light receiving side to stabilize the output voltage. For this purpose, a primary-side control circuit 5 for controlling the main switch 4 is provided.

Referring to FIG. 3 and FIG. 4 which is an operation time chart of the circuit shown in FIG.
The operation of the FET synchronous rectifier circuit will be described. FIG. 4 shows V0 as the drive signal of the main switch 4, VS as the secondary winding voltage waveform of the transformer, and V1 as the rectifying MOSFET, corresponding to V0, VS, V1 and V2 shown in FIG. The drive waveform, V2, is shown as the drive waveform of the MOSFET on the commutation side.

The voltage Vin input between the positive input terminal 1 and the negative input terminal 2 is applied between the secondary terminal of the transformer 3 by turning on the main switch 4 controlled by the control circuit 5 during the mode 1m. (The secondary winding voltage waveform VS of the transformer) VS turns on the MOSFET 7 on the rectifying side and accumulates magnetic energy in the smoothing coil 8.
The DC voltage smoothed by the smoothing capacitor 9 is applied to the load 10
Output to

Next, mode 2 in which the main switch 4 is off
During the period m, the excitation energy of the transformer 3 is released, and a resonance voltage determined by the parasitic capacitance of the main switch 4 and the excitation energy is generated. This resonance voltage also occurs on the secondary side of the transformer 3 and turns on the MOSFET 6 on the commutation side,
At the same time, the MOSFET 7 on the rectifying side is turned off.

[0006] By this operation, the electromagnetic energy stored in the smoothing coil 8 is output via the MOSFET 6 on the commutation side.

Mode 3 in which the resonance voltage drops to the input voltage
m, the secondary voltage of the transformer 3 becomes zero volts, the MOSFET 6 on the commutation side is turned off, and the smoothing coil 8 is turned off.
Is output via the parasitic diode of the MOSFET 6 on the commutation side.

Next, the operation of stabilizing the output voltage will be described. The output voltage is detected by the voltage detection circuit 11 and the detection signal is transmitted by the photocoupler 12-1 on the light emission side and the photocoupler 12-2 on the light reception side. To supply a stable output voltage to the load 10.

As described above, the conduction period of the commutation-side MOSFET 6 that emits electromagnetic energy of the smoothing coil 8 during the off period of the main switch 4 in the modes 2m and 3m is the period of the mode 2m in which the resonance voltage is generated. However, during the remaining mode 3m, the power is released via the parasitic diode of the MOSFET 6 on the commutation side, and there is a problem that excessive power loss is caused by the forward voltage of the parasitic diode.

FIG. 5 shows a second example of the prior art (Japanese Patent Laid-Open No. 7-194104) for solving the above problem. The description of the same circuit portions as those in the first example of the above-described prior art will be omitted.

The MOSFET synchronous rectifier circuit in the second example has a secondary-side control circuit 16 that outputs a signal synchronized with a control signal for driving the main switch 4. The photocoupler 17-2 on the light receiving side is connected to the control circuit 16 on the secondary side. The synchronization signal from the control circuit 16 for driving the primary side main switch 4 is supplied to the photocoupler 17- on the light receiving side.
2 and a commutation-side MOSF for synchronous rectification
A secondary-side control circuit 16 for driving the MOSFET with the ET 6 and the MOSFET 7 on the rectifying side in synchronization with the main switch 4 is provided on the secondary side. In addition, the photocoupler 17-1 on the light emitting side is connected to the control circuit 5 on the primary side together with the photocoupler 12-2 on the light receiving side.

The operation of FIG. 5 showing a second example of the prior art will be described with reference to a time chart shown in FIG. When the control circuit 5 on the primary side turns on the main switch 4, at the same time, the photocoupler 17-1 on the light emitting side and the photocoupler 17 on the light receiving side are turned on.
A signal synchronized with the turning-on of the main switch 4 is transmitted to the control circuit 16 on the secondary side via -2. Upon receiving this signal, the control circuit 16 turns on the rectifying MOSFET 7 and turns on the commutating M
OSFET6 is turned off. In the mode 1m, the load 1 is transferred from the secondary side of the transformer 3 through the MOSFET 7 on the rectifying side to the smoothing coil 8 while accumulating electromagnetic energy.
Generates a voltage at zero.

Next, when the control circuit 5 on the primary side turns off the main switch 4, at the same time, the photocoupler 17 on the light emitting side is turned off.
-1, A signal synchronized with the turning off of the main switch 4 is transmitted to the control circuit 16 on the secondary side via the photocoupler 17-2 on the light receiving side. The control circuit 16 on the secondary side receives this signal, turns off the MOSFET 7 on the rectification side, and turns off the MOSFET 7 on the commutation side.
Turn 6 on. In the modes 2 m and 3 m, the electromagnetic energy stored in the smoothing coil 8 is released through the MOSFET 6 on the commutation side in the ON state, and a voltage is generated in the load 10.

Synchronous rectification is performed by the main switch 4 and M for synchronous rectification.
The method of synchronizing the OSFET 6 and the MOSFET 7 is important. However, this circuit of the second conventional example is an ideal circuit for theoretical study, and this circuit uses a signal synchronized with the drive signal of the primary switch 4 on the secondary side and a MOS for synchronous rectification on the secondary side.
When the FETs 6 and 7 are driven, a time delay occurs between the drive signal of the main switch 4 and the voltage generated on the secondary side of the transformer 2 unless the components are ideal components. For this reason, a period occurs in which the MOSFET 6 and the MOSFET 7 are simultaneously turned on, and the secondary side of the transformer 3 is short-circuited, resulting in excessive power loss. In order to prevent this, the control circuit 16 on the secondary side becomes complicated. Regarding this, the second conventional example does not mention any specific contents.

FIG. 7 shows a third example of the prior art (Japanese Patent Laid-Open No. 7-67343) for solving the above-mentioned problem of the prior art.

The description of the same circuits as those in the first and second examples of the prior art will be omitted. The MOSFET synchronous rectifier circuit of the third conventional circuit is for supplying the voltage generated in the secondary winding of the transformer 3 when the main switch 4 is turned on to the load 10 while accumulating electromagnetic energy in the smoothing coil 8. A first synchronous rectification MOSFET 6;
Second synchronous rectification MOSFET 7 for releasing electromagnetic energy of smoothing coil 8 when main switch 4 is turned off
A third MOSFET 18 for discharging charges stored in a gate of the second MOSFET, and a third MOSFET
A fourth MOSFET 19 for turning on the FET 18 is provided.

The operation of the third example of the prior art will be described with reference to a time chart shown in FIG. In the ON period mode 1m of the main switch 4, the voltage V generated on the secondary side of the transformer 3
By S, both the MOSFETs 7 and 19 are turned on, and a voltage is generated in the load 10 while accumulating electromagnetic energy in the smoothing coil 8. Next, when the main switch 4 is turned off, a resonance voltage due to the excitation energy stored in the transformer 3 and the parasitic capacitance of the main switch 4 is generated during the mode 2m. This resonance voltage is also generated on the secondary side of the transformer 3 and the MOSFET
7 and 19 are turned off. At the same time, the resonance voltage is MOS
MOSFET 6 via parasitic diode of FET 18
To turn on the MOSFET 6.

As a result, the electromagnetic energy stored in the smoothing coil 8 is released through the MOSFET 6 in the ON state, and a voltage is generated in the load 10. During the period of mode 3, even if the resonance voltage drops to zero volt,
Since the charge stored in the gate capacitance of the MOSFET 6 is held without discharging because T18 is in the OFF state, the MOSFET 6 continues to maintain the ON state.

Next, the main switch 4 is turned on again, and a voltage is again generated on the secondary side of the transformer.
By turning on the FET 18 and discharging the above-mentioned electric charge, the MO
The SFET 6 is turned off. At the same time, MOSFET
7 and 19 are turned on, and a voltage is generated in the load 10.
Here, when only the MOSFET 7 as the rectifying element is used, the voltage generated on the secondary side of the transformer 3 during the transition from the mode 3 m period to the ON mode 1 m period of the main switch 4 is a parasitic diode of the MOSFET 6 and the MOSFET 7 in the ON state. Is returned via. Therefore, MOSFET
18 does not rise to the gate potential to turn on
There is a disadvantage that the ON state of T6 is maintained. In order to solve this problem, a MOS
By connecting the FETs 19 in series by common connection of the drains, it is possible to prevent a secondary side short circuit caused by the parasitic diode of the MOSFET 7, and to surely turn on the MOSFET 18 and turn off the MOSFET 6. That is, the MOSFET 19 operates to prevent a short circuit on the secondary side via the parasitic diode of the MOSFET 7.

[0020]

However, in the MOSFET synchronous rectification drive circuit according to the third example of the prior art, the MOSFET 19 is replaced with the MOSFET 18 as described above.
Is turned on to surely turn off the MOSFET 6, but this is a problem that power is lost due to conduction of the MOSFET 19 because the MOSFET 7 is always on when the MOSFET 7 is on.

Therefore, the reduction of the conduction loss of the rectifying element is missed and is not a good measure.

It is therefore an object of the present invention to provide a MOSFET synchronous rectification drive circuit capable of reducing conduction loss of a rectifier.

[0023]

According to the present invention, in a synchronous rectifier circuit of a switching power supply, a transformer, a main switch connected to a primary side of the transformer, and a drive signal line of the primary side main switch are connected. A primary-side control circuit including a synchronous signal output line for outputting a signal synchronized with the driving of the primary side main switch, and having a detection signal input line for stabilizing an output voltage; and a synchronous signal output line. A light emitting side photocoupler for transmitting a signal synchronized with the driving of the main switch, a smoothing coil connected between an output line at one end of a secondary side of the transformer and a positive side of the output, A drain is connected to a connection point between the output line and the smoothing coil, a source is connected to a negative output terminal, and a first MOSFET is connected to an output line at one end on a secondary side of the transformer. A second MOSFET having a drain connected to the other end of the transformer secondary side and a source connected to a negative output terminal, and a gate and a drain of the first MOSFET connected to the secondary side of the transformer. A third MOSFET connected to the other end and a source for discharging the charge stored in the gate of the first MOSFET; an emitter connected to the gate of the third MOSFET and the first MO;
A collector is connected to the gate of the SFET, and the third M
A driving circuit for MOSFET synchronous rectification, comprising: a photocoupler on the light receiving side for transmitting a signal synchronized with the primary switch on the primary side to turn on the OSFET, is obtained.

[0024]

The driving circuit for MOSFET synchronous rectification according to the present invention comprises:
High frequency switching of input voltage by main switch,
The secondary voltage obtained by converting this high frequency by a transformer is represented by M
The DC voltage is rectified by a synchronous rectifier circuit using an OSFET, a smoothing coil, and a smoothing capacitor to output a stable voltage.

[0025]

FIG. 1 shows an embodiment of a switching power supply of the present invention, that is, a MOSFET synchronous rectification drive circuit which is a synchronous rectification circuit of an input / output insulated switch power supply.

The prior art MOSFE shown in FIG.
The same parts as those of the T-synchronous rectification drive circuit are denoted by the same reference numerals and described.

Referring to FIG. 1, the MOSFET synchronous rectification drive circuit includes a transformer 3, a main switch 4 connected to the primary side of the transformer 3, and a drive signal line of the main switch 4, A primary-side control circuit 5 having a synchronization signal output line for outputting a signal synchronized with the driving of the main switch 4 on the side, and having an input line for a detection signal for stabilizing the output voltage; A light emitting side photocoupler 15-1 for transmitting a signal synchronized with a drive signal to a signal line, a capacitor 14 connected in series with the light emitting side photocoupler 15-1, and a detection signal for stabilizing an output voltage The drain is connected to the connection point between the photocoupler 12-2 on the light receiving side connected to the input line and the output line at one end on the secondary side of the transformer 3 and the smoothing coil 8 on the secondary side. M commutation side over scan are respectively connected
An OSFET (first MOSFET) 6 and a rectifying side in which a gate is connected to an output line at one end of a secondary side of the transformer 3, a drain is connected to the other end of the secondary side of the transformer 3, and a source is connected to a negative output terminal. MOSFET7 (second MOSFET)
T), the gate of the MOSFET 6 on the commutation side and the transformer 3
Between the other end of the secondary side and the MOSFET on the commutation side
A discharge MOSFET 13 (third MOSFET) for discharging the electric charge accumulated in the gate of T6, and an emitter and a commutation MO on the gate of the discharge MOSFET 13
The collector is connected to the gate of the SFET 6 and the discharge M
Primary switch 4 for turning on OSFET 13
15-2 on the light receiving side receiving a signal synchronized with
A secondary-side smoothing capacitor 9 connected between the positive and negative sides of the output, and a detection circuit 11 for detecting the output voltage
And a photocoupler 12-1 on the light emitting side that transmits a detection signal from the detection circuit 11.

Next, the operation of the MOSFET synchronous rectification drive circuit of the present invention will be described with reference to FIG. 2 which is an operation time chart of the circuit of FIG. FIG. 2 shows FIG.
So as to correspond to V0, VS, V1 and V2 attached to
V0 is a drive signal of the main switch 4, VS is a secondary winding voltage waveform of the transformer, V1 is a drive waveform of the rectification side MOSFET, and V2 is a drive waveform of the commutation side MOSFET.

The voltage Vin input between the positive side input terminal 1 and the negative side input terminal 2 is primary-secondary when the main switch 4 controlled by the primary side control circuit 5 is turned on during the mode 1m. The voltage VS (secondary winding voltage waveform VS of the transformer 3) VS generated between the secondary terminals of the transformer 3 turns on the MOSFET 7 and accumulates magnetic energy in the smoothing coil 8 due to the delay of the time t due to the propagation delay between them. The DC voltage smoothed by the capacitor 14 is output to the load 10.

During the mode 2m in which the main switch 4 is on, the excitation energy of the transformer 3 is released, and a resonance voltage determined by the parasitic capacitance of the main switch 4 and the excitation energy is generated. This resonance voltage is generated on the secondary side of the transformer 3 and is connected to the
The voltage is applied to the gate of the SFET 6 to turn on the MOSFET 6 and turn off the MOSFET 7 at the same time.

The electromagnetic energy stored in the smoothing coil 8 is released via the MOSFET 6 in the ON state, and the load 1
Generates a voltage at zero. In the period of mode 3m, even if the resonance voltage drops to zero volt, the charge accumulated in the gate capacitance of MOSFET 6 is held without discharging because MOSFET 13 is in the off state.
OSFET 6 keeps on state.

Next, when the main switch 4 is turned on again, the photocouplers 15-1 and 15-2 operate via the capacitor 14 in response to a signal synchronized with the drive signal of the main switch, and the MOSFET 13 is turned on for t time to turn on. By discharging the charge, the MOSFET 6 is turned off. At the same time, a voltage is generated on the secondary side of the transformer 3, the MOSFET 7 is turned on, and a voltage is generated on the load 10.

Here, the MOSFET 13 is connected to the commutation side MO.
To charge and discharge the SFET 6 and discharge the charge accumulated at the gate of the MOSFET 6 and turn it off,
An element having a gate cutoff threshold voltage lower than the cutoff gate threshold voltage of T6 than the conduction voltage drop of the photocoupler 15-2 on the light receiving side is required.

Next, the operation of stabilizing the output voltage will be described. The output voltage is detected by the detection circuit 11 and the detection signal is transmitted by the photocouplers 12-1 and 12-2, and the control circuit 5 of the main switch 4 controls the drive pulse width of the main switch 4 to stabilize the output voltage. Is supplied to the load 10.

[0035]

As described above, according to the embodiment of the present invention, according to the MOSFET synchronous rectification drive circuit of the present invention, the input voltage is switched at a high frequency by the main switch, and the high frequency is converted by the transformer. The obtained secondary-side voltage is rectified into direct current by a synchronous rectifier circuit using MOSFETs, a smoothing coil and a smoothing capacitor, so that a stable voltage can be output, and conduction loss of the rectifying element can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a drive circuit for MOSFET synchronous rectification of the present invention.

FIG. 2 is a time chart showing an operation of the MOSFET synchronous rectification drive circuit shown in FIG. 1;

FIG. 3 is a circuit diagram showing a MOSFET synchronous rectification drive circuit as a first example of the prior art.

FIG. 4 is a time chart showing an operation of the MOSFET synchronous rectification drive circuit shown in FIG. 3;

FIG. 5 is a circuit diagram showing a MOSFET synchronous rectification drive circuit as a second example of the related art.

FIG. 6 is a time chart illustrating an operation of the MOSFET synchronous rectification drive circuit illustrated in FIG. 5;

FIG. 7 is a circuit diagram showing a drive circuit for MOSFET synchronous rectification as a third example of the prior art.

8 is a time chart showing an operation in the MOSFET synchronous rectification drive circuit shown in FIG. 7;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Positive side input terminal 2 Negative side input terminal 3 Transformer 4 Main switch 5 Primary side control circuit 6 Commutation side MOSFET 7 Rectification side MOSFET 8 Smoothing coil 9 Smoothing capacitor 10 Load 11 Detection circuit 12-1, 15-1 , 17-1 Light-emitting side photocoupler 12-2, 15-2, 17-2 Light-receiving side photocoupler light-receiving side 13 MOSFET for discharging 14 Capacitor 16 Secondary-side control circuit V0 Drive signal of main switch VS transformer V1 Rectifier side MOSFET drive waveform V2 Commutation side MOSFET drive waveform

Claims (3)

[Claims] In a synchronous rectifier circuit of a switching power supply, a transformer, a main switch connected to a primary side of the transformer, and a drive signal line of the primary side main switch, connected to a drive of the primary side main switch and synchronized with the drive of the primary side main switch. A control signal on the primary side, including a synchronization signal output line for outputting a detected signal, and a detection signal input line for stabilizing an output voltage; and a signal synchronized with driving of the main switch to the synchronization signal output line. A light emitting side photocoupler for transmitting
A smoothing coil connected between an output line at one end of the secondary side of the transformer and the positive side of the output, a drain at a connection point between the output line and the smoothing coil, and a source at a negative output terminal. A first MOSFET connected to each of the second MOSFETs, a gate connected to an output line at one end of the secondary side of the transformer, a second drain connected to the other end of the secondary side of the transformer, and a source connected to a negative output terminal. A MOSFET, a gate and a drain of the first MOSFET, a second end of the transformer and a source connected to each other,
A third MOSFET for discharging the electric charge stored in the gate of the ET, an emitter connected to the gate of the third MOSFET, and a collector connected to the gate of the first MOSFET, turning on the third MOSFET; A photocoupler on the light receiving side for transmitting a signal synchronized with the primary switch on the primary side.
Drive circuit for ET synchronous rectification. 2. The drive circuit for MOSFET synchronous rectification according to claim 1, further comprising: a capacitor connected in series to the light-emitting side photocoupler for transmitting a signal synchronized with a drive signal to the drive signal line of the main switch. A MOSFET synchronous rectification drive circuit, comprising: 3. The drive circuit for MOSFET synchronous rectification according to claim 1, wherein a secondary-side smoothing capacitor connected between a positive side and a negative side of the output, a detection circuit for detecting an output voltage, A MOSF having a light emitting side photocoupler for transmitting a detection signal from the detection circuit.
Drive circuit for ET synchronous rectification.