JPH10225114A - Synchronous rectifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid a short-circuit current, caused by the operation delay of a MOSFET in the secondary side of a transformer, by providing a control (discharging) circuit separately for switching off a commutating MOSFET and to prevent the decline of efficiency by making it possible to turn on the MOSFET over the total range of the commutation interval, which is a merit of the original choke coil driving system. SOLUTION: This circuit is a synchronous rectifier circuit of a converter, in which rectifying MOSFET 4 and commutating MOSFET 6 are connected to the secondary coil 2B of a transformer in a voltage converter circuit and which filters the output with a choke coil 8 and an output capacitor 9. The drive circuit 11 for turning on the commutating MOSFET 6 and a control (discharging) circuit 13 for turning it off are provided. The drive circuit 11 is made up of the auxiliary windings 8B of the choke coil 8 with one end connected to the source of the MOSFET 6 and a diode 12. The control circuit 13 contains at least a control switch 14, which operates synchronized with a main switching element 3, and its other end is connected to the drain of the commutating MOSFET 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulated one-stone forward converter, and more particularly to a synchronous rectifier circuit using an MO (2) SFET on the secondary side.

[0002]

2. Description of the Related Art In a voltage converter such as a DC-DC converter, a synchronous rectifier circuit in which a rectifier diode is replaced with a MOSFET has an advantage that the voltage drop in a conductive state can be reduced and the efficiency of the circuit can be improved. FIG. 6 shows a conventional example of a one-stone forward converter using a synchronous rectifier circuit. 3 is a main switch element, 2 is a transformer, 4 is M for rectification.
OSFET, 6 is a commutation MOSFET, 8 is a choke coil, 1 is an input capacitor, and 9 is an output capacitor. Parasitic diodes incorporated in the rectification and commutation MOSFETs 4 and 6 are indicated by reference numerals 5 and 7, respectively.

FIG. 7 is a time chart of the synchronous rectifier circuit shown in FIG. 6 during the conduction period (t1) of the primary side main switch element 3 via the transformer 2 from the primary side input capacitor 1 to the output capacitor. Power is transmitted to 9. At this time, the voltage generated in the secondary winding of the transformer 2 is a rectifying MOSFET.
4 is made conductive, and a predetermined output voltage is generated in the output capacitor 9 while storing electromagnetic energy in the choke coil 8. Next, when the main switch element 3 is cut off, the excitation energy stored in the transformer generates a resonance voltage between the output capacity of the main switch element 3, the input capacity of the commutation MOSFET 6, and the like.

The commutation MOSFE is generated by the resonance voltage.
T6 conducts, and at the same time, the rectifying MOSFET 4 shuts off. With this operation, the electromagnetic energy stored in the choke coil 8 is released to the output capacitor 9 via the commutation MOSFET 6. (3) However, since the resonance voltage ends in the period t2 and the resonance voltage is an AC voltage, the commutation MOSFET 6
Can be conducted only during the period of Δt2 at which the resonance voltage becomes equal to or higher than the gate threshold voltage Vth of the MOSFET 6 for commutation, and during the remaining periods of Δt1, Δt3 and t3, the M
The OSFET 6 is cut off, and the electromagnetic energy remaining in the choke coil 8 is discharged to the output capacitor 9 through the parasitic diode 7.

As described above, this circuit includes the main switch element 3
During the cutoff period, the conduction period of the commutation MOSFET 6 that emits the electromagnetic energy of the choke coil 8 is only a part Δt2 of the generation period of the resonance voltage, and the remaining electromagnetic energy is the other period Δt1. At Δt 3 + t 3, the power is released via the parasitic diode 7 of the MOSFET 6 for commutation, so that the power loss due to the forward voltage VF of the diode is large.

FIG. 8 shows a second example of the prior art for improving the above-mentioned disadvantage. As disclosed in Japanese Patent Application Laid-Open No. 55-66281, this conventional example has an auxiliary winding in which a gate of a commutation MOSFET 6 is provided in a choke coil 8 with respect to the elements of the first conventional synchronous rectifier circuit described above. 8B. In FIG. 8, since a P-type MOS is used as the rectifying MOSFET, the circuit configuration is slightly different from that of FIG. 6, but the function is not changed.

As shown in FIG. 7 (e), a positive or negative voltage is generated in the choke coil 8 in conjunction with the on / off operation of the main switch element 3. A switching waveform of a square wave optimal for driving the commutation MOSFET 6 is obtained. Further FIG.
According to the method described above, since the resonance voltage at the time of resetting the transformer is not used, the delay time Δt until the gate threshold is reached when the commutation MOSFET 6 is turned on as shown in FIG.
Since 1 is eliminated, the loss caused by flowing through the diode is reduced by (4), and the period t2 of the resonance voltage determined by the exciting inductance of the transformer and the parasitic capacitance of the circuit is reduced by the input capacitance of the MOSFET 6 for commutation. There are advantages. As a result, there is an advantage that the conversion frequency can be increased and the size can be further reduced.

In the above-described conventional synchronous rectifier circuit shown in FIG. 8, the circuit example shown in FIG.
Although there is an advantage that the commutation MOSFET can be conducted for 3) and the resonance cycle can be shortened, this method is practically hardly used because of the following problems. First, since the voltage generated in the auxiliary winding 8B of the choke coil 8 changes from positive to negative, ΔV during the voltage change
Since L is large and the driving loss of the commutation MOSFET 6 due to this is C (△ VL) ² f, the loss is nearly four times greater than the change from positive to zero.

Secondly, when the main switch element 3 is turned on and the commutation MOSFET 6 is turned off, the gate charge is passed to the source side via the auxiliary winding 8B of the choke coil 8, and the turn-off time tOFF of the commutation MOSFET 6 is turned off. Therefore, a delay occurs, and the MOSFET 6 cannot be turned off immediately. However, current MOSFET 4 or diode 5
, A current short-circuiting the secondary of the transformer flows through the commutation MOSFET 6 and the rectification MOSFET 4 or the diode 5 for approximately tOFF time, and an excessive loss occurs. At this time, in a DC-DC converter having an overcurrent protection function or the like, the protection function may be activated and may not be able to operate normally.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to solve these problems and to provide a more efficient one-stone forward converter. (5)

[0011]

In order to achieve the above object, the present invention connects a main switch element and a drive circuit thereof to a first winding of a transformer having at least a first winding and a second winding. A choke coil and a capacitor are connected to the second winding as a rectifying element, a commutating element and an output filter, and a MOSF is used as the rectifying element and the commutating element.
In a one-piece forward converter using an ET and turning on / off the MOSFET by a voltage generated in the transformer by turning on / off the main switch element, it is necessary to turn on the commutation element. A drive circuit and a control (discharge) circuit for turning off are separately provided, and the drive circuit applies an on signal to a commutation element via a diode from an auxiliary winding added to the choke coil. A means is provided for connecting to a connection point between the rectifying element and the secondary winding of the transformer via a control switch operating in synchronization with the main switching element to turn off the commutating element. .

[0012]

According to the present invention, the present invention utilizes the excitation energy of the choke coil as the driving energy of the gate of the commutation MOSFET, and forcibly discharges the gate charge using the control switch as the control (discharge) means. Short circuit current caused by the commutation MOSFET can be suppressed.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 shows a MOS of a forward converter according to the present invention.
FIG. 2 is a circuit diagram showing a first embodiment of the FET synchronous rectifier circuit. A series circuit in which the sources of the two MOSFETs 4 and 6 are common is connected in parallel with the secondary winding 2B of the main transformer so that the drains of the MOSFETs 4 and 6 are connected, and an output capacitor is connected between the drain and the source of the MOSFET 6 for commutation. 9 and a choke coil (6) An output filter circuit composed of 8 is connected. A drive circuit 11 for turning on the FET 6 and a control (discharge) circuit 13 for turning off the FET 6 are connected to the gate of the FET 6 for commutation. The drive circuit 11 connects the auxiliary winding 8B wound around the choke coil 8 to the diode 12 with the polarity shown. The control circuit 13 has a control switch 14 that operates in synchronization with the main switch element 3, and the other end is a commutation MOSFET
6 sources.

Next, the operation of the circuit of the first embodiment will be described with reference to FIG. When the main switch element 3 shifts from on to off, a resonance voltage is generated in the transformer 2 for a period of t'2. This period t′2 is smaller than the resonance voltage generation period t2 shown in FIG. 7 because the input capacitance Ciss of the commutation MOSFET 6 does not contribute. Since the commutation MOSFET 6 is turned on by the voltage generated in the auxiliary winding 8B of the choke coil 8 during the period (t'2 + t'3) when the main switch element is off, the electromagnetic energy accumulated in the choke coil is reduced by the FET6. Through the output capacitor 9. Next, when the main switch element 3 shifts from off to on, the control switch 14 is immediately turned on and the commutating MOSF is turned on.
The gate charge of ET6 can be rapidly discharged.

At this time, no current flows through the commutation MOSFET 4 or the diode 5 while the gate charge of the commutation MOSFET 6 remains, so that the commutation MOSFET 6
Then, the rectifying MOSFET 4 or the diode 5 does not turn on at the same time and a short-circuit current flows. Therefore, in the circuit of the embodiment shown in FIG. 1, even though the gate of the commutation MOSFET 6 is driven by the voltage of the auxiliary winding 2B of the choke coil 8, excessive loss that occurs when the commutation MOSFET 6 is turned off can be prevented. In addition, since the diode 12 prevents a negative voltage from being applied to the gate of the MOSFET, the commutation MOS (7) FET 6 can be driven at an optimum voltage.

FIG. 3 shows a second embodiment of the present invention in which the control circuit 13 of the embodiment of FIG. 1 is represented by a specific circuit. Reference numeral 15 denotes a control FET, and 16 denotes the control FET 15.
Is a diode for blocking a current flowing through a parasitic diode (not shown). Since the control FET 15 is connected so as to be turned on in synchronization with the on operation of the main switch element 3, it simultaneously discharges the gate charge of the commutation MOSFET 6 and turns it off. In this embodiment, the influence of the input capacitance Ciss of the commutation MOSFET 6 is eliminated during the reset period of the transformer, so that the period of generation of the resonance voltage is shortened. However, the input capacitance of the control FET 15 is replaced by the transformer secondary winding. Since they are connected in parallel, the generation period is slightly extended by that much.

FIG. 4 shows a third embodiment of the present invention.
Is a capacitor, and 18 is a resistor. By installing a series circuit of a capacitor 17 and a resistor 18 between the secondary windings 2B of the transformer and obtaining the gate signal of the control FET 15 from the middle point, the input capacitance of the control FET viewed from the transformer can be reduced. .

FIG. 5 shows a fourth embodiment of the present invention, which is a specific circuit example when a MOSFET is used as a main switch element. 20 is a MOSFET as a main switch element,
21 is a capacitor, 22 is a pulse transformer, 23 is MO
This is a drive signal source for the SFET. By supplying the gate signal of the control FET 15 from the drive signal source of the MOSFET via the pulse transformer 22 and the capacitor 21,
Since the resonance voltage generation period of the transformer 2 is not affected at all by the input capacitance of the commutation FET 6 and the control FET 15, the frequency can be higher than the conversion frequency. (8)

FIG. 9 is a circuit diagram showing another embodiment of the present invention. In the figure, ST is a saturable transformer, and a control MOSFET 15 is provided.
Is supplied from the current of the main switch element 3.

[0020]

As described above, the present invention is a synchronous rectification system in which the auxiliary winding is wound around the output choke coil and the commutation MOSFET is driven by the voltage generated in the auxiliary winding. The control (discharge) circuit is separately provided to avoid the short-circuit current on the secondary side of the transformer caused by the operation delay of the FET, and the conduction of the MOSFET over the entire commutation period, which is an advantage of the original choke coil driving method. Is possible, and a decrease in efficiency can be prevented. Furthermore, since the resonance voltage generation period required for resetting the transformer can be shortened, the conversion frequency can be increased,
This is an effective means for miniaturization.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a synchronous rectifier circuit according to the present invention.

FIG. 2 is a waveform diagram showing operation waveforms of the first embodiment of the synchronous rectifier circuit of the present invention.

FIG. 3 is a circuit diagram showing a second embodiment of the synchronous rectifier circuit of the present invention.

FIG. 4 is a circuit diagram showing a third embodiment of the synchronous rectifier circuit of the present invention.

FIG. 5 is a circuit diagram showing a fourth embodiment of the synchronous rectifier circuit of the present invention.

FIG. 6 is a circuit diagram showing a first example of a conventional synchronous rectifier circuit.

FIG. 7 is a waveform diagram showing operation waveforms in the conventional synchronous rectifier circuit.

FIG. 8 is a circuit diagram showing a second example of a conventional synchronous rectifier circuit.

FIG. 9 is a circuit showing another embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 input capacitor 2 transformer 3 main switch element 4 rectifying MOSFET 5 rectifying FET parasitic diode 6 commutating MOSFET 7 commutating FET parasitic diode 8 choke coil 9 output capacitor 10 drive circuit 12, 16 diode 13 control (discharge) circuit Reference Signs List 14 control switch 15 control FET 17, 21 capacitor 18 resistor 20 MOSFET 22 pulse transformer 23 drive signal source ST saturable transformer

Claims (6)

[Claims] 1. A main switch element is connected to a primary winding of a transformer, and a series circuit of a rectifying MOSFET and a commutating MOSFET common to a source is connected in parallel to a secondary winding of the transformer, and a rectified output is choked. In a synchronous rectifier circuit configured to be smoothed by a coil and an output capacitor, an auxiliary winding is wound around the choke coil, and a part of the auxiliary winding is connected to a gate of the commutation MOSFET via a diode. A synchronous rectifier circuit comprising: a control switch that operates in synchronization with the main switch element between a gate of a commutation MOSFET and a drain of the rectification MOSFET. 2. The synchronous rectifier circuit according to claim 1, wherein a MOSFET is used as the control switch. 3. The synchronous rectifier circuit according to claim 2, wherein a gate of the MOSFET is connected to one end of a secondary winding of the transformer. 4. A synchronous rectifier according to claim 2, wherein a series circuit of a capacitor and a resistor is connected in parallel between the secondary windings of the transformer, and a gate of a MOSFET is connected to a connection point of the capacitor and the resistor. circuit. 5. The synchronous rectifier circuit according to claim 2, wherein the gate signal of the MOSFET is supplied from a saturable transformer driven by the current of the main switch element. 6. A pulse transformer for driving a main switch element is provided, and a control switch is driven by a drive signal source of the pulse transformer.
Or 2 synchronous rectifier circuits.