JP3818435B2 - Synchronous rectifier circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in a low-voltage output synchronous rectifier circuit, and more particularly to a synchronous rectifier circuit capable of improving efficiency.
[0002]
[Prior art]
FIG. 3 shows a first conventional example of a synchronous rectifier circuit.
[0003]
This is known as US Pat. No. 5,528,482, and a metal oxide semiconductor field effect transistor (MOSFET) Q1 is connected to the primary winding N1 of the transformer 1 and the secondary winding N2 of the transformer 1. Are connected in series to MOSFETs Q2 and Q3, diodes D1 and D2 are connected in parallel to MOSFETs Q2 and Q3, respectively, and a series circuit of a reactor L1 and a capacitor Co is connected in parallel to MOSFET Q2.
[0004]
In this circuit, when MOSFET Q1 is on, voltage V1 is applied to primary winding N1 of transformer 1 in the direction of the solid line arrow, and voltage V2 is applied to secondary winding N2 in the direction of the solid line arrow. . In addition, the current i _L that has flowed through the reactor L1 flows through the capacitor Co → the diode D2 → the transformer secondary winding N2. At this time, the secondary winding voltage V2 is applied between the gate and source of the MOSFET Q3, and the MOSFET Q3 is turned on. Since the diode D2 and the MOSFET Q3 are turned on between the gate and the source of the MOSFET Q2, the voltage becomes zero and the circuit is turned off.
[0005]
Next, when MOSFET Q1 is OFF, voltage V1 is applied to primary winding N1 of transformer 1 in the direction of the dotted arrow, and voltage V2 is applied to secondary winding N2 in the direction of the dotted arrow. In addition, the current i _L that has flowed through the reactor L1 flows through the capacitor Co → the diode D1. At this time, the secondary winding voltage V2 is applied between the gate and source of the MOSFET Q2, and the MOSFET Q2 is turned on. A transformer secondary voltage V2 is applied in the reverse bias direction between the gate and source of the MOSFET Q3, and the MOSFET Q3 is turned off.
[0006]
The diode has a PN junction threshold voltage and is a forward voltage of about 0.5 to 1 V. However, since the MOSFET is a unipolar device and has no threshold voltage and is an on-resistance element, the on-voltage is set to 0.1 V. Can also be reduced. For example, in a power supply of 3V output, the forward voltage of 1V is reduced by 25% at 1V / (3V + 1V), but if this can be reduced to 0.1V, 3% at 0.1V / (3V + 0.1V) The efficiency is reduced and the efficiency of the apparatus can be increased.
[0007]
FIG. 4 shows a second conventional example of a synchronous rectifier circuit.
[0008]
This is known as Japanese Patent Application Laid-Open No. 08-223906. The difference from FIG. 3 is that the transformer 1 is provided with a tertiary winding N3 and the gates of the MOSFETs Q2 and Q3 are driven by the tertiary winding. The MOSFET Q2 has a gate connected to a resistor R1 and a capacitor C1 filter circuit and a diode D3, and the MOSFET Q3 has a gate connected to a resistor R2 and a capacitor C2 filter circuit and a diode D4.
[0009]
In the circuit of FIG. 4, when MOSFET Q1 is on, voltage V1 is applied to primary winding N1 of transformer 1 in the direction of the solid line arrow, and voltage V3 is applied to tertiary winding N3 in the direction of the solid line arrow. Is done. In addition, the current i _L that has flowed through the reactor L1 flows through the capacitor Co → the diode D2 → the transformer secondary winding N2. At this time, the tertiary winding voltage V3 is applied between the gate and source of the MOSFET Q3, and the MOSFET Q3 is turned on. Since the diode D3 is turned on between the gate and the source of the MOSFET Q2, the voltage becomes zero and the device is turned off.
[0010]
Next, when MOSFET Q1 is off, voltage V1 is applied to primary winding N1 of transformer 1 in the direction of the dotted arrow, and voltage V3 is applied to tertiary winding N3 in the direction of the dotted arrow. In addition, the current i _L that has flowed through the reactor L1 flows through the capacitor Co → the diode D1. At this time, the tertiary winding voltage V3 is applied between the gate and source of the MOSFET Q2, and the MOSFET Q2 is turned on. The gate and the source of the MOSFET Q3 are turned off when the diode D4 is turned on.
[0011]
[Problems to be solved by the invention]
At present, the operating voltage of the CPU (central processing unit) has been lowered to 1.5V to 2.5V, and accordingly, the output voltage of the power source that supplies power to the CPU is also lowered to 1.5V to 2.5V. It has become. On the other hand, a voltage of 5 to 10 V is required for the gate voltage of the power MOSFET. On the other hand, the circuit shown in FIG. 3 has a problem that the transformer secondary voltage is as low as 2 to 5 V, so that the MOSFET cannot be driven sufficiently.
[0012]
On the other hand, the circuit shown in FIG. 4 requires a tertiary winding in the transformer, which increases the size of the transformer. In other words, the transformer used in these devices is generally a planar transformer using a printed board as the winding, so providing a tertiary winding leads to an increase in size of the transformer and is expensive. is there.
[0013]
Accordingly, an object of the present invention is to realize high efficiency in a small size and at a low cost.
[0014]
[Means for Solving the Problems]
In order to solve such a problem, in the invention of claim 1, a MOSFET as an on-resistance element is used as a rectifying element of a synchronous rectifier circuit, and a first diode and a first capacitor are connected in series between the drain and source of the MOSFET. A second series circuit in which a second diode and a second capacitor are connected in series between the drain of the MOSFET and the first capacitor is connected between the source of the MOSFET and the second capacitor. A series circuit of a third diode and a third capacitor is connected to each other, and the third capacitor is double-charged with a voltage applied to the MOSFET, so that the MOSFET can be driven with the voltage charged to the third capacitor. It is characterized by that.
[0015]
In the first aspect of the present invention, the voltage applied to the MOSFET can be boosted multiple times by using a plurality of the first and second series circuits as a set to obtain the driving power of the MOSFET. 2), or a series circuit of a P-type MOSFET and an N-type MOSFET and a series circuit of first and second resistors, respectively, in parallel with the third capacitor, and a series connection of first and second resistors. The points are connected to the gates of the P-type MOSFET and N-type MOSFET, and the gates of the P-type MOSFET and N-type MOSFET and the drain of the MOSFET are connected via a diode. Either type or N-type MOSFET can be connected and charged and discharged (invention of claim 3).
[0016]
That is, the voltage applied between the drain and source of the MOSFET Q4 is charged in the capacitor C3 of the first series circuit of the diode D6 and the capacitor C3. When the voltage between the drain and source of MOSFET Q4 becomes zero, the voltage charged in capacitor C3 is transferred to capacitor C4 by the second series circuit of diode D7 and capacitor C4. Next, when a voltage is applied between the drain and source of the MOSFET Q4, the capacitor C5 is charged with the voltage of the MOSFET Q4 and the voltage of the capacitor C4. By using this voltage as the driving power for the MOSFET Q4, the MOSFET can be sufficiently driven even in a rectifier circuit with a low voltage output.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a circuit diagram showing an embodiment of the present invention.
[0018]
As shown, a series circuit of a diode D6 and a capacitor C3 is provided in a parallel circuit of the MOSFET Q4 and the diode D5, and a series circuit of a diode D7 and a capacitor C4 is provided between the drains of the capacitor C3 and the MOSFET Q4. A series circuit of a diode D8 and a capacitor C5 is connected between the sources. A series circuit of MOSFETs Q5 and Q6 and a series circuit of resistors R3 and R4 are connected to the capacitor C5. Further, a connection point between the MOSFETs Q5 and Q6 is connected. Is configured such that the gate of the MOSFET Q4 is connected to each other, the gates of the MOSFETs Q5 and Q6 are connected to the connection point between the resistors R3 and R4, and the diode D9 is connected between the gates of the MOSFETs Q5 and Q6 and the drain of the MOSFET Q4. ing.
[0019]
In the circuit of FIG. 1, when the voltage V _Q4 is applied to the MOSFET Q4, the capacitor C3 is charged to the voltage V _Q4 via the diode D6. When the voltage V _{Q4 of the} MOSFET _Q4 becomes zero, the capacitor C4 is charged with the voltage of the capacitor C3 via the diode D7 and the MOSFET Q4. Next, when the voltage V _Q4 is applied to the MOSFET Q4, the capacitor C4 and the V _Q4 voltage of the MOSFET _Q4 are charged to the capacitor C5 via the diode D8. Therefore, the voltage of the capacitor C5 is a double voltage of the voltage _VQ4 .
[0020]
When the diode D5 is turned on, the diode D9 is turned on, and the gates of the MOSFETs Q5 and Q6 both have a negative potential. However , the MOSFET Q5 is turned on because it is a P-type MOSFET, and the MOSFET Q6 is turned off because it is an N-type MOSFET. As a result, the voltage of the capacitor C5 is applied to the gate of the MOSFET Q4 via the MOSFET Q5, and the Q4 is turned on.
[0021]
When a voltage is applied to the MOSFET Q4, the diode D9 is turned off, a voltage obtained by dividing the voltage of the capacitor C5 by the resistors R3 and R4 is applied between the source and gate of the MOSFET Q5, and the capacitor C5 is also connected between the gate and source of the MOSFET Q6. Is divided by resistors R3 and R4. Then, the MOSFET Q5 is turned off because it is a P-type MOSFET, and the MOSFET Q6 is turned on because it is an N-type MOSFET. As a result, the gate of MOSFET Q4 is discharged via MOSFET Q6, and Q4 is turned off.
[0022]
Thus, in the drive circuit of FIG. 1, MOSFET Q4 is turned on when diode D5 is turned on, and MOSFET Q4 is turned off when diode D5 is turned off, so that the MOSFET can be used in the same manner as a diode (rectifier element).
[0023]
FIG. 2 shows a configuration example when the drive circuits 2 and 3 as described above are applied to a forward converter.
[0024]
As shown, a MOSFET Q1 is connected in series to the primary winding of the transformer 1, a series circuit of MOSFETs Q2 and Q3 is connected to the secondary winding of the transformer 1, and a diode D1 and a driving circuit are connected in parallel to the MOSFETs Q2 and Q3, respectively. 2, a diode D2 and a drive circuit 3, and a series circuit of a reactor L1 and a capacitor Co are connected in parallel to the MOSFET Q2.
[0025]
Also in the circuit of FIG. 2, the MOSFET is turned on when the diode is turned on, and the MOSFET is turned off when the diode is turned off. Therefore, the MOSFET can be used in the same manner as the diode. Even when the output voltage is as low as 1.5 to 2.5 V and the transformer secondary voltage is as low as 2 to 5 V, the MOSFET drive voltage is boosted to a double voltage, so that it can be driven at a high voltage of 4 to 10 V. Is possible. Further, by providing a plurality of series circuits of the diode D6 and the capacitor C3 and a series circuit of the diode D7 and the capacitor C4 in FIG.
[0026]
【The invention's effect】
According to the present invention, even when the transformer secondary voltage is as low as 2 to 5 V, the MOSFET can be driven with a drive voltage of 4 to 10 V, which is the double voltage, so that the on-voltage can be sufficiently reduced and the generated loss can be reduced. There are advantages.
[0027]
Further, since it is not necessary to wind a tertiary winding around the transformer, it is possible to reduce the size of the apparatus. Further, if the drive circuit is integrated, it can be used as a two-terminal element like a diode, and the wiring between the circuits can be simplified, the number of assembly steps can be reduced, and the cost of the apparatus can be reduced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of the present invention.
FIG. 2 is a block diagram showing an application example of the present invention to a forward converter.
FIG. 3 is a circuit diagram showing a first conventional example.
FIG. 4 is a circuit diagram showing a second conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Transformer, 2, 3 ... Drive circuit, Q1-Q6 ... Metal oxide semiconductor type field effect transistor (MOSFET), D1-D9 ... Diode, L1 ... Reactor, R1-R4 ... Resistance, Co-C5 ... Capacitor.

Claims (3)

A MOSFET as an on-resistance element is used as a rectifier of the synchronous rectifier circuit, and a first series circuit in which a first diode and a first capacitor are connected in series between the drain and source of the MOSFET is connected to the drain of the MOSFET and the first A second series circuit in which a second diode and a second capacitor are connected in series is connected between the capacitors, and a series circuit of a third diode and a third capacitor is connected between the source of the MOSFET and the second capacitor, respectively. A synchronous rectifier circuit characterized in that a MOSFET can be driven by a voltage charged in a third capacitor by double-charging the third capacitor with a voltage applied to the MOSFET. 2. The synchronous rectifier circuit according to claim 1, wherein a plurality of sets of the first and second series circuits are used to boost a voltage applied to the MOSFET by a plurality of times to obtain driving power for the MOSFET. A series circuit of a P-type MOSFET and an N-type MOSFET and a series circuit of first and second resistors are connected in parallel with the third capacitor, respectively, and the series connection point of the first and second resistors is connected to the P-type MOSFET, N The gates of the P-type MOSFETs and the gates of the P-type MOSFETs and N-type MOSFETs and the drains of the MOSFETs are respectively connected via diodes. 2. The synchronous rectifier circuit according to claim 1, wherein the two are connected to charge and discharge.

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