System and method for synchronous rectifier

摘要

A synchronous rectification circuit for a power supply includes a power switch for coupling to a secondary winding of a transformer of the power supply and an output capacitor of the power supply. The power switch includes a four-terminal MOSFET having a source, a drain, a gate, and a body. A body diode is formed by a junction between the body and the drain or by a junction between the body and the source. The body diode is coupled in parallel to the source and drain of the power transistor. A control circuit is configured to provide a control signal for controlling an on/off state of the power switch in response to a voltage condition of two terminals of the power switch. When the body diode makes a transition from reverse bias to forward bias, the control circuit causes the MOSFET to turn on, and when the forward bias voltage drops below a reference voltage, the control circuit causes the MOSFET to turn off. When the body diode makes a transition from forward bias to reverse bias, the control circuit causes the MOSFET to turn off.

主权项

1. A power converter having a synchronous rectifier, the power converter comprising:
a transformer with a primary winding for receiving a DC input voltage and a secondary winding for providing an output voltage through an output capacitor; a synchronous rectifier coupled to the secondary winding of the transformer and the output capacitor, the synchronous rectifier including:
a power switch for coupling to a secondary winding of a transformer of the power supply and an output capacitor of the power supply, wherein the power switch comprises a four-terminal MOSFET having a source, a drain, a gate, and a body, and wherein a body diode is formed by a junction between the body and the drain or by a junction between the body and the source, the body diode being coupled in parallel to the source and drain of the power transistor; and a control circuit configured to provide a control signal for controlling an on/off state of the power switch in response to a voltage condition of two terminals of the power switch, wherein:when the body diode makes a transition from reverse bias to forward bias, the control circuit causes the MOSFET to turn on, and when the forward bias voltage drops below a reference voltage, the control circuit causes the MOSFET to turn off; andwhen the body diode makes a transition from forward bias to reverse bias, the control circuit causes the MOSFET to turn off, wherein the control circuit comprises: first and second differential inputs, configured for coupling to a positive and a negative terminals of the power switch and sensing a voltage across that power switch; an output terminal, configured for outputting a signal to drive the power switch; a voltage comparing unit, configured to act in response to the first and second inputs, wherein the voltage comparing unit is configured to output a logic signal according to the voltage difference between the sensed voltage drop across the power switch and a reference threshold voltage; and
a logic processing circuit, coupled to the voltage comparing unit and configured to provide a first voltage signal and a second voltage signal of the voltage across the power switch;wherein the logic process unit comprises:an input terminal for receiving a input signal from the voltage comparing unit;a delay module coupled to the input terminal;an SR flip-flop coupled to the input terminal and the delay module;a NAND logic gate having a first input coupled to the input terminal and a second input coupled to an output of the SR flip-flop; anda drive circuit coupled to the output of the logic processing unit and the output terminal of the synchronous rectification circuit,wherein the delay module is configured to provide a delay time between the determination of the first voltage signal and the second voltage signal of the voltage across the power switch.