Patent ID: 6442048
Filing Date: 2002-08-27
Classification: H02M,Y02B

Abstract:
A flyback converter with synchronous rectifying function, comprisinga flyback converter having a transformer, the transformer having a primary winding with a negative pole connected to a thyristor, the transformer having a secondary winding having a first auxiliary winding with a positive pole connected to a drain of a MOS transistor, the MOS transistor having a source connected to ground, the first auxiliary winding having a negative pole connected to a positive pole of a primary winding of a current transformer, the current transformer having a secondary winding connected to a diode and a resistor, the secondary winding of the current transformer having a positive pole connected to ground, the secondary winding of the transformer having a second auxiliary winding with a positive pole connected to ground, when the thyristor is turned off, a voltage with a high level is induced at the secondary winding of the transformer and a current flows through the diode and the resistor; a controllable unit connected between a negative pole of the second auxiliary winding of the secondary winding of the transformer and a gate of the MOS transistor, the controllable unit having a control terminal and being turned on or off by a voltage at the control terminal thereof; a controller connected between the gate of the MOS transistor and ground, the controller having a control terminal connected to a negative pole of the second winding of the current transformer, the controller being turned on or off by a voltage at the control terminal thereof; a comparator having one input for providing a check voltage and connected to a junction of the diode and the resistor, another input for providing a reference voltage, and an output connected to the control terminal of the controllable unit; when the output load of the flyback converter is low and the check voltage is lower than the reference voltage, the comparator turns off the controllable unit to turn off the MOS transistor.