Patent ID: 7209372

Claim:
A flyback converter with a synchronous rectifier, comprising: a flyback switching controller for switching a main switch in a critical conduction mode on the basis of a feedback voltage to convert an input voltage into a high-frequency pulse; a transformer having a primary coil for receiving the high-frequency pulse and a secondary coil; a synchronous switch including a MOSFET having a source and drain connected to any one of first and second output lines connected respectively to both ends of the secondary coil; a feedback controller for detecting an output voltage between the first output line and the second output line and outputting the detected voltage as the feedback voltage to the flyback switching controller; a current detector having primary and secondary coils for current detection, the current detector detecting load current flowing in any one of the first and second output lines according to a turn ratio between the primary and secondary current detection coils thereof; a current/voltage converter for converting the load current detected by the current detector into a voltage and outputting the converted voltage; a first switch connected to the first output line through a first resistor and to the second output line for performing an on/off switching operation in response to the output voltage from the current/voltage converter; a second switch connected to the first output line through a second resistor and to the second output line for performing an off/on switching operation in a reverse manner to the on/off switching operation of the first switch in response to the on/off switching operation of the first switch; a constant voltage source connected to the first output line through the second resistor and to the second output line for converting a voltage on the first output line into a predetermined constant voltage and supplying the converted constant voltage; a turn-on switch for switching a connection between the first output line and a gate of the synchronous switch in response to the constant voltage from the constant voltage source; and a turn-off switch for switching a connection between the gate and source of the synchronous switch in association with the switching operation of the second switch.