Synchronous rectifier controller for offline power converter and method therefor

An offline power converter comprising:

• a transformer having a primary winding having a first end for receiving an input voltage, and a second end, and a secondary winding having a first end for providing an output voltage, and a second end;
• a power transistor having a drain coupled to said second end of said primary winding, a gate, and a source coupled to a primary ground terminal;
• a primary controller having an input for receiving a feedback signal, and an output coupled to said gate of said power transistor;
• a synchronous rectifier transistor having a drain coupled to said second end of said secondary winding, a gate, and a source coupled to a secondary ground terminal;
• and
• a synchronous rectifier controller having a power supply terminal coupled to said first end of said secondary winding for receiving a power supply voltage, a drain terminal coupled to said second end of said secondary winding, a ground terminal coupled to said secondary ground terminal, and a gate terminal coupled to said gate of said synchronous rectifier transistor, wherein said synchronous rectifier controller includes circuitry that discharges a voltage on said gate of said synchronous rectifier transistor after power-on that is powered by a voltage on said drain terminal and said secondary ground terminal but not by said power supply voltage.

A method for use in an offline power converter having a synchronous rectifier transistor with a drain coupled to a secondary winding of a transformer, comprising:

• detecting whether a voltage at the drain of the synchronous rectifier transistor exceeds a first voltage;
• and
• in response to detecting that said voltage at the drain of the synchronous rectifier transistor exceeds said first voltage, biasing a first transistor having a drain coupled to a gate of the synchronous rectifier transistor and a source coupled to ground to be conductive, wherein said biasing comprises:
• providing a voltage to a gate of said first transistor that is greater than a turn-on voltage of said first transistor and less than an overvoltage of said first transistor using a protection circuit powered only by a voltage on the drain terminal and ground.