Synchronous rectifier control circuit and method

A synchronous rectifier (SR) controller comprising:

• an output terminal configured to be coupled to a control terminal of an SR transistor of a flyback converter;
• and
• an input terminal configured to receive an output voltage of the flyback converter, wherein the SR controller is configured to:
• determine a first voltage across conduction terminals of the SR transistor;
• assert a turn-on signal when a body diode of the SR transistor is conducting current;
• assert a turn-off signal when current flowing through the conduction terminals of the SR transistor decreases below a first threshold;
• generate a gating signal based on the output voltage of the flyback converter and on the first voltage;
• turn on the SR transistor based on the turn-on signal and on the gating signal;
• and
• turn off the SR transistor based on the turn-off signal.

A flyback converter comprising:

• a transformer having first and second windings;
• an output terminal coupled to the second winding;
• a first primary transistor coupled to the first winding;
• a primary controller having an output coupled to a control terminal of the first primary transistor;
• a synchronous rectifier (SR) transistor coupled to the second winding;
• and
• an SR controller configured to:
• determine a first voltage across conduction terminals of the SR transistor,
• assert a turn-on signal when a body diode of the SR transistor is conducting current,
• assert a turn-off signal when current flowing through the conduction terminals of the SR transistor decreases below a first threshold,
• generate a gating signal based on an output voltage at the output terminal and on the first voltage;
• turn on the SR transistor based on the turn-on signal and on the gating signal;
• and
• turn off the SR transistor based on the turn-off signal.