Circuit for generating a dynamic drain-source voltage threshold and method thereof

A synchronous rectifier control circuit, used with a synchronous rectification circuit having a primary switch and a synchronous rectifier, comprising:

• a drain-source threshold setting circuit, configured to receive a drain-source voltage across the synchronous rectifier, and to provide a dynamic drain-source voltage threshold based on the drain-source voltage;
• a primary switch detecting circuit, configured to receive the drain-source voltage and the dynamic drain-source voltage threshold, and to provide a primary switch state indicating signal based on a comparison result of the drain-source voltage and the dynamic drain-source voltage threshold;
• and
• an on-control circuit, configured to receive the primary switch state indicating signal, the drain-source voltage and a turn-on threshold, on the premise that the primary switch state indicating signal indicates an on state of the primary switch, the on-control circuit provides a synchronous on signal to turn on the synchronous rectifier when the drain-source voltage decreases to the turn-on threshold.

A control method for controlling a synchronous rectifier of a synchronous rectification circuit, wherein the synchronous rectification circuit having a primary switch, the control method comprising:

• generating a dynamic drain-source voltage threshold based on the drain-source voltage;
• determining if the primary switch is in an on state by comparing the drain-source voltage with the dynamic drain-source voltage threshold;
• detecting if the drain-source voltage is lower than a turn-on threshold;
• and
• after the drain-source voltage reaches the dynamic drain-source voltage threshold, turning on the synchronous rectifier when the drain-source voltage is lower than the turn-on threshold.