METHOD FOR OPERATING POWER CONVERTER, SEMICONDUCTOR CHIP AND POWER CONVERTER

A power converter includes a voltage transformer, a clamp capacitor, a main switch, a clamp switch, and a control circuit. The clamp capacitor is coupled to the primary winding of the voltage transformer, and the main switch is coupled in series between the primary winding and a ground terminal. The clamp switch is coupled in series between the clamp capacitor and the main switch, and the clamp switch, the main switch, and the primary winding are coupled to a common node. The control circuit turns on the main switch periodically, and turns on the clamp switch before turning on the main switch. After turning off clamp switch and before turning on the main switch, the control circuit determines a length of time that the clamp switch should be turned on next time according to a voltage of the common node sensed right before the main switch is turned on and a threshold voltage.