A converter includes a transformer, a main switch, an active clamp circuit, and a control circuit. The transformer includes a primary winding and a secondary winding, and is configured to receive an input voltage and output an output voltage to a load. The main switch is coupled between the primary winding and a primary ground terminal. The active clamp circuit includes an auxiliary switch and a clamp capacitor. The auxiliary switch is coupled to the clamp capacitor in series, and the active clamp circuit is coupled in parallel to the two terminals of the primary winding or the main switch, and is configured to clamp the voltage across the main switch when it is OFF. The control circuit outputs an auxiliary switch control signal to turn on the auxiliary switch when the voltage across the main switch is at its first peak of the resonant voltage.