A converter includes a transformer, an active clamp circuit, a primary side switch, a secondary side switch, a load detection circuit, a state detection circuit and a control circuit. The active clamp circuit is electrically coupled to the primary winding of the transformer, and includes a clamp switch. The primary side switch is electrically coupled to the primary winding and a primary ground terminal. The secondary side switch is electrically coupled to a secondary winding of the transformer and a load. The control circuit outputs a control signal to turn on or turn off the clamp switch. The control circuit sets a blanking time according to the load state signal, such that the clamp switch is turned on when a drain-source voltage of the primary side switch is at a peak value of a resonance after the blanking time starting from the reference time point.