A converter includes a transformer, a primary side switch, a load detection circuit, a state detection circuit and a control circuit. The transformer is configured to output a voltage to a load. The primary side switch is coupled to a primary winding and a primary ground terminal. The load detection circuit is configured to detect a load state of the load and output a load state signal. The state detection circuit is configured to detect a reference time point. The control circuit is configured to output a control signal to turn on or off the primary side switch. The control circuit further sets a blanking time according to the load state signal, such that the primary side switch is turned on when a drain-source voltage of the primary side switch is at a valley of the resonance after the blanking time starting from the reference time point.