A trigger circuit includes an off-time controller configured to receive a sensing voltage by sensing a driving current and to compare the sensing voltage to first and second certain voltages that are close to a zero voltage value and symmetrical to the zero voltage value to control a turn-off time of a driving switch in order for the sensing voltage to correspond to the zero voltage value at the turn-on time point of the driving switch and a switching controller configured to provide a switching control signal for turning on the driving switch at the turn-on time point of the driving switch.