NOVELTY - The chopper circuit has an input unit that inputs a main turn-on signal for turning on a main switching element and an auxiliary turn-on signal for turning on an auxiliary switching element; and a prohibiting unit that prohibits the main switching element from turning on unless the auxiliary turn-on signal is input. The prohibiting unit includes a delay circuit that prohibits the main switching element from turning on unless a predetermined period of time has elapsed after the auxiliary turn-on signal is input to the auxiliary switching element.    USE - Chopper circuit for soft switching converters. Uses include but are not limited to a DC/DC converter (claimed) for fuel cell system (claimed) for electric vehicles or hybrid vehicles, such as fuel cell hybrid vehicle, an alternating current (AC)/DC converter, a power factor correction (PFC) circuit, an uninterruptible power supply (UPS), a power conditioner and a frequency converter.    ADVANTAGE - Breakdown of the main switching element is avoided since the prohibiting unit prohibits the main switching element from turning on when the auxiliary turn-on signal is not normally input to the auxiliary switching element because of abnormality, thus main switching element is not subjected to hard switching operation. Chopper circuit includes a delay circuit, thus, time interval is corrected to a normal time interval circuit even when the auxiliary turn-on signal and the main turn-on signal are input at a short time interval between the auxiliary turn-on signal and the main turn-on signal. Switching of the main switching element is controlled with faster determination because the prohibiting unit is formed of a logic circuit.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a DC/DC converter; and    (2) af fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic diagram of a switch control unit.    Control unit (20)    Switch control unit (50)    Soft switching converter (60)    Gates (S1,S2)    Wire harness (WH)