Abstract: A semiconductor switch is provided with a main element having reverse conductivity and serving as a voltage-driven switching element having a high withstand voltage, an auxiliary element serving as a voltage-driven switching element having a withstand voltage lower than that of the main element, and a high-speed freewheel diode having a withstand voltage equal to that of the main element, wherein a negative pole of the main element is connected to a negative pole of the auxiliary element to define the positive pole of the main element as a positive pole terminal and the positive pole of the auxiliary element as a negative pole terminal, and the high-speed freewheel diode is parallel-connected between the positive pole terminal and the negative pole terminal so that a direction from the negative pole terminal toward the positive pole terminal constitutes a forward direction.

Abstract: An electric power conversion system including a direct current power supply, plural main circuit switching devices bridge-connected to convert the direct current to alternating current, including free wheeling diodes connected in reverse parallel thereto, respectively, and a reverse voltage application circuit applying a reverse voltage smaller than the direct current power supply to each free wheeling diode, upon cutoff of back-flow current of the free wheeling diode. The reverse voltage application circuit includes in series connection a low-voltage direct current power supply having a voltage lower than the direct current power supply, a reverse voltage application switching device having holes as a majority current and a lower withstand voltage than the main circuit switching devices and adapted to turn on upon reverse recovery of the free wheeling diode, and an auxiliary diode having a shorter reverse recovery time than the free wheeling diode.