The load 42 is a three-phase electrical motor driven by the AC power supplied from the power converter 41. The load 42 is not limited to specific use but is an electrical motor mounted on various types of electrical devices. Thus, the load 42 is used as an electrical motor for, for example, a hybrid car, an electrical car, a rail vehicle, an elevator, or air-conditioning equipment.

The power converter 41 will be described in detail hereinafter. The main conversion circuit 400 includes switching elements and free-wheeling diodes (not illustrated). Switching of the switching element causes the DC power supplied from the power supply 40 to be converted into the AC power. The AC power is then supplied to the load 42. The specific circuit configuration of the main conversion circuit 400 is of various types. The main conversion circuit 400 according to Embodiment 4 is a three-phase full-bridge circuit having two levels, and can include six switching elements and six free-wheeling diodes anti-parallel connected to the respective switching elements. Each of the switching elements of the main conversion circuit 400 may include the circuit board 1 according to Embodiment 1 or the circuit board 2 according to Embodiment 2. The semiconductor device 3 according to Embodiment 3 may be applied to each of the switching elements of the main conversion circuit 400. The six switching elements form three pairs of upper and lower arms in each pair of which the two switching elements are serially connected to each other. The three pairs of upper and lower arms form the respective phases (U-phase, V-phase, and W-phase) of the full-bridge circuit. Output terminals of the respective pairs of upper and lower arms, i.e., three output terminals of the main conversion circuit 400 are connected to the load 42.