To obtain a power supply in which the size is prevented from increasing due to the necessity of breakdown strength corresponding to the switching voltage.

Transformers 11-1N corresponding to semiconductor device groups 331-33n,..., 3N1-3Nn comprise input terminals A1, B1 receiving AC voltages, a plurality of terminals U1-Vn for generating output voltage to drive circuits 211-2Nn for the semiconductor devices 331-3Nn, and output terminals A2, B2 for generating voltages substantially identical to the AC voltages being fed to the input terminal. The transformers are connected in multistage cascade. The transformers 11-1N have output terminals being connected sequentially with the input terminal of next stage transformer and an AC voltage is outputted to the input terminal of next stage. Each transformer bares the potential substantially equal to the voltage being born by a corresponding semiconductor device group 331-33n,..., 3N1-3Nn thus lowering the breakdown strength.