PURPOSE: To reduce the required capacity of a smoothing capacitor by connecting plural sets of DC/DC converters having the same output voltage and the same switching period in parallel on respective output sides and by setting the mutual phase difference in each switching operation at a value obtained by dividing said switching period by the number of converter parallel sets.

CONSTITUTION: DC/DC converters composed of (n) sets of switching elements SW1-SWn, transformers Tr1-Trn and diodes D1-Dn having the same output voltage and the same switching period are connected in parallel on the respective output sides to have an output smoothing capacitor for common use, and the mutual phase difference in the switching operation of each converter is set at a value obtained by division of said switching period by the number n of converter parallel sets, That is, the desirable required capacity of each converter is 1/n of the required output capacitance for a DC power supply. Thus, it is possible to decrease an AC component itself in the output current composite value of each converter and to reduce the capacity of said output smoothing capacitor.