PURPOSE: To enable the power supply stable against the input source fluctuation by making the input AC control rectifier element as the phase control subject while providing a battery at the prestage of the switching section.

CONSTITUTION: When throwing in the input power source, the phase angle of the AC control rectifier element 2 is set to produce the min. output by means of the pulse phase shifter 21, where the phase angle is increased as the time elapses and the battery 24 is maintained to the regular voltage. Consequently the excessive surge current is suppressed. The DC voltage rectified by the input rectifier 5 charges the battery 24 and further smoothed by the reactor 23 and the input capacitor 6 then converted at the switching section 7 and supplied as DC output from the output rectifier 8 and the output smoothing section 9 to the load. While during the power interruption, the power is supplied from the battery 24. The influence of the input source voltage fluctuation is eliminated by controlling the AC controlled rectifier element 2 by means of the pulse phase shifter 21.