PURPOSE:To eliminate a special overcurrent protecting circuit and an irregular magnetization preventing circuit by selecting the capacity of a coupling capacitor to limit the primary current of an output transformer to the degree not exceeding the current capacity of a switch element at load shortcircuiting time. CONSTITUTION:An inverter 19 is connected with an input side rectifier 2. The inverter 19 has a pair of switch elements 15, 16 provided between the rectifier 2 and an output transformer 20, and coupling capacitors 17, 18. A smoothing condenser 3 is provided between the inverter 19 and the rectifier 2. The capacities of the capacitors 17, 18 are selected to limit the primary current of an output transformer in the degree of sufficiently outputting to a rated load but not exceeding the current capacity of the elements 15, 16, at load shortcircuiting time.
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