POWER UNIT AND METHOD FOR CONTROLLING ITS VOLTAGE

PROBLEM TO BE SOLVED: To provide a power unit which is enabled to reduce its input harmonic current and, at the same time, to supply stable electric power to a load with a simple constitution and a method for controlling the voltage of the power unit. SOLUTION: A power unit is provided with a first voltage source which takes in a pulsating current generated by rectifying the alternating current of a commercial power source, and a second voltage source which takes in a direct current obtained by switching the pulsating current by means of a transistor Q901 as an input and smoothing the current by means of a diode D904, a choke coil L901, and a capacitor C901. A voltage source upon which the pulsating current from the first voltage source and direct current from the second voltage source are superposed is impressed upon an insulating transformer T1 by switching the voltage by means of a transistor Q1. An output of the insulating transformer T1 is smoothed to generate A constant voltage central is performed by controlling the switching duty ratio or frequency of the driving pulse of the transistor Q1 based on the DC voltage detected by means of a voltage detecting circuit 102.