Power supply having a chopper with an improved power factor

Abstract

A power supply with a full-wave rectifier and a chopper having an inductor and a transistor connected in series across the rectifier. The transistor is switched on and off providing an interrupted voltage which is applied across a smoothing capacitor, via a blocking diode, to produce a smoothed DC voltage for driving a load. When the transistor is turned on, current from the rectifier is drawn through the inductor. When the transistor is turned off, the current through the inductor is conducted, in a positive direction, to the smoothing capacitor. The inductor and parasitic capacitance inherently present in the chopper create oscillations which allow current to flow through the inductor in a negative direction. A controller turns the transistor on at a time when the current flows in the negative direction and turns the transistor off after a predetermined on-period so as to release the current through the inductor, thereby causing the current to first flow in the negative direction and then in the positive direction through the inductor during the on-period. An input current wave shaping circuit is provided so that for each on-period, the amount of current flowing continuously through the inductor in the negative direction occurring immediately after the turning on of the transistor is less than the amount of current flowing through the inductor in the positive direction.