Power supply.

摘要

A power supply has a fullwave rectifier (22) providing a rectified pulsating DC voltage from an AC power source (21) and a chopper (30) having an inductor (31) and a switching transistor (32) which are connected in series across the rectifier (22). The transistor (32) is turned on and off for providing an interrupted voltage to be applied through a blocking diode (33) across a smoothing capacitor (34) to produce a smoothed DC voltage for driving a load (23). The inductor (31) stores energy from the rectifier (22) and release the energy in response to the transistor (32) being turned on and off respectively while a current flows through the inductor (31) in a positive direction. The inductor (31) cooperates with a parasitic capacitance inherently present in the chopper (30) to form an oscillation circuit which admits the current to flow through the inductor (31) in the negative direction. A controller (41) turns on the transistor (32) at a timing when the current flows in a negative direction and subsequently turns off the transistor (32) after a predetermined ON-period so as to release the current from the inductor (31), thereby causing the current to flow firstly in the negative direction and then in the positive direction through the inductor (31) during the ON-period. An input current wave shaping circuit (47, 50, 61, 62) is provided to establish a relation that, for each ON-period, an amount of current flowing continuously through the inductor (31) in the negative direction occurring immediately after the turn on of the transistor (32) is always smaller than that of the current subsequently flowing through the inductor (31) in the positive direction. <IMAGE>