Switching power supply capable of ac to dc conversion

Abstract

A switching power supply capable of translating a.c. voltage of sinusoidal waveform into d.c. voltage, having a rectifier circuit connected to a pair of a.c. input terminals for rectifying the incoming commercial a.c. voltage. A first main switch is connected between the pair of outputs of the rectifier circuit via a main inductor. A smoothing capacitor is connected in parallel with the first main switch via a rectifying diode. A second main switch is connected in parallel with the smoothing capacitor via the primary winding of a transformer. The first and the second main switch are each provided with a soft-switching capacitor connected in parallel therewith. A first and a second ancillary inductor, electromagnetically coupled respectively to the main inductor and to the transformer primary, are connected in parallel with the first and the second soft-switching capacitor via a first and a second ancillary switch, respectively. A switch control circuit is connected to the main switches for on-off control thereof at a repetition frequency higher than the frequency of the input a.c. voltage, and to the ancillary switches for on-off control thereof at such a repetition frequency, and with such conducting periods, as to assure soft turn-on of the main switches.