Low-noise switching power supply

Abstract

A low-noise, high-power-factor power supply includes a transformer having a primary winding connected to a pair of a.c. input terminals via a rectifier circuit, and a secondary winding connected to a pair of d.c. output terminals via a rectifying and smoothing circuit. Connected between the pair of outputs of the rectifier circuit via at least part of the transformer primary, a switch is turned on and off to keep the d.c. output voltage constant. A smoothing capacitor is connected between the pair of outputs of the rectifier circuit via at least part of the transformer primary and a serial connection of a reverse blocking diode and an inductor. For noise reduction a bypass capacitor is connected between the pair of outputs of the rectifier circuit and in parallel with the serial circuit of the inductor and the reverse blocking diode and at least part of the transformer primary and the smoothing capacitor. The bypass capacitor is less in capacitance than the smoothing capacitor.