Switching power supply unit

Abstract

A switching power supply unit comprising: a transformer having a primary winding and a secondary winding, a capacitor coupled to the transformer, a first switching circuit connected in series with a series circuit of the primary winding and an input power source, the first switching circuit comprising a parallel circuit comprising a first switching element, a first diode and a first capacitor, a second switching circuit connected in parallel with the primary winding, the second switching circuit comprising a parallel circuit comprising a second switching element, a second diode and a second capacitor, first and second switching controlling circuits connected to the first and second switching circuits, respectively for controlling the first and second switching elements so as to turn on/off alternately with a period in which the first and second switching elements turn off being interposed therebetween, a rectifying and smoothing circuit connected to the secondary winding; and voltage detection and voltage reduction circuit having a control signal input terminal and connected to the rectifying and smoothing circuit, the voltage detection and voltage reduction circuit detecting an output voltage of the rectifying and smoothing circuit and outputting a feedback signal to the first switching controlling circuit such that an output voltage of the rectifying and smoothing circuit is stabilized at one of a first voltage and a second voltage lower than the first voltage based on a signal applied to the control signal input terminal.

Dependencies are inferred from the claim text.