Patent ID: 7362601

Claim:
A power supply device comprising: first and second input ports for receiving alternating current having an input voltage; a transformer having a primary winding and a secondary winding, the primary winding having a pair of primary winding terminals, one of which is connected to the first port; and a switching circuit connected between another of the pair of primary winding terminals and the second port, the switching circuit comprising first, second, third, and forth switching elements and a capacitor, the first and second switching elements being connected to the other of the pair of primary winding terminals, respectively, the third switching element being connected between an other end of the first switching element and the second port, and the forth switching element being connected between an other end of the second switching element and the second port, the capacitor being connected between a connecting point of the first and third switching elements and another connecting point of the second and forth switching elements, wherein the capacitor is charged with energy obtained by the primary winding during a normal operation of the power supply device, the energy stored in the capacitor is supplied to the transformer through a current path provided between the first input port and the second input port to continue feed of electric power to the secondary winding when the input voltage decreases to or less than a predetermined value.