Patent ID: 7405952

Claim:
A power supply device comprising: a direct current (DC) generating unit for generating a direct current, the DC generating unit having a pair of output terminals; a first switching element; a transformer having a primary winding and a secondary winding, the primary winding being connected in series to the first switching element, the series-connected first switching element and primary winding being connected to the pair of output terminals of the DC generating unit; an auxiliary power supply unit for storing power and feeding power to the primary winding, the auxiliary power supply unit comprising a second switching element, and a capacitor connected in series with the second switching element, the series-connected second switching element and capacitor being connected in parallel to the first switching element; a detecting unit for detecting an effective voltage value of the direct current; a third switching element connected between the pair of output terminals; and a controller for switching the first and second switching elements complementarily to transfer power from the primary winding to the secondary winding and charge the capacitor, wherein the controller sends a first control signal to maintain the third switching element open when the effective voltage value is more than a predetermined value, and the controller sends a second control signal to close the third switching element to provide a current path for a current to flow through the primary winding when the effective voltage value decreases to or less than the predetermined value, thereby continuing energizing the transformer.