Patent ID: 7719859

Claim:
A switching power supply device comprising: a first switching element adapted to be connected to a negative electrode of a DC power supply; a second switching element connected to the first switching element and adapted to be connected to a positive electrode of the DC power supply; a transformer including a primary winding, a secondary winding, and a tertiary winding; a series circuit, including a capacitor, an inductor, and the primary winding connected in series, connected in parallel with one of the first switching element or the second switching element, adapted to rectify and smooth a positive or negative voltage developed at the secondary winding by alternately turning on and off the first switching element and second switching element and provide a DC output; and a drive circuit adapted to turn on or off the second switching element according to a positive or negative voltage developed at the tertiary winding, wherein the drive circuit includes a control unit adapted to detect a development time, during which the negative voltage is developed at the tertiary winding, as the on time of the first switching element, and make the on time of the second switching element become substantially equal to the on time of the first switching element.