Patent ID: 6137698
Filing Date: 2000-10-24
Classification: H02M

Abstract:
A switching power supply comprising:a transformer having a primary winding and a secondary winding which are magnetically coupled to each other;a primary transistor for supplying a switching current to said primary winding to induce a voltage across said secondary winding; anda secondary rectifying circuit for rectifying the voltage induced across said secondary winding to output a DC voltage from an output terminal;said secondary rectifying circuit comprising:a secondary transistor which comprises a MOS transistor; anda secondary control circuit for controlling a voltage at a gate terminal of said secondary transistor;said secondary control circuit being arranged to apply a voltage to the gate terminal of said secondary transistor in synchronism with a voltage induced across said secondary winding to operate said secondary transistor in a third quadrant for rectifying the voltage induced across said secondary winding; andsaid secondary control circuit comprising an ON timer for establishing predetermined ON period in which operating said secondary transistor in third quadrant.