Patent ID: 8199530

Claim:
A power supply unit including: a transformer, a full bridge circuit provided for the primary side of said transformer, a rectifier and smoothing circuit with two synchronous rectifier switches provided for the secondary side of said transformer, output terminals provided for said rectifier and smoothing circuit, and a control means to which output states are input from said output terminals and which performs ON/OFF controls of switches provided for said full bridge circuit and the two synchronous rectifier switches in said rectifier and smoothing circuit, wherein a resonant inductor constituted by a leakage inductor of said transformer and a parasitic inductor component of wirings, and a resonant capacitor constituted by a parasitic capacitance of top and bottom arm switches at the side necessary for a resonant operation for performing zero volt switching in said full bridge circuit are included, wherein the zero volt switching during a light load is performed in such a manner that through the ON/OFF control of the switches provided for said full bridge circuit and the two synchronous rectifier switches in said rectifier and smoothing circuit, a resonant peak voltage necessary for the zero voltage switching determined by the output current flowing to said output terminals, said resonant inductor and said resonant capacitor is ensured so that an energy accumulated in said rectifier and smoothing circuit is returned to said full bridge circuit so as to act as equivalent as when said output current is increased and to increase the current flowing through said full bridge circuit.