Patent ID: 7212416

Claim:
A switching power supply device comprising: a transformer with a primary and a secondary winding; a main switching element which switchcs a current flowing through said primary winding; a controller which controls an operation of said main switching element; a smoothing circuit; a rectifying switching element which connects and disconnects between said secondary winding and said smoothing circuit; and a rectifying-element drive circuit which drives said rectifying switching element, and includes a reactor of which one end is connected to one end of said secondary winding stores energy during an on period of said main switching element, and releases said stored energy during an off period of said main switching element, a current detection circuit which is connected between an other end of said reactor and an other end of said secondary winding, detects the current value of the current flowing through the reactor, and which generates a signal indicating the current value, and a drive circuit which receives said signal indicating said current value, and turns on said rectifying switching element in said off period of said main switching element when said current flowing through said reactor is greater than or equal to a predetermined value, and turns off said rectifying switching element when said current flowing through said reactor is less than said predetermined value.