Patent ID: 7570498

Claim:
A switching power supply apparatus comprising: a transformer having at least a primary winding connected to a direct current power supply, and a secondary winding; a switching device, connected serially to the primary winding of the transformer, for supplying an electric current from the direct current power supply to the primary winding of the transformer; a current measurement circuit for measuring an electric current flowing through the switching device; a rectifying and smoothing circuit connected to the secondary winding of the transformer; and a switching device control circuit for performing overcurrent protection by controlling switching-on/off of the switching device so as to make a voltage output from the rectifying and smoothing circuit have a specific value, and switching off the switching device when the electric current flowing through the switching device exceeds a value as an overcurrent detection point, wherein the switching device control circuit includes: a current variation measurement circuit for measuring a gradient of the electric current flowing through the switching device, based on a current measurement signal output from the current measurement circuit; and an overcurrent detection-point correction circuit for correcting the overcurrent detection point with respect to the electric current flowing through the switching device, based on a signal output from the current variation measurement circuit.