Patent ID: 8742693

Claim:
A switching power supply circuit comprising: a full-wave rectification circuit that performs full-wave rectification of an AC input voltage so as to generate a primary voltage; a transformer that transforms the primary voltage into a secondary voltage utilizing electromagnetic induction between first and second isolated windings; a rectifying and smoothing circuit that generates a DC output voltage from the secondary voltage so as to supply the DC output voltage to a load; a primary current control circuit that performs on/off control of primary current based on a result of comparison between a primary current detection voltage corresponding to the primary current flowing in the first winding and a first reference voltage; and a reference voltage correction circuit for monitoring an on-duty ratio of secondary current flowing in the second winding so as to correct the first reference voltage, wherein the transformer includes a third winding besides the first winding and the second winding, and the reference voltage correction circuit includes a secondary current time detection circuit that monitors a winding voltage in the third winding so as to output a secondary current time detection signal corresponding to an on-duty ratio of the secondary current, a first multiplier circuit that multiplies the secondary current time detection signal by the first reference voltage so as to generate a multiplied voltage, an error amplifier that generates an error voltage corresponding to a difference between the multiplied voltage and a second reference voltage, a voltage dividing circuit that divides the primary voltage so as to generate a divided voltage, and a second multiplying circuit that multiplies the divided voltage by the error voltage so as to generate the first reference voltage.