Patent ID: 6922345

Claim:
A switching power supply comprising: a transformer including a primary winding, a secondary winding and a feedback winding; a switching device connected in series to the primary winding and having a control terminal; a control circuit connected between the feedback winding and the control terminal of the switching device; a rectifying and smoothing circuit connected to the secondary winding, for rectifying and smoothing a voltage generated in the secondary winding and outputting a resultant rectified and smoothed voltage; and an output voltage detecting circuit for detecting the voltage output from the rectifying and smoothing circuit and outputting a feedback signal to the control circuit; wherein in an operation under a non-low load condition, the control circuit controls the output voltage at a constant value in accordance with the feedback signal from the output voltage detecting circuit by controlling the on-period such that the on-period decreases with decreasing load within a range greater than a predetermined minimum on-period, and in an operation under a low load condition, the control circuit controls the output voltage at a constant value in accordance with the feedback signal from the output voltage detecting circuit by fixing the on-period to the minimum on-period and controlling the off-period such that the off-period increases with decreasing load.