A switching power supply includes a control circuit disposed between a feedback
winding and a switching device. The control circuit includes an on-period control
circuit that, in an operation under a non-low load condition, controls an on-period
of the switching device such that the on-period decreases with decreasing load,
a minimum on-period setting circuit that disables the on-period control circuit
in an operation under a low load condition so that the on-period of the switching
device does not become shorter than a predetermined minimum on-period, and an off-period
control circuit that controls an off-period of the switching device in the operation
under the low load condition such that the off-period increases with decreasing
load. As a result, the output voltage is maintained at a constant value in accordance
with a feedback signal.
