An output voltage detection circuit is provided for accurately detecting a
secondary side output voltage based on a voltage induced to an auxiliary
winding in an isolated switching power supply including a transformer
having the auxiliary winding. A voltage waveform processing circuit
(10-1) processes the waveform of the voltage induced to the auxiliary
winding (7-3), a hold circuit (10-2) holds the voltage peak value after
the processing, a secondary side current detection circuit (24) detects a
period during which current passes through a secondary side winding
(7-2), a correction circuit (22) performs correction to decrease the peak
voltage which has been held by the hold circuit (10-2) during the period,
and the corrected voltage is fed back to a control circuit (9).
Meanwhile, a reset circuit (23) resets, for a fixed period after the
current passes through the secondary side winding (7-2), the peak voltage
having been held by the hold circuit (10-2).