A primary side is provided with an oscillation circuit arranged to turn on a power switch at a constant cycle. The secondary side is provided with an on period control circuit arranged to output an off signal for turning the power switch off by detecting output voltage and comparing with a reference triangle wave signal. An isolated signal transfer circuit is provided between the primary side and secondary side to transfer an on signal. The primary side is provided with a power switch off circuit arranged to turn off the power switch based on the on signal.