Patent ID: 7426121

Claim:
An output voltage detection circuit incorporated into an isolated switching power supply comprising: a transformer having a primary side winding, a secondary side winding, and an auxiliary winding, a switching element connected to the primary side winding, and a control circuit for controlling on/off of the switching element, the output voltage detection circuit generating detection voltage as a detected value of secondary side output voltage based on voltage generated on the auxiliary winding, and feeding back the detection voltage to the control circuit, the output voltage detection circuit, comprising: a secondary side current detection circuit connected to the auxiliary winding and detecting a period during which current passes through the secondary side winding, a voltage waveform processing circuit connected to the auxiliary winding and processing a waveform of the voltage generated on the auxiliary winding, a hold circuit connected to an output stage of the voltage waveform processing circuit and holding a peak voltage of the voltage having been processed by the voltage waveform processing circuit, a correction circuit connected to an output stage of the hold circuit, and correcting the peak voltage having been held by the hold circuit and generating the detection voltage in the period having been detected by the secondary side current detection circuit, and a reset circuit for resetting, for a fixed period of time after the current starts passing through the secondary side winding, the detection voltage having been generated by the correction circuit.