Patent ID: 8503196

Claim:
A feedback circuit for an isolated power converter including a controller to switch a power switch to convert an input voltage to an output voltage, the feedback circuit comprising: an opto-coupler coupled to an output of the isolated power converter, operative to amplify a first current related to the output voltage to generate a second current; a current-to-voltage converter connected to the opto-coupler, operative to generate a first voltage according to the second current; a reversed polarity regulator connected to the opto-coupler, operative to decrease the first current in response to an increase in the output voltage at light load; a voltage source providing a second voltage; and a start up circuit coupled to the current-to-voltage converter and the voltage source, operative to select one of the first voltage and the second voltage as a feedback signal for the controller; wherein the start up circuit comprises: a first switch connected between the current-to-voltage converter and the controller; second switch connected between the voltage source and the controller; a first comparator connected to the current-to-voltage converter and the voltage source to compare the first voltage with the second voltage to generate a first comparison signal; a second comparator configured to compare a supply voltage to the isolated power converter with a reference voltage to generate a second comparison signal; and a flip-flop connected to the first comparator and the second comparator to switch the first switch and the second switch according to the first comparison signal and the second comparison signal.