Patent ID: 7907425

Claim:
A circuit for predicting a valley timing for a voltage across a switching device, comprising: a voltage detector for detecting the voltage across the switching device to produce a detection voltage as a function of the voltage across the switching device; a first comparator for comparing the detection voltage with a first threshold to produce a first comparison signal; a second comparator for comparing the detection voltage with a second threshold to produce a second comparison signal; a logic circuit for producing a first logic signal and a second logic signal according to the first and second comparison signals; a timer for predicting a valley timing for the detection voltage according to the first and second logic signals, to thereby determine a valley signal used to trigger turn on of the switching device; and, a blanking signal for selectively blanking the valley signal; wherein the logic circuit further provides a third logic signal and a fourth logic signal according to the first and second comparison signals; wherein the logic circuit further comprises: a first negative-edge triggering circuit for triggering a first setting signal in response to the first comparison signal; a second negative-edge triggering circuit for triggering a resetting signal in response to the second comparison signal; a positive-edge triggering circuit for triggering a second setting signal in response the second comparison signal; a first flip-flop for determining the first and second logic signals according to the first setting signal and the resetting signal; and a second flip-flop for determining the third and fourth logic signals according to the second setting signal and the resetting signal.