Control circuit with burst mode and extended valley switching for quasi-resonant power converter

A control circuit for a power converter, comprising:

• a PWM circuit generating a switching signal coupled to switch a transformer of the power converter;
• a feedback input circuit coupled to an output of the power converter for generating a feedback signal, in which the feedback signal is coupled to turn off the switching signal;
• a detection circuit coupled to the transformer for generating a valley signal in response to a waveform of the transformer;
• a frequency-variation circuit coupled to receive the feedback signal and the valley signal for generating a frequency-variation signal, in which the frequency-variation signal is coupled to turn on the switching signal;
• and
• a burst circuit coupled to receive the feedback signal for generating a burst signal to disable the switching signal;
• wherein the feedback signal is correlated to an output load of the power converter.