Patent ID: 7426120

Claim:
A switching control circuit having a valley voltage detector for a power converter, comprising: a control circuit coupled to a feedback signal of the power converter to generate a switching signal for controlling a switching device responsive to a valley voltage generated across the same for regulating an output of the power converter; an output circuit receiving the switching signal and generating an output signal responsive thereto to turn said switching device ON, said switching device switching a transformer of the power converter; an input terminal coupled to an auxiliary winding of the transformer through a resistor, in which a minimum voltage on the input terminal is regulated; a first resistor; a current mirror coupled to the input terminal for generating a voltage signal at the first resistor in response to a current flowed through the resistor while said switching device is OFF; the valley voltage detector including: a waveform detector, coupled to the auxiliary winding through said input terminal to detect a variation of a voltage of said auxiliary winding and to produce a beginning signal in response to a valley voltage generated across said switching device and detected by said waveform detector while said switching device is OFF; and a phase lock circuit coupled to the first resistor to generate a control signal to enable the switching signal in response to the peak value of the voltage signal and the beginning signal; wherein the switching device is switched on in response to the valley voltage across the switching device.