Active clamping flyback circuit and control method thereof

An active clamping flyback circuit, comprising a flyback circuit, a clamping circuit and a clamping control circuit, wherein said flyback circuit comprises a transformer, a main transistor, and a freewheeling diode or a synchronous rectifier, an output feedback circuit is coupling to an auxiliary winding of said transformer and outputs a feedback voltage through a divided voltage;

• said clamping circuit comprises a first capacitor and a first transistor coupling in series, one end of said first capacitor is coupling to a high voltage end of an input voltage, one end of said first transistor is coupling to a common end of said main transistor and a primary winding of said transformer;
• when said active clamping flyback circuit operates in a discontinuous conduction mode, said clamping control circuit starts timing when said first transistor is off, and ends timing until said feedback voltage is reduced from positive voltage to zero voltage, wherein time elapsed for said timing is a first time, and said clamping control circuit adjusts a turn-off time of next switching cycle of said first transistor so that said first time of said next switching cycle is configured to be substantially the same as a first threshold,
• wherein said first threshold is a quarter of an oscillation period of an oscillation, the oscillation is generated based on a sum inductance of a magnetizing inductance and a leakage inductance of said transformer, and a sum capacitance of a drain-source parasitic capacitance of said first transistor and a drain-source parasitic capacitance of said main transistor.