Flyback converter and control method thereof

A power supply method for a flyback converter, comprising a first switching transistor, a second switching transistor, a transformer and a first capacitor, wherein a resonant loop circuit is formed by connecting the first capacitor a primary side winding of the transformer and the second switching transistor, wherein the power supply method comprises:

• providing a supply voltage by a power supply device, which is connected to a common node of the first switching transistor and the second switching transistor to provide the supply voltage;
• wherein the power supply device comprises a controllable switching transistor and a charging capacitor, in a first operating mode, the controllable switching transistor is turned on before the second switching transistor is turned off, and after the second switching transistor is turned off, the charging capacitor is charged and stores energy;
• in a second operating mode, before the first switching transistor is turned on, the controllable switching transistor is turned on, and the charging capacitor is charged and stores energy;
• the supply voltage is obtained according to a voltage across the charging capacitor, wherein the power supply method further comprises:
• adjusting an on time of the controllable switching transistor according to whether the first switching transistor is turned on by zero voltage switching;
• wherein in a case that the first switching transistor is turned on by zero voltage switching, the on time of the controllable switching transistor is decreased;
• in a case that the first switching transistor is turned on by non-zero voltage switching, the on time of the controllable switching transistor is increased, wherein the first operating mode is a boundary conduction mode, and the second operating mode is a discontinues conduction mode.