Zero-voltage-switching flyback converter

A control circuit for a flyback circuit, wherein the flyback circuit has a transformer having a primary winding and an auxiliary winding, and a primary switch coupled to the primary winding, the primary winding and the auxiliary winding having opposite dot orientations with each other, the control circuit comprising:

• an auxiliary switch, coupled to the auxiliary winding;
• a first control unit, configured to provide a first control signal to control turning-on of the auxiliary switch, wherein, the auxiliary switch is turned on when, after an elapse of a predetermined waiting time counted from a predetermined moment in an immediate switching cycle, an auxiliary switch voltage signal decreases to a valley of the auxiliary switch voltage signal;
• a second control unit, configured to provide a second control signal to control turning-off of the auxiliary switch, wherein the auxiliary switch is turned off when the auxiliary switch current signal increases to a current level of a reference current;
• a third control unit, configured to provide a third control signal to control turning-on of the primary switch, wherein the primary switch is turned on when a primary switch voltage signal decreases to a valley of the primary switch voltage signal;
• and
• a logic circuit, configured to provide a primary switching signal based on the third control signal and a fourth control signal and an auxiliary switching signal based on the first control signal and the second control signal, wherein the primary switch is controlled by the primary switching signal, and the auxiliary switch is controlled by the auxiliary switching signal.

An isolation voltage converter, comprising:

• a flyback circuit, comprising a transformer having a primary winding and an auxiliary winding, and a primary switch coupled to the primary winding, wherein the primary winding and the auxiliary winding have opposite dot orientations with each other;
• and
• a control circuit, comprising:
• an auxiliary switch, coupled to the auxiliary winding;
• a first control unit, configured to provide a first control signal to control turning-on of the auxiliary switch, wherein, the auxiliary switch is turned on when, after elapse of a predetermined waiting time counted from a predetermined moment in an immediate preceding switching cycle, an auxiliary switch voltage signal decreases to a valley of the auxiliary switch voltage signal;
• a second control unit, configured to provide a second control signal to control turning-off of the auxiliary switch, wherein the auxiliary switch is turned off when the auxiliary switch current signal increases to a current level of a reference current;
• a third control unit, configured to provide a third control signal to control turning-on of the primary switch, wherein the primary switch is turned on when a primary switch voltage signal decreases to a valley of the primary switch voltage signal;
• and
• a logic circuit, configured to provide a primary switching signal based on the third control signal and a fourth control signal and an auxiliary switching signal based on the first control signal and the second control signal, wherein the primary switch is controlled by the primary switching signal, and the auxiliary switch is controlled by the auxiliary switching signal.