Isolated power supply, control circuit and method thereof

An isolated power supply, comprising:

• an isolated converter comprising a primary side and a secondary side, wherein the primary side comprises a primary main transistor switch, and wherein the secondary side comprises a secondary synchronous rectifier;
• and
• a control circuit of an isolated power supply,
• wherein the control circuit comprises:
• a primary original signal generator configured to receive a voltage feedback signal of an output voltage of the isolated power supply to generate a primary original turn-on signal, wherein the primary original turn-on signal contains information about an expected turn-on instant for the primary main transistor switch in the isolated power supply;
• and
• a secondary control signal generator configured to receive a secondary voltage signal from a secondary winding in the isolated power supply and the primary original turn-on signal to obtain:
• information about an expected turn-on instant for the secondary synchronous rectifier in the isolated power supply;
• a freewheeling time of a body diode in the secondary synchronous rectifier;
• and a dead time, thereby generating a secondary control signal,
• wherein in a current switching period, if the expected turn-on instant for the primary main transistor switch is earlier than the expected turn-on instant for the secondary synchronous rectifier, the secondary control signal provides an indication to turn on the secondary synchronous rectifier;
• or if a value of the secondary voltage signal is higher than a turn-off threshold for the current switching period, the secondary control signal provides an indication to turn off the secondary synchronous rectifier,
• wherein the secondary control signal generator further adjusts the turn-off threshold for a next switching period based on the freewheeling time and the dead time of the current switching period, so that the freewheeling time gets closer to a freewheeling reference value and the dead time gets closer to a dead reference value.

A control method for an isolated power supply, comprising:

• receiving a voltage feedback signal of an output voltage of the isolated power supply to generate a primary original turn-on signal, wherein the primary original turn-on signal contains information about an expected turn-on instant for a primary main transistor switch in the isolated power supply;
• and
• receiving a secondary voltage signal from a secondary winding in the isolated power supply and the primary original turn-on signal to obtain:
• information about an expected turn-on instant for a secondary synchronous rectifier in the isolated power supply;
• a freewheeling time of a body diode in the secondary synchronous rectifier;
• and a dead time, thereby generating a secondary control signal,
• wherein in a current switching period, if the expected turn-on instant for the primary main transistor switch is earlier than the expected turn-on instant for the secondary synchronous rectifier, the secondary control signal provides an indication to turn on the secondary synchronous rectifier;
• or if a value of the secondary voltage signal is higher than a turn-off threshold for the current switching period, the secondary control signal provides an indication to turn off the secondary synchronous rectifier, and
• wherein the turn-off threshold for a next switching period is adjusted by a secondary control signal generator based on the freewheeling time and the dead time of the current switching period, so that the freewheeling time gets closer to a freewheeling reference value and the dead time gets closer to a dead reference value.