Switching power supply circuit with synchronous rectification and associated control method

A control method for a switching power supply circuit with synchronous rectification, wherein the switching power supply circuit comprises an energy storage component, a primary switch coupled to a primary side of the energy storage component, and a rectifier switch coupled to a secondary side of the energy storage component, and the switching power supply circuit is configured to provide an output voltage at the secondary side of the energy storage component, the control method comprising:

• providing a driving signal according to a drain-source voltage across the rectifier switch to control the rectifier switch;
• turning on the rectifier switch when the drain-source voltage across the rectifier switch is less than a first threshold value;
• controlling the driving signal to be a maximum voltage for a predetermined duration after the rectifier switch is turned on;
• adjusting the driving signal according to the drain-source voltage across the rectifier switch and a second threshold value after an on-time period of the rectifier switch reaches the predetermined duration;
• and
• turning off the rectifier switch when the drain-source voltage across the rectifier switch is greater than a turn-off threshold value, wherein the turn-off threshold value is greater than the second threshold value.

A switching power supply circuit with synchronous rectification, comprising:

• an energy storage component, comprising a primary side coupled to a primary switch and a secondary side configured to provide an output voltage;
• a rectifier switch, coupled to the secondary side of the energy storage component;
• a secondary side control circuit, configured to provide a driving signal to control the rectifier switch according to a first threshold value, a second threshold value, a turn-off threshold and a drain-source voltage across the rectifier switch;
• wherein
• when the drain-source voltage across the rectifier switch is less than the first threshold value, the secondary side control circuit is configured to control the rectifier switch being fully on for a predetermined duration;
• after the predetermined duration expired, the secondary side control circuit is configured to control the driving signal according to the second threshold value and the drain-source voltage across the rectifier switch, wherein the second threshold value is less than the first threshold value;
• and
• when the drain-source voltage across the rectifier switch is greater than the turn-off threshold, the secondary side control circuit is configured to control the rectifier switch to be turned off.