Switching power supply circuit with synchronous rectifier and associated control circuit and control method

A control method for a switching power supply circuit with a synchronous rectifier, wherein the switching power supply circuit receives an input voltage and provides an output voltage, the control method comprising:

• receiving the input voltage at a primary side of an energy storage component;
• periodically turning ON and OFF a synchronous rectifier switch to provide the output voltage, wherein the synchronous rectifier switch is coupled to a secondary side of the energy storage component;
• sampling a maximum value of a drain-source voltage across the synchronous rectifier switch, and providing a dynamic reference voltage based on the maximum value of the drain-source voltage across the synchronous rectifier switch;
• judging whether a primary switch coupled to the primary side is turned ON based on the drain-source voltage across the synchronous rectifier switch, the dynamic reference voltage, a preset window time period, and a working mode of the switching power supply circuit;
• and
• after the primary switch is judged as turned ON, turning ON the synchronous rectifier switch based on the drain-source voltage across the synchronous rectifier switch.

A synchronous rectifier control circuit for a switching power supply circuit, wherein the switching power supply circuit comprises an energy storage component having a primary side and a secondary side, a primary switch coupled to the primary side, a synchronous rectifier switch coupled to the secondary side, the synchronous rectifier control circuit comprising:

• a driving logic circuit, configured to provide a driving signal to control the synchronous rectifier switch, wherein after the primary switch is judged as turned ON, the synchronous rectifier switch is turned ON at a following falling edge of a drain-source voltage across the synchronous rectifier switch;
• wherein
• when the switching power supply circuit is not operating in a preset mode, the primary switch is judged as turned ON when the drain-source voltage across the synchronous rectifier switch remains larger than a dynamic reference voltage during a preset window time period;
• and
• when the switching power supply circuit is operating in the preset mode, the primary switch is judged as turned ON once the drain-source voltage across the synchronous rectifier switch is larger than the dynamic reference voltage.