Synchronous rectification controller and control method thereof

A synchronous rectifier controller in use of a power supply with primary and second sides isolated from each other, wherein on the secondary side the power supply comprises a synchronous rectifier with a control terminal, a control signal at the control terminal turns ON the synchronous rectifier to provide a channel approximately short-circuited if the control signal exceeds a threshold voltage, the control signal substantially turns OFF the synchronous rectifier to turn the channel into an open circuit if the control signal is between the threshold voltage and a ground reference voltage, the synchronous rectifier controller comprising:

• a primary-side ON detector, comparing a detection signal at one end of the channel with a predetermined level, wherein the predetermined level is higher than the ground reference voltage, an excitation time is a period when the detection signal exceeds the predetermined level, and the primary-side ON detector provides a satisfaction signal if the excitation time lasts a preset time;
• and
• a precharge circuit precharging the control signal to a sub-threshold voltage between the threshold voltage and the ground reference voltage in response to the satisfaction signal.