A synchronous rectifier applied to a secondary side of a power converter includes a forbidden turning-on time generation circuit, a green mode signal generation circuit, and a controller. The forbidden turning-on time generation circuit generates a forbidden turning-on time corresponding to the secondary side according to a synchronous signal of the secondary side. The green mode signal generation circuit enables a green mode signal when the forbidden turning-on time is greater than a reference value by at least one time, and disables the green mode signal when the forbidden turning-on time is continuously less than a reference value by a predetermined number of times. The reference value is changed with an output voltage of the secondary side. The controller enters a green mode when the green mode signal is enabled, and leaves the green mode when the green mode signal is disabled.