A flyback converter includes a transformer, a sensing impedance, a switch and a control circuit. The sensing impedance is coupled between the transformer and an output terminal of the flyback converter. The switch is coupled to the transformer. The transformer is charged when the switch activates. The transformer is discharged when the switch deactivates. The control circuit is arranged to detect if the sensing impedance is bypassed, and further arranged to adjust an operating frequency of the switch when the sensing impedance is bypassed.