A power supply includes a transformer winding, a switching circuit, a controller and a filter circuit. The transformer winding is configured to provide a first voltage. The switching circuit is coupled to the transformer winding and includes a first and a second switching unit. On the condition that the power supply is operated under a standby mode, the controller controls the first and the second switching units to provide a discharging path between two terminals of the transformer winding. On the condition that the power supply is operated under an operating mode, the controller controls the switching circuit such that the switching circuit provides a second voltage according to the first voltage. The filter circuit is coupled to the switching circuit and configured to filter the second voltage to provide an output voltage.