A power supply has a normal operation mode and a standby operation mode. The power supply includes a main circuit, a first control circuit, a second control circuit, and a switching controller. The main circuit has at least one output port for converting an input voltage into an output voltage at the output port. The first control circuit is used for controlling the main circuit under the normal operation mode. The second control circuit is used for controlling the main circuit under the standby operation mode. And, the switching controller processes a control signal to control the first control circuit and the second control circuit under either of the normal operation mode and the standby operation mode in response to a load status of the output port.