A power supply method includes the following steps: detecting whether a battery is installed in a system; delivering a control signal to a power conversion circuit by a controller when it is determined that the battery is not installed in the system; turning on part of a plurality of transistors and turning off the other part of the plurality of transistors by the power conversion circuit according to the control signal and a reference signal, so that an input voltage is delivered to a receiving terminal of the system from a sending terminal of the system through a current path.