A power transfer device includes an input terminal, an output terminal, a control unit, and a drive unit. The input terminal can receive an input voltage. The output terminal can provide an output voltage. The control unit can control a switch between the input and output terminals to adjust the output voltage according to the input voltage and a reference voltage, wherein said control unit is deactivated if the reference voltage reaches the input voltage. The drive unit can connect the control unit and the switch if the control unit is activated, and can maintain the output voltage at or near the input voltage if the control unit is deactivated.