A power backup circuit provides a plurality of input power sources to back up a load. The power backup circuit includes a first switch, a second switch, and a control unit. The input power sources at least includes a first input power source and a second input power source. If the input power source of the load needs to be changed from the first input power source to the second input power source, the control unit controls the first switch to be coupled to the second input power source and controls the second switch to be coupled to the second input power source after the control unit effects a supply current flowing through a first power supply path and a second power supply path both coupled to the first input power source and the load to be reduced below a current threshold.