A power backup circuit (100, 100', 100") configured to provide a plurality of input power sources (Vin1, Vin2) to back up a load, wherein the input power sources (Vin1, Vin2) at least comprise a first input power source and a second input power source, the power backup circuit (100, 100', 100") comprising a first mechanical switch (12, 12') comprising a plurality of first ends and a second end; the first ends respectively coupled to first nodes of the input power sources (Vin1, Vin2), the second end coupled to one end of the load, and a first power supply path formed by coupling the first end to the second end, a second mechanical switch (14, 14') comprising a plurality of third ends and a fourth end; the third ends respectively coupled to second nodes of the input power sources (Vin1, Vin2), the fourth end coupled to the other end of the load, and a second power supply path formed by coupling the third end to the fourth end, and a control unit coupled to the first switch (12, 12') and the second switch (14, 14'), wherein if the input power source (Vin1, Vin2) of the load is determined to be changed from the first input power source to the second input power source, the control unit is configured to effect a supply current flowing through the first power supply path and the second power supply path to be reduced below a current threshold, and then the control unit is configured to control the second end of the first switch (12, 12') to be coupled to the first end corresponding to the second input power source by a first control signal for a first switching time period, and to control the fourth end of the second switch (14, 14') to be coupled to the third end corresponding to the second input power source by a second control signal for a second switching time period, characterized in that the first switching time period is defined between a time point that the first end corresponding to the first input power source is disconnected from the second end and a time point that the first end corresponding to the second input power source is connected to the second end, and the second switching time period is defined between a time point that the third end corresponding to the first input power source is disconnected from the fourth end and a time point that the third end corresponding to the second input power source is connected to the fourth end; the first switching time period and the second switching time period are approximately equal to a mechanical delay time of switching the first switch (12, 12') and the second switch (14, 14'). a first mechanical switch (12, 12') comprising a plurality of first ends and a second end; the first ends respectively coupled to first nodes of the input power sources (Vin1, Vin2), the second end coupled to one end of the load, and a first power supply path formed by coupling the first end to the second end, a second mechanical switch (14, 14') comprising a plurality of third ends and a fourth end; the third ends respectively coupled to second nodes of the input power sources (Vin1, Vin2), the fourth end coupled to the other end of the load, and a second power supply path formed by coupling the third end to the fourth end, and a control unit coupled to the first switch (12, 12') and the second switch (14, 14'), wherein if the input power source (Vin1, Vin2) of the load is determined to be changed from the first input power source to the second input power source, the control unit is configured to effect a supply current flowing through the first power supply path and the second power supply path to be reduced below a current threshold, and then the control unit is configured to control the second end of the first switch (12, 12') to be coupled to the first end corresponding to the second input power source by a first control signal for a first switching time period, and to control the fourth end of the second switch (14, 14') to be coupled to the third end corresponding to the second input power source by a second control signal for a second switching time period, characterized in that the first switching time period is defined between a time point that the first end corresponding to the first input power source is disconnected from the second end and a time point that the first end corresponding to the second input power source is connected to the second end, and the second switching time period is defined between a time point that the third end corresponding to the first input power source is disconnected from the fourth end and a time point that the third end corresponding to the second input power source is connected to the fourth end; the first switching time period and the second switching time period are approximately equal to a mechanical delay time of switching the first switch (12, 12') and the second switch (14, 14').