BACKUP POWER SUPPLY DEVICE AND CONTROL METHOD THEREFOR

A backup power supply device () includes a first switch (), a second switch (), a third switch (), a fourth switch (), an auxiliary power supply (), and a control circuit (), wherein the first switch () includes a first diode () provided in a direction in which a current supplied from the main power supply () flows to the second switch () when the first switch () is off, and when a power supply that supplies a DC voltage to the load () is switched from the main power supply () to the auxiliary power supply (), the control circuit () executes a first step (S) of turning off the first switch, a second step (S) of turning on the third switch () and the fourth switch () after the first step (S); and a third step (S) of turning off the second switch () after the second step (S).