Patent ID: 7345380

Claim:
A backup power supply system with a null transfer time, comprising: a first static switch connected between a first ac power source and a load; a second mechanical switch; and a dc/ac power inverter including a dc terminal connected to a dc power source at its upstream side for converting the dc power source into a second ac power source, and an ac terminal connected to the second switch so that the second switch is connected between the dc/ac power inverter and the load; wherein when the first power source is normal, the first switch and the second switch are closed, and the dc/ac power inverter is controlled to supply an approximately null current; and wherein when the first ac power source is abnormal or fails, the first switch is opened and the second switch is continuously closed so that the dc/ac power inverter rapidly supplies a sine wave voltage to the load; whereby the second switch does not require switching operation at the instant when the first ac power source is abnormal or fails.