Line Interactive UPS

The Line Interactive UPS is the most common UPS used for small business, Web, and departmental servers. In this design, the battery-to-ac power converter (inverter) is always connected to the output of the UPS. Battery charging is provided by operating the inverter in reverse during times when the input AC power is normal. When the input power fails the transfer switch opens and the power flow is from battery to the UPS output. The fact that the inverter is always connected to the output provides additional filtering and yields reduced switching transients when compared with the standby type UPS. The inverter also provides regulation, operating to correct brownout conditions, which would otherwise force the UPS to switch to battery operation. This allows the UPS to operate at sites with very poor power. The inverter can be designed such that its failure will still permit power flow from the AC input to the output, which eliminates the potential of single point failure and effectively provides for two independent power paths. This topology is inherently very efficient, which leads to high reliability while at the same time providing superior power protection. High efficiency, low cost, high reliability coupled with the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5kVA (Volt-Ampere) power range.