Online Double Conversion Systems are so called, as they convert (rectify) the incoming AC mains to DC and then convert (invert) it back to AC again.

This may seem a strange thing to do, but the advantage is that the output from the UPS is now independent of the input and as a result provides superb power protection.

The basic operation modes can be explained in the following diagram. Use the buttons to create a failure of the mains, or the UPS itself:

Notice that if the mains is present, and the UPS should fail, the load is still powered. This is due the "bypass function" inherent in online double conversion topology.

Advantages of double conversion technology include:

• High Quality Output Waveform
• Wide Input Voltage Range (without reverting to battery)
• Protection against practically all power anomalies
• The capability of increasing run time
• Higher system reliability due to inbuilt bypass function

Online double conversion systems are also known as VFI systems an acronym for Voltage and Frequency Independence.

VFI systems are considered to be the best power protection design, depsite this there are some disadvantages:

• Efficiency - As the inverter is constantly running there is losses in the system, typically from 8-10% at full load
• Noise - Since the inverter is running it needs to be cooled. This fan cooling generates noise, generally too much for an office environment