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If you are trying to make a single phase UPS, the power level will determine the topology you should use. This will lead you to the proper control device. The 8051 peripherals will be the determining factor in your selection. Based upon the dynamics of the design, a 8051 might not have enough power to do the job.

Fill in the following:
Power: _____ WATTS
Wave format: modulated sine or true sine
Phase: 1 or 3

The single-phase design will need the controls of 3 distinct blocks:
1. A rectifier/PFC (DC rail voltage)
2. A battery re-charger
3. An inverter

Richard