Achieving high efficiency in a double conversion transformerless UPS

This paper describes the design of a transformerless UPS that uses two high frequency PWM power converters in series without sacrificing efficiency compared to their transformed forerunners. It has long been accepted, however, that a high frequency PWM converter will have higher losses than a diode rectifier or SCR phase control rectifier and that the loss in efficiency is simply one way to bare the cost for the increased performance. The advantages of using a high frequency PWM converter for the AC-to-DC stage are generally well known: improved input power factor; improved input current THD; and improved performance on a generator. This paper presents, as a case study, how one UPS design addressed the efficiency issue and controlled the losses to produce a product with efficiency equal to or better than the SCR forerunner.