High-power-factor Sinusoidal-output-waveform Converter Based on the Flyback Topology for Ac Ups

I.Introduction. Everyday, the number of critical loads connected to the utility line increases. These critical loads need a highly reliable power supply. There has been reported several solutions to this need, some of these solutions include ferroresonant voltage regulators, variable inductance regulators, tap changing regulators and uninterruptible power supplies (UPS) [1]. Among the UPS solutions reported, [2-7] offer different approaches to provide high quality voltage to the load. Each solution has advantages and disadvantages, proving that there is no “best solution” and keeping open the interest on developing new solutions. The solution presented in [2] covers only the inverter stage of the UPS. The topology offers good dynamic response, high efficiency and low components count, but it does not address the front end and the battery charger stages. In [3], a simple and highly efficient approach is presented, however, since the front end and the inverter stages share power switches, there is no isolation between the main line and the load. On the other hand, the approach described in [4] provides isolation to the battery set, soft switching and PFC, but the solution uses a high number of switches. [5] and [6] present an alternative which provides isolation between the main line, the battery set and the load, good dynamic response and near unity input power factor, but the power topology is complex. The solution proposed in this paper uses the natural output waveform of the PFC converters before filtering: a semisinusoidal waveform. This approach has been reported in [7] and [8] using a forward converter. The semisinusoidal waveform allows the inverter stage working at low frequency (twice the main line frequency) with high efficiency.