Study of SEPIC and Ćuk Converters Working as Automatic Power Factor Corrector when Operating in Unusual Discontinuous Conduction Modes

It is very well known that the SEPIC and Ćuk converters can operate as Automatic Power Factor Correctors (APFCs) when the converter diode stops conducting during the transistor OFF-state interval, thus achieving operation in the well-known Discontinuous Conduction Mode (DCM). In order to reduce de APFC size, the input inductance can be reduced. However, if that inductance is not high enough, not only the converter diode can stop conducting during the transistor OFF-state interval, but also the input rectifier diodes. Considering the number of diodes that can stop conducting, the aforementioned scenario leads to three DCMs and, consequently, the operation as APFC is not guaranteed anymore. In this paper, the line current waveform of SEPIC and Ćuk converters operating in any of the three aforementioned DCMs is studied in order to evaluate their operation as APFC. The results show that even in the unusual DCMs, the converters operate as quasi-ideal APFCs, thus allowing the design of these converters with lower value of the input inductance and, consequently, with higher power density. The analysis is validated with simulation and experimental results.