Off-line Balanced Forward-Flyback Converter

Due to the flybacks' indirect characteristic of energy transfer, the transformer size increases for high power levels. Providing an additional direct energy transfer path can decrease its size. Parallel forward-flyback converters offer such functionality and outperform forward converters with regard to PFC functionality. A balanced forward-flyback converter is a variation on this kind of parallel converter. The forward and flyback sub-converters share a transformer winding and an additional balancing capacitor enables even better AC line voltage utilization. This paper starts with an in-depth analysis of the balanced forward-flyback converter, and introduces ten operating modes in which this converter can operate. A boundary-conduction mode (BCM) controller for the magnetization current is developed and presented, and through simulations the PFC performance of the converter is tested with a constant switch on-time controller. Moreover, a prototype is designed and built with both a dissipative R-C-D snubber and two-switch clamp configuration. Experimental results from the 100W/120-373V AC/DC prototype were obtained to prove the converter operation and BCM controller concept. The balanced forward-flyback converter in BCM offers good PFC performance and manages a THD of input current between 1.69% and 4.38%.