The Asymmetrical Half-Bridge Flyback Converter: a Reexamination

Isolated Zero-Voltage-Switching (ZVS) dc-dc converter topologies are attractive solutions in the continuous ride toward higher switching frequencies, allowing more compact power supplies. Among them, the Asymmetrical Half-Bridge Flyback Converter (AHBFC) represents an interesting solution, featuring a simple duty-cycle control at a constant switching frequency, as opposed to the popular LLC converter. The majority of the papers dealing with this topology, present an approximated voltage gain which is similar to an isolated Buck converter, i.e. proportional to the duty-cycle. However, when the converter is designed for a resonant operation, so as to eliminate any reverse recovery problem of the rectifier diode, its voltage gain can be quite different, becoming non monotonic and a function of the switching frequency. This paper investigates this aspect, deriving a theoretical framework capable of capturing its real voltage gain behavior. The proposed analytical model has been verified through simulations as well as experimental measurements taken on a 160W prototype working at 400kHz.