Analysis and Design of Freewheeling Current Commutating Methods to Low Voltage-Rated Diode

A center-tapped rectifier is widely employed in applications requiring low output voltage and high output current. However, the rectifier diodes experience high voltage stresses, resulting in high conduction losses. To solve this problem, a low voltage-rated freewheeling diode was adopted in previous studies. It improves the overall efficiency by commutating the freewheeling current in the secondary side to the low voltage-rated diode instead of the high voltage-rated diodes. However, the detailed commutation process was not thoroughly explained in the previous studies. Actually, only a small portion of the total freewheeling current flows through a low voltage-rated diode, and most of it still flows through the high voltage-rated diodes, leading to slight improvement in efficiency. Therefore, this article offers the operation principle during the freewheeling interval in detail. Furthermore, it presents two simple methods for increasing the freewheeling current commutated to the low voltage-rated diode. These methods also reduce the circulating current in the primary side, resulting in low overall conduction losses. The article also explains the features and design considerations of each method and topological extensions for other types of rectifiers. Finally, a 250-W prototype is implemented, achieving a 94.8% peak efficiency.