Analysis on half-bridge LLC resonant converter by using variable inductance for high efficiency and power density server power supply

The half-bridge (HB) LLC resonant converter is widely used in the middle power applications, because it has many advantages such as wide zero-voltage-switching (ZVS) range and low component counts. However, in wide input voltage applications such as a server power supply with a hold-up time requirement, the HB LLC resonant converter should be designed with low k factor, which indicates small magnetizing inductance. It causes large conduction and turn-off switching loss in the primary side, which results in low efficiency in nominal input voltage. To overcome these problems, this paper proposes a HB LLC resonant converter applying the variable magnetizing inductance. In nominal status, the proposed converter operates with large magnetizing inductance to reduce conduction and turn-off loss of the primary switches. On the other hand, during the hold-up time, the proposed converter operates with low k factor by decreasing the magnetizing inductance, which enables it to meet the hold-up time requirement. The proposed method has simple structure and high efficiency and is implemented with high power density. The feasibility of the proposed method is verified with a prototype converter with 350W (56V/6.25A) output.