Design Optimization of Unregulated LLC Converter with Integrated Magnetics for Two-Stage 48V VRM

In this paper, a high-efficiency and power-density unregulated LLC converter (DCX) is proposed for a first-stage converter in two-stages 48V voltage regulator module (VRM) for data center applications. An optimized matrix transformer structure including an enhanced termination method for both primary and secondary side winding, in addition, a PCB winding structure for equal current sharing among all transformer parallel layers is proposed, by which a significant reduction in the total conduction loss for the high frequency transformer is achieved. The proposed optimized design is applied for designing two LLC-DCX converters with different fixed transformation ratios of (4:1) and (8:1) to study and evaluate the impact of the intermediate bus voltage variation on the overall two-stage 48V VRM performance. The designed LLC-DCX with GaN devices can provide a continuous output power of 900W with maximum efficiencies of 98.4% for the (4:1) conversion and 98.0% for the (8:1) conversion with high-power-densities of 1600 W/in ³ and 1200W/in ³ respectively.