Optimal Design of MHz LLC Converter for 48V Bus Converter Application Yinsong Cai, Student Member, IEEE, Mohamed H. Ahmed, Student Member, IEEE, Qiang Li, Member, IEEE, and Fred C. Lee, Life Fellow, IEEE

Optimal Design of MHz LLC Converter for 48V Bus Converter Application Yinsong Cai, Student Member, IEEE, Mohamed H. Ahmed, Student Member, IEEE, Qiang Li, Member, IEEE, and Fred C. Lee, Life Fellow, IEEE

 Abstract—Intermediate bus architecture employing 48V bus converters is widely used in power supply applications. With the rapid increase of demanded power by these loads, higher efficiency and power density are driving better performance power management solutions. In this paper, a Gallium Nitride (GaN) based design of a two-stage solution is proposed. The first stage is a multi-phase Buck for regulation. The second stage is an LLC converter with fixed switching frequency for isolation. The detailed design and optimization of the LLC converter are studied. To achieve high power density and high efficiency, the transformer design becomes critical at MHz frequency. The matrix transformer concept is applied and a merged winding structure is used for flux cancellation, which effectively reduces the AC winding losses. A novel primary termination and via structure is proposed, resulting in great loss reduction. In addition, to study the current sharing of parallel winding layers, a 1-Dimensional analytic model is proposed, and a symmetrical winding layer scheme is used to balance the current distribution. Finally, the prototype for the two-stage bus converter is developed, with the peak efficiency of 96% and power density of 615W/in3.

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