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Optimized Design of Integrated Printed-circuit-board-winding Transformer for MHz LLC Converter

Circuit diagram
Fig.1. Circuit diagram of the second-stage LLC
Intermediate bus architecture employing 48 V 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 sought-after for 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 applied for regulation. The second stage is an LLC converter with fixed switching frequency applied for isolation. The design and optimization of an LLC matrix transformer are studied. Fig. 1 shows the circuit diagram of the studied LLC. First, a novel termination and via structure of the primary windings is proposed resulting in significant loss reduction. In this structure, both terminations and vias are interleaved. The leakage flux is reduced and as a result the ac losses caused by an eddy current are reduced. Second, to minimize the winding loss, an overlapped winding structure is proposed. The new structure achieves improved interleaving and less current crowding. The loss reduction is verified by finite element analysis simulation results. Third, the traditional structure with parallel windings also suffers from poor current sharing between layers. To address this issue, a symmetrical winding layer arrangement with alternating magnetomotive force is proposed. Better current sharing is achieved and proven in the simulation. In a summary, the design and optimization of a high frequency, high current bus converter is developed. The hardware prototype is built with a 14 layer-PCB board and a customized planar transformer. The prototype is shown in Fig. 2, with the LLC part populated. The efficiency curve of the second-stage LLC is shown in Fig. 2. 97.8% peak efficiency is achieved. Finally, the two-stage converter is developed and tested. It achieves a high peak efficiency of 96% and a power density of 615 W/in3.
Converter prototype and LLC efficiency
Fig. 2. Two-stage converter prototype and LLC efficiency
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