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10 KW Transformer and DC Stage Design for Electrical Vehicle Charging System

Percent efficiency with respect to output voltage. Both the full brige parallel switches, and full brige diade case are shown
Fig. 1. Efficiency comparison
The vehicle charging system used in this design includes an ac/dc PFC stage and a dc/dc stage to accommodate the wide range of output voltage. The output voltage ranges from 200V to 500V and the thermal design is considered at full power level. Paralleling GaN devices is used at both stages to increase the power level and efficiency at the same time. This paper focuses on the design of the DC stage, including LLC converter operation, efficiency estimation and high-power, high-frequency transformer design. The dc input for the dc stage will be fixed at 400V, so the operation frequency should be tuned to fit different output voltages. A full compar-ison and selection has been made to compare different operation frequency ranges based on peak current and RMS current. The transformer is designed after the operation frequency is de-termined. In the first prototype, the transformer loss can be controlled to be below 50W for the entire range, and the peak efficiency of the LLC stage can reach 98.3% using diodes and 98.7% is using switches as the secondary side.

When designing the converter operation mode, in order to regulate the output voltage, the opera-tion frequency range should be below the resonant frequency so a high gain can be achieved without any additional loss. In this range there is no secondary side switching loss, and the pri-mary switches' turn-off loss can be minimized as well. Peak current and RMS current are com-pared for different resonant frequencies, and the final operation range is selected to be 200 KHz to 400 KHz.

The transformer design is based on this frequency range. Because the switches (or diodes) can create a great deal of power loss, the transformer should have a better performance as it will determine the whole converter's power density. Ferrite 3C97 is selected as the material for this operation range, and the first prototype is designed by first limiting the loss. The volume is still large, and further efforts will be made to design a better high-power-density transformer.

The first LLC converter with open-loop control will be tested, and more work will be done on transformer design. Either a matrix transformer or a planar structure will be studied as well.

Design of inductor core
Fig. 2. Core design overview
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