Planar Transformer Design for a LLC DC-DC Converter with Variable Input and Wide Output Voltage Range

DC-DC converters are fundamental to electric vehicle (EV) fast charging infrastructures, where the LLC DC-DC resonant converter is widely adopted due to its soft-switching capability, high efficiency, galvanic isolation, and wide output voltage regulation. The high-frequency transformer in the LLC converter plays a key role in achieving such features, as well as for ensuring high power density. This study presents the design of a planar transformer that incorporates detailed modeling of critical parameters, including magnetizing and leakage inductance, flux density, losses, and parasitic capacitances. A custom ferrite core structure is proposed, consisting of ferrite tiles, ferrite bars, and a central ferrite block for a 25 kW LLC converter power module operating with an input voltage range of 650 V to 850 V and an output voltage varying between 200 V and 500 V. The proposed design was validated through electromagnetic simulations in ANSYS Maxwell, employing the finite element method (FEM), while the performance of the LLC converter was analyzed using PLECS simulations.