This article presents a fast numerical calculation method of realistic power losses for high-frequency litz wires. Explicitly, the imperfect structure of litz wires is considered when calculating losses due to an excitation current (skin losses) and external magnetic fields (proximity losses). Calculations of litz wires with more than 1000 strands were performed on a personal computer and have been validated by measurements up to 10 MHz. In the calculation, the impact of the bundle structure on skin and proximity losses is examined. The method allows to select a suitable litz wire for a specific application or to design a litz wire considering realistic twisting structures.

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This paper proposes a portable 11 kW off-board charger for electric vehicles. In the ac/dc stage, a three-phase power factor correction (PFC) in Vienna topology is chosen. The loss and volume of the PFC inductance are calculated over a wide range of parameters and optimized with regard to design, winding, and core material. A three-phase LLC resonant converter operating at 1 MHz is chosen for the galvanically isolated dc/dc stage. A parametrizable loss model of the high-frequency transformer and the resonance inductor is developed to minimize volume, weight, and losses. With the help of an automated algorithm using these loss models, the inductive components are optimized in terms of winding specification, magnetic material, and core geometry, verified by finite element analysis and measurements. For the ac/dc stage, 900 V SiC devices are adopted, and 1200 V SiC devices are used in the primary and secondary sides of the dc/dc stage. A variable dc-link voltage is utilized to adjust the charging profile and to operate the LLC resonant converter at the most efficient point near the series resonance frequency. A mechatronically integrated portable air-cooled off-board charger prototype with 11 kW, three-phase 400 VAC input, and 620–850 VDC output is realized and tested. The prototype demonstrates a power density of 2.3 kW/liter (37.7 W/in³), a peak efficiency of 96%, and 95.8% efficiency over the defined battery voltage range.

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