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AC Resistance Evaluation in Litz-Wire Planar Spiral Coils for Wireless Power Transfer
Xiaona Wang, Pan Sun, Qijun Deng, and Wengbin Wang
Abstract A relatively high operating frequency is required for an efficient wireless power transfer (WPT). However, with the raising of the operating frequency, the coil AC resistance increases sharply which possibly degrades the whole efficiency. Hence, evaluating the coil AC resistance is very important for choosing the operating frequency to obtain a good efficiency. With regard to a given Litz-wire coil, the AC resistance is attributed to the magnetic field which leads to skin-effect and proximity-effect, and corresponding conductive resistance and induction resistance in it. A numerical calculation method based on the Biot-Savart law is proposed to calculate the magnetic field strength over strands in the Litz-wire planar spiral coils for its AC resistance evaluation. Based on the predicted resistance, the optimized frequency can be found to achieve the maximum efficiency for a WPT system. Sample coils are manufactured to verify the resistance analysis method. A prototype WPT system is setup to conduct the experiments. Experiments show that the proposed method can accurately predict the AC resistances of Litz-wire planar spiral coils and the optimized operating frequency for the maximum efficiency.
Keyword Wireless power transfer, Litz-wire, AC resistance, Biot-Savart law, efficiency
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