Circuit Models and Fast Optimization of Litz Shield for Inductive-Power-Transfer Coils

The metal shields with plates or rings are usually added to the inductive-power-transfer coils to attenuate the stray magnetic field. However, the coil-to-coil efficiency is reduced owning to the extra losses on the shields. This paper introduces the litz shield which attenuates the field with smaller shielding loss thanks to the uniform distribution of the shield current. Two types of litz shields, i.e., shorted litz shield and driven litz shield are discussed in detail. Their performances are analyzed with the circuit model composed of four coupled inductors. The efficiency and the field attenuation for the coils with litz shields are optimized with Pareto fronts. A fast method to derive the fronts using a lumped-loop model is described. Only tens of finite-element simulations are required in the entire optimization. The coils with different shields were optimized to transfer 100-W power across 40-mm gap with the same efficiency. The measurement results prove that the shorted litz shield is more effective than the other shields. It attenuated 23% more of the stray magnetic field compared to the coils with traditional metal shield.