Precise Modeling Approach of Litz-Wire based Planar Rectangular Spiral Coils with Ferrite Core and Lateral Misalignment for WPT Applications

The wireless power transfer (WPT) coils are built with litz wires to minimize the skin effect and eddy loss. Precise modeling of litz wires leads to enormous meshes and takes lengthy simulation time in finite element analysis (FEA). Various equivalent analytical models often use solid conductors with uniform current distribution to realize litz wires, which fail to include the twisting and stranding effect. Also, for mutual inductance calculation, various geometrical over-simplifications are made to reduce the complexity of the analytical model. These simplifications add significant errors in the self and mutual inductance parameter estimation. This paper proposes improved analytical modeling for the litz wire-based rectangular planar spiral coils with a ferrite core for precise parameter calculation. The proposed modeling method also addresses lateral misalignment between WPT coils. The accuracy of the proposed modeling is compared with existing modeling methods, FEA simulation, and experimental results obtained from the rectangular spiral coil hardware prototypes. The proposed model is 5 to 6 times more accurate than the existing solid conductor analytical method.