Accurate Modeling Approach of Litz-Wire Based Planar DD Spiral Coils for WPT Applications

The litz wires are used in wireless power transfer (WPT)to address the skin effect and eddy losses. The WPT coils are designed with the help of finite element analysis (FEA) software like ANSYS Maxwell 3D. Due to the twisting and stranding effect, the litz wire structure becomes complex and requires an efficient and massive computation effort to design a WPT coil. Several traditional analytical models treat Litz wire as equivalent solid conductors with uniform current distribution models, leading to unacceptable low accuracy in the self-inductance calculation. At the early stage of coil design, an analytical method including geometrical features of the litz wire will accelerate the design process. Also, for mutual inductance calculation, over-simplifications in coil layout are made to reduce the complexity of the analytical model, which in turn gives erroneous results. This paper proposes improved analytical modeling for the Litz wire-based DD-type planar spiral coils to calculate self and mutual inductance precisely. The accuracy of the proposed modeling is compared with existing modeling methods and the simulation results obtained from the ANSYS Maxwell 3D. The experimental results obtained from the 380mm * 240mm spiral coil hardware prototypes are reported to validate the proposed model.