AC Impedance Modeling of Composite Litz Wire with Non-ideal Transposition Based on Improved Discrete Transposition Model

Litz wire is a widely used winding solution for high-frequency magnetic components, effectively mitigating eddy current losses in windings. However, practical Litz wires often exhibit non-ideal transposition (NIT), making accurate AC impedance calculation a challenging problem. This paper proposes an improved Discrete Transposition Model (iDTM) based on weighted segmentation for composite Litz wires with NIT. First, the 3D structure of the Litz wire is analyzed using micro-CT imaging. Based on this, an energy-minimized Litz 2D model (EM-Litz2D) is developed to simulate the cross-section of highly filled composite Litz wires. Next, accounting for layered transposition in concentric structures and multi-level transposition in composite structures, the iDTM with weighted segmentation is introduced. This method is applicable to both concentric and composite Litz wires with NIT. Finally, the proposed method is experimentally validated using Litz wire samples at frequencies up to 1 MHz and HFT winding cases at frequencies up to 500kHz. Compared to existing methods, the proposed approach achieves higher accuracy.