Loss Analysis of Multi-stranded Litz Wire and Its Bundle Separation Compensation Method for Wireless Power Transfer System

In low and medium frequency wireless power transfer (WPT) system, the loosely coupled transformer (LCT) is often wound with Litz wire. Incomplete stranding of Litz wire leads to unbalanced currents in each bundle, so this paper analyzes this phenomenon in detail, describes the effect of this phenomenon on coil loss, and proposes a novel bundle separation compensation method. Eliminating the requirement for extra components, this approach enables current equalization across all strand bundles of the Litz wire while preserving the system's inherent output characteristics, ultimately boosting overall transmission efficiency. First, the twisted structure of Litz wire is studied, and the reasons for unbalanced current in each bundle are analyzed by circuit modeling of a traditional S/S compensated WPT system. Then, a separate bundle compensation method is proposed, similarly by circuit modeling, the effect of balancing current and efficiency improvement of each bundle is analyzed. Finally, a 2kW prototype is built to verify the effectiveness and correctness of the proposed method. The experiments show that the output current of the system with the traditional and novel methods varies less than 1% under different load conditions. The light-load and full-load efficiency can be improved by 3.1% and 1.1%, respectively, while keeping the constant current output characteristics.