Stackless Onboard Coil for Dynamic Wireless Power Transfer

Dynamic wireless power transfer (DWPT) technology is emerging as a key enabler for in-motion charging of electric vehicles (EVs), allowing continuous energy transfer without the need for dedicated charging stops. This capability can reduce the required battery capacity and vehicle weight, thereby lowering the vehicle cost, extend battery life by minimizing deep discharge cycles, and support distributed charging that helps reduce grid peak loads. However, conventional receiver coil structures, such as the double-layered Double-D Quadrature (DDQ) configuration, require additional underbody space due to their two-layer winding structure, which limits their applicability in passenger EVs where the battery pack occupies most of the available area. To address this issue, a novel stackless onboard coil composed of four segmented coils is proposed. The midpoints of adjacent coil pairs are connected to create an additional current path, forming a composite magnetic structure within a single magnetic layer. This configuration eliminates the need for stacked windings and reduces the onboard coil thickness by 35% compared with conventional DDQ coils, while maintaining power transfer performance and efficiency without significant degradation. Theoretical analysis based on an equivalent circuit model, finite-element method, and dynamic circuit simulations was conducted to compare the proposed stackless and conventional DDQ systems. A 1.2 kW prototype with a 220 mm air gap was constructed, and experimental results confirmed continuous power transfer without a stacked structure. Compared with the DDQ coil, the proposed design increased the average output power by 13%, decreased the average efficiency by 1.8 percentage points, and improved power fluctuation by 21.5 percentage points. The proposed stackless coil is particularly suitable for passenger EV applications of DWPT systems, where minimizing the onboard coil thickness is prioritized over maximizing power transfer efficiency.