Vertical Bifilar Self-Resonant Coil for Electric Vehicle Wireless Power Transfer

This article proposes a vertical bifilar self-resonant coil to replace the Litz wire and external compensation capacitors in the transmitter of a wireless electric vehicle (EV) charging system. Unlike traditional planar self-resonant coils, the vertical coil aligns with the magnetic field orientation, which reduces the impact from eddy and proximity effects, thereby reducing the ac copper loss. The vertical structure yields a high capacitance area, effectively lowering the resonant frequency. This makes self-resonance at 85 kHz viable, as required for EV charging. Meanwhile, the dielectric layer can be thicker than in planar coils due to the larger capacitance area, providing the high ac dielectric strength required for high-power charging. The vertical bifilar self-resonant transmitter coil is optimized through design procedures to minimize copper loss while meeting constraints such as resonant frequency, current density, and layer-layer voltage stress. The proposed coil structure is validated in a 3.3 kW EV wireless charging system, achieving a minimum dc efficiency of 92.47% at a lateral receiver misalignment of 150 mm.