Spiral Capacitive Coupler and Optimum Impedance Matching for Capacitive Wireless Power Transfer

This paper improves the coupling capacitance of capacitive wireless power transfer using spiral-shaped slots and presents the optimal parameter selection strategy to improve overall efficiency. Regarding the capacitive coupling plate, it consists of a very thin but dense spiral pattern to increase the internal self-inductance of the capacitor. This self-inductance significantly compensates for the coupling capacitance, thereby increasing the effective net coupling capacitance value at the same physical outer dimensions. Regarding the optimum impedance transformation network to maximize the overall efficiency, the optimum matching impedance is analyzed for given coupling capacitance and its parasitic resistance. It is also found that the benefit of high coupling capacitance saturates beyond certain limit, and this limit is also analytically derived to design optimum coupling capacitance. Compared to a conventional capacitive-inductive hybrid coupler, our proposed system does not need the litz-wire coupler and the heavy/brittle ferrite materials. The optimum matching inductance value which is required for the highest efficiency is reduced. Moreover, the current through the inverter and TX-side matching network is also reduced, thereby reducing losses at TX-side as well. The measurement results achieve 88.46% efficiency with 200 W load power at a frequency of 2.8 MHz.