Design of wireless power transfer coils to minimise capacitor stress and AC resistance

Abstract

A Wireless Power Transfer system for charging electric vehicles must have high efficiency to compete with wired charging. Although efficiency can be increased using a number of ways, in the lumped circuit model the only loss comes from parasitic resistances of the system. In a WPT system, AC resistance of power transfer coils is the most dominant source of power loss and it increases considerably with frequency. Therefore AC resistance of coils is the most important factor of efficiency. Additionally, most WPT systems work using resonant coupling. Due to resonance, reactive voltage in a series resonant tank and current in a parallel resonant tank can be several times the input and output voltages and currents, depending on the topology used. This work presents a design procedure that attempts to address those two problems in the design of a WPT system. First is to minimise capacitor stress by quantifying the voltage and current stresses in relation to the operating conditions of the WPT system and second to minimise the absolute resistance of a WPT coil. This design procedure results in the lightest coil designs that can be chosen from commercially available Litz wires. Experimental validation has achieved 10 kW wireless power transfer using 0.6 kg coils with 18 cm air-gap, 98.6 % coil-to-coil efficiency and 94.8 % DC-to-DC efficiency, with the measured values agreeing well with the design calculations. Sensitivity analysis has also been done to explore the effects of coupling factor in the system.