Design of a 3 kW primary power supply unit for inductive charging systems optimized for the compatibility to receiving units with 20 kw rated power

This paper presents a power unit for use in the primary side of 3 kW rated inductive charging system. The objective is the design of 3 kW rated primary sides which are compatible to 20 kW rated secondary side receiving units inside electric vehicles. The motivation arises from the consideration that 3 kW and 20 kW charging stations should be kept compatible. Compatible especially in the way that vehicles with 20 kW rated receiving units should be able to use 3 kW charging stations. The basis for the design of the 3 kW system is a series-compensated secondary receiving coil without an additional DC/DC-converter stage between a B4 rectifier and battery with a rated power of 20 kW. The absence of a DC/DC stage allows lower weight and installation space in the vehicle, but the possibility for impedance matching is missing. The system behavior is investigated analytically, so that all currents and voltages, as well as the reactive power in both compensation networks can be calculated depending on the given systems parameter, such as the inductance of the secondary coil, the coupling factor, the operation frequency and the transferred power. It is shown that the operation point with the lowest reactive power, and thus least losses in the coils and compensation capacitors, strongly depends on the design of the receiving unit. Since the receiving unit is designed for a transferred power of 20 kW, an operation with 3 kW will result in a significantly lower efficiency. Therefore a solution is presented, to shift the ideal operation point of the secondary side system from 20 kW to a much lower value. That way, the 3 kW system is able to operate very close to the shifted ideal operation point.