The η-α-Pareto front of inductive power transfer coils

Inductive power transfer (IPT) has been proposed as an alternative charging technology for electric and hybrid electric vehicles (EV/HEV). Because the low magnetic coupling of the transmission coils potentially limits the achievable transmission efficiency η, the losses in the inductors are of key importance in the design of an IPT system. The available area for the receiver coil is usually limited due to the vehicle construction and thus a high area related power density α of the IPT system is needed. In this paper, it is shown that η is maximized if the resonant compensation is chosen such that only active power is transmitted through the air gap. It is shown that a physical boundary termed the η-α-Pareto front exists, where a trade-off between high efficiency and high area related power density is encountered. A thermal model is used to include the effect of an increased winding temperature into the efficiency calculation. The comparison of the η-α-Pareto fronts of three example designs shows that the shielding of the inductor yields a lower η due to additional losses in the shielding materials and that a higher transmission frequency does not necessarily lead to an efficiency improvement.