As the wireless power transfer (WPT) technology has been proved to be a convenient and reliable charging method to plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV), the loosely coupled transformer structure and size are the primary and fundamental concern to design an efficient WPT system. In this paper, a double D (DD) coil and a unipolar coil are selected to conduct the study. We focus on the coil structure design to achieve the maximum coupling coefficient as well as efficiency with two situations: (a) with no misalignment, and (b) with a 75mm doorto-door, and 100mm front-to-back misalignment at which the maximum operating capability (MOC) can still be achieved. A coil size optimization process is proposed for both the DD coil and the unipolar coil configurations. The relationship between the size of the secondary (receiving) coil, which determines the weight of the pad on the vehicle, and achievable maximum efficiency is studied for both coil topologies. The interoperability between the two coil topologies is studied. The proposed transformer structures with aluminum shielding meet human exposure regulations of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines as a foundation. Finally experiments validated the analyses.Index Terms-Wireless power transfer, electric vehicle, loosely coupled transformer, interoperability, safety, electromagnetic radiation. 0885-8993 (c)