MHz frequencies, kW, 30 cm gap wireless power transfer with low air gap flux density and high efficiency using surface spiral winding coils

MHz operation is required for kW, sub-meter gap wireless power transfer (WPT) to maintain the electromagnetic field strength in the air gap at a very low, intrinsically safe level. To effectively reduce losses associated with skin and proximity effects, surface spiral winding (SSW) coils have been developed for MHz WPT. However, over multi-kV voltage stress has to be sustained by the power amplifier for kW transmission with the use of SSW coils. A design methodology of an efficient power converter for MHz, kW WPT is developed and introduced, with the implementation of an excitation coil to effectively reduce the voltage stress on the power amplifier. The Class E power amplifier is selected for the design and evaluated on a 1 kW, 30 cm gap, 3.5 MHz WPT test system. Up to 96.7% efficiency is obtained on the power amplifier and an overall 90.6% DC-to-DC efficiency is achieved. Magnetic field strength within the air gap is maintained at a very low, intrinsically safe level.