Evaluation of Wireless Resonant Power Transfer Systems With Human Electromagnetic Exposure Limits

This study provides recommendations for scientifically sound methods of evaluating compliance of wireless power transfer systems with respect to human electromagnetic exposure limits. Methods for both numerical analysis and measurements are discussed. An exposure assessment of a representative wireless power transfer system, under a limited set of operating conditions, is provided in order to estimate the maximum SAR levels. The system operates at low MHz frequencies and it achieves power transfer via near field coupling between two resonant coils located within a few meters of each other. Numerical modeling of the system next to each of four high-resolution anatomical models shows that the local and whole-body SAR limits are generally reached when the transmit coil currents are 0.5 A_RMS - 1.2 A_RMS at 8 MHz for the maximal-exposure orientation of the coil and 10-mm distance to the body. For the same coil configurations, the exposure can vary by more than 3 dB for different human models. A simplified experimental setup for the exposure evaluation of wireless power transfer systems is also described.

I. Introduction

Recently, efficient wireless high power transfer over relatively long distances between the transmitter and receiver has been achieved with coupled magnetic resonators [1], [2]. Resonant wireless power transfer systems typically operate in the low MHz range (approximately 1–20 MHz). At frequencies below this range, it is difficult to realize high-Q resonant coils with dimensions appropriate for mobile devices, whereas at higher frequencies, the shorter wavelengths limit the spatial extent of the near-field coupling.