Digital Coil: Transmitter Coil with Programmable Radius for Wireless Powering Robust Against Distance Variation

A wireless powering system robust against distance variation is required. In this paper, a digital transmitter (TX) coil, whose radius depends on the distance (d) between the TX and receiver (RX) coils, is proposed that maximizes the efficiency of wireless powering. Firstly, we analytically derived the optimum TX coil radius (r
_{TX, OPT}
) to maximize the coil-to-coil efficiency (1) by cooptimizing the coupling coefficient and the quality factors of the coils. It was found that r
_{TX, OPT}
was approximately equal to d. Then, a practical implementation of the digital TX coil, whose radius was electrically varied without mechanical motion, was proposed. It was found by a measurement that, compared with a conventional TX coil with a constant radius, the proposed digital TX coil increased η from 12% to 20% when d was four times the RX coil radius.