Single-Turn Air-Core Coils for High-Frequency Inductive Wireless Power Transfer

Abstract

The efficiency of megahertz-frequency inductive wireless power transfer designs is currently limited by the quality factor of the resonant tanks and the coils. In this article, we use two-dimensional finite-element analysis to optimize the cross section of single-turn air-core inductors for this application and demonstrate coils for wireless power at high-frequency and very-high-frequency industrial, scientific, and medical (ISM) bands. Our resonant tanks achieve diameter-normalized quality factors (Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> ) that are 25%-70% better than the state of the art for air-core coils at 6.78 MHz (Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> = 73 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and 13.56 MHz (102), and we show significant improvements at the 27.12 MHz (123) and 40.68 MHz (137) ISM bands. We highlight the importance of capacitor selection and the tank configuration for designing high-Q resonant tanks and recover the increase of Q with frequency that is missing from the existing literature. Furthermore, we show that maximizing height is not ideal for wireless power transfer applications and build high-performance coils at small heights. We integrate these coils into a wireless power transfer system at 6.78 MHz and 500 W output power, showcasing a maximum dc-dc efficiency above 89% with a gap (3 cm) equal to the coil radius, a significant efficiency improvement over existing work.