Comprehensive Impedance Analysis and Region-to-Region Transformation for Megahertz Wireless Power Transfer Systems

This article presents a comprehensive impedance analysis of megahertz (MHz) wireless power transfer (WPT) systems considering load variations, coil misalignment and non-ideal factors including device parasitics and coil compensation error. The analysis shows that for the MHz operation, the above two non-ideal factors substantially enlarge the variation region of power amplifier's (PA) load impedance, which may adversely affect the stable operation of the MHz WPT systems in practical scenarios. This observation results in the development of a new region-to-region impedance matching network (IMN) design to enhance the robustness of the MHz WPT systems rather than existing ideal trajectory-to-trajectory or point-to-point impedance transformation. Specifically, the Möbius transformation is chosen as a tool to calculate the region-to-region IMN parameters due to its circularity-preserving property. The determination of initial and target PA impedance regions and solution of the Möbius transformation-based IMN design are further discussed from a system-level perspective. Experimental results show significantly improved stable operation of an actual 6.78 MHz WPT system with the proposed region-to-region impedance matching. With a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm 3\%$</tex-math></inline-formula> coupled coil compensation error, the experimental MHz WPT system achieves a system efficiency of 82–87% under a 15-50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Omega$</tex-math></inline-formula> dc load variation, and the system efficiency ranges between 81.18-87.50% with a varying mutual inductance coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$k$</tex-math></inline-formula> between 0.086–0.161.