Analysis and Design of a Self-Resonant Rectenna for Small-Size and Ultraloosely Coupled MHz Wireless Power Transfer Applications

Abstract

Wireless power transfer (WPT) working at multimegahertz is widely considered a promising technology for a more compact and lighter system. Thanks to these features, the small-size megahertz (MHz) WPT systems are desired for the portable or implantable electronic devices. This article proposed a self-resonant rectenna for small-size and ultraloosely coupled MHz WPT applications. Different from the receiver using the Class-E rectifier, the rectenna can achieve the self-resonant operation without additional series compensation capacitor and bulky inductor, which helps to minimize the components count and receiver size. By the self-resonant operation and the corresponding design method, the rectenna also helps to achieve the high efficiency for the small-size MHz WPT systems when working at ultraloosely coupled conditions. A prototype of the self-resonant rectenna is built and tested in an example 6.78-MHz WPT system. The misalignment and the transfer distance of the prototype are up to 35 and 10 mm when the rectenna diameter is only 10 mm. The experimental results show that the self-resonant rectenna-based WPT system can achieve up to 21% dc–dc efficiency and 1.05-W output power when the 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> is ranging from 0.009 to 0.038, namely the ultraloosely coupled condition. In the same working condition, the traditional single-diode rectenna-based WPT system can only deliver 0.18-W output power with a 4.3% dc–dc efficiency.