An Inductive Link with Integrated Receiving Coil—Coupling Coefficient and Link Efficiency

Abstract

Inductive powering can free microelectromechanical systems (MEMS) devices from tethering to a power supply, thus expanding their scope of applications. We investigate inductive powering for MEMS using a microfabricated coil as a receiver operating at and below 1 MHz. The microcoil is designed to enclose the MEMS functions to achieve maximum coupling, and is built into a silicon substrate by our inlaid electroplating process, so that the microcoil has a small resistance despite its long trace. Non-negligible parasitic effects from the silicon substrate affect the characterization of an inductive link and its optimization. Taking into account the parasitic effects of Si microcoils, the coupling coefficient of the link is determined by a coil-model scheme that is different from conventional strategies. Furthermore, an equivalent circuit is developed for our link and used to analyze link operation over the frequency range of 4 kHz to 4 MHz. With measured link parameters, link efficiency is calculated with our equivalent circuit, and the results agree better with experiments than do conventional models. The equivalent circuit also indicates that microcoils with high quality factor at low frequencies, such as our inlaid electroplated coils, improve link performance, while parasitic capacitance has little effect.