Optimization of coil geometry using strongly coupled magnetic resonance at 13.56 MHz ISM band

Abstract

There are an estimated 2 billion smartphones that need to be charged at least once a day. The most convenient and comfortable way of charging them would be to use wireless charging technology that offers full-mobility to users. Unfortunately, present wireless charger is still limited to close contact operation. Commercially available wireless chargers use inductive power transfer (IPT) which is only efficient at close distance. Because of this, most wireless power transfer (WPT) researches are focused on strongly coupled magnetic resonance (SCMR). Unlike IPT which transfer efficiency precipitously drops as the distance increases, SCMR can provide an acceptable efficiency at distances few times its resonator diameter. Many studies about SCMR are centered to resonator design to maximize quality factor and to achieve a high figure-of-merit (FOM) to obtain optimal transfer efficiency. In this paper, the FOM of spiral coils were studied to determine the optimum geometry of coils at 13.56 MHz which can fit in smartphones and chargers. While most studies in SCMR using spiral coil determined optimum coil geometry from arbitrarily sized coils, the size of the coils were based on the average size of the said devices. Several geometries of spiral coils were exhaustively examined using MATLAB and validated using sets of transmitter and receiver coils. Results showed that the pair of transmitter and receiver coil exhibiting maximum quality factor individually also provided optimal magnetic coupling. Based on simulation, it is possible to achieve high transfer efficiency even at a distance twice the resonator device size. This implies that users would not be any more limited to close contact charging.