A Self-Resonant Two-Coil Wireless Power Transfer System Using Open Bifilar Coils

Usually, self-resonant coils are restricted to use on the intermediary circuits of multiple-coil wireless power transfer (WPT) systems. This is because a source sees a parallel resonance when connected to a typical coil at self-resonance, and parallel associations in the primary and secondary sides of a two-coil system are not as practical as the respective series associations. However, many applications require two-coil systems due to their reduced size and simplicity. In this work, a self-resonant two-coil WPT system is proposed. The presented system uses open bifilar coils that lead to a series resonance at the resonant frequency. In this way, no capacitor is used in the whole system and it behaves as a series association of coil self-capacitance and inductance at the resonant frequency. Thus, the proposed system has a reduced size and also avoids capacitor damage due to high individual voltages caused by resonance effects. Practical measurements and theoretical results of the system efficiency and relative power transfer show good agreement and validate the proposed scheme as a promising solution for a two-coil self-resonant WPT system.