A Self-tuning resonant-inductive-link transmit driver using quadrature symmetric phase switched fractional capacitance

Abstract

Inductive coupling for power transfer is increasingly popular in many applications such as RFID and wireless charging. While much recent work has focussed on receivers, less consideration has been given to the transmit function. High-Q antenna circuits are beneficial for several reasons. Activation of a link at a distance requires a large magnetic field from the transmitter, so for a given antenna current, lower driver voltages may be used, simplifying the driver and its power supplies, and improving overall efficiency. Further, the inherent filtering allows a high-efficiency switching driver to be used while reducing harmonics in the current. However, the consequent narrow bandwidth requires precise tuning to resonance. The excitation frequency may be varied in some applications, but this transfers the tuning problem to the receiver. Any transmit tuning circuitry must be linear with large voltages (from a few V to kV) and currents (mA to many A). A conventional technique is to use multiple external capacitors selected by large switches or even relays. The number of selectable elements needed depends on the Q factor, component tolerances, and environmental effects, with a typical system requiring 5 or more extra capacitors and associated HV switches (Fig. 22.1.1), plus extra IC pins, adding to system cost and volume.

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