Abstract-Inductively-coupled systems used in applications such as RFID and wireless power often require high Q factor resonant transmitters to maximise the magnetic field and achieve high overall efficiency.However, these are sensitive to environmental detuning as well as component tolerances. Existing methods for accurate tuning require search algorithms, usually requiring the suspension of normal operation in order to calibrate the resonant inductor-capacitor circuit, thus reducing power throughput and increasing system complexity.We describe here how zero-voltage switched fractional capacitance techniques may be used to achieve continuous and real-time adaptive tuning of large-signal resonant inductorcapacitor circuits. Minimal additional circuitry is required and tuning is maintained without disrupting normal operation. Many variants are possible for the implementation of the system, and some trade-offs relating to the available tuning range and operating voltages are analysed for two alternative topologies. Experimental results are presented for a 125kHz demonstration system.

The efficiency of inductively-coupled power transfer systems is increased when high-Q inductor-capacitor circuits are used, maximising the magnetic field strength at the transmitter for a given drive amplitude. Such circuits require precise tuning to compensate for environmental effects and component tolerances which modify the resonant frequency. A single zero-voltageswitched fractional capacitance may be used to accurately tune the circuit to resonance, reducing implementation costs compared to classical tuning techniques. However, integration onto a chip presents challenges which must be addressed, such as operating with large voltage excursions and compensating for high-voltage driver delays.We describe here the operation of a self-tuning LC resonant circuit driver using a symmetrically-switched fractional capacitance. An architecture for a fully integrated system for operation at 75kHz-2.6MHz is presented. Implemented in a 0.18µ µ µ µm 1.8V-50V CMOS/LDMOS technology, the integrated circuit uses high voltage interfaces for capacitance switching and sampling inputs, and includes digital phase trimming to compensate for propagation delays in large driver devices. Correct operation of the self-tuning functionality is verified across the available frequency range, with results presented for static and dynamic tuning responses.

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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 [1,2], 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 [3] 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.An alternative tuning approach common at RF is to intermittently connect a capacitance to the LC circuit such that the time-averaged fractional value achieves the desired resonant frequency. At GHz operation, synchronism between the oscillation and the switching is not realistic; the switching rate is set significantly below the resonant frequency and phase-noise shaping is applied. In low-frequency power applications, harmonics and losses due to such switching are more important, but now it is possible to impose zerovoltage synchronous switching on a fractional capacitance (Fig. 22.1.1). Hence a single additional capacitor C2 and HV switch SW1 can provide a large continuous tuning range, (of the order of 50%, depending on capacitor values), and if switching is ideally at the zero voltage instants, losses, transients and harmonics will be minimised. Zero-voltage switching at resonance implies that the switch SW1 opens and closes at symmetrical time points in quadrature with the antenna-loop drive voltage, i.e. either side of the current minima. This is easy to arrange if the circuit is already resonating, but the issue of determining precise resonance still remains. To avoid the need for lossy sense resistors, one can vary the drive frequency and measure the peak voltage VC on the capacitors, but the system must go offline.Instead of trying to determine the required timing from an unknown state, if we simply impose the ideal quadrature switching for the fractional capacitance, then observation of the voltage across the capacitor switch SW1 provides a direct means of detecting any tuning error [4]. Figure 22.1.2 shows the concept; SW1 is opened and closed symmetrically by φSW in quadrature w...

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