22.1 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 [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...

An Overview of Regulation Topologies in Resonant Wireless Power Transfer Systems for Consumer Electronics or Bio-Implants

“…To simplify the design of the binary-weighted capacitor array used in the resonant frequency control, a single phase switched fractional capacitor can be adopted for TX regulation by using the resonant frequency trimming technique as given in [45].…”

28.4 A High-Q Resonant Inductive Link Transmit Modulator/Driver for Enhanced Power and FSK/PSK Data Transfer Using Adaptive-Predictive Phase-Continuous Switching Fractional-Capacitance Tuning

“…Furthermore, a large number of capacitors, HV switches and control lines are required for precise tuning to resonance at high-Q to compensate for detuning effects and component tolerances. Adaptive real-time tuning of a driven resonant circuit can be achieved by synchronously switching a capacitor in and out of circuit within each cycle, where the duty cycle determines the average capacitance [2]. Resonance is maintained with external magnetic loading and across PVT variations by sensing the voltage across the switch each cycle at the closure instant, and determining the direction of error from resonance to close a control loop ( fig.…”

“…This gives a min:max tuning range of 1:1.41, sufficient to permit both compensation of tolerance/environmental effects as well as the required frequency shifting for FSK/PSK data modulation. Capacitive dividers isolate the high voltages on the tuning capacitors from the tuning control input [2]. The timing reference is derived from a current integrator oscillator, creating VTR and VDR, the currents generated by a digital V-to-I function.…”