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The Class-D power amplifier (PA) is widely used to drive the primary coil of wireless power transfer (WPT) systems. Due to coupling and loading variations, the PA is required to adjust the transmission power quickly, precisely, and efficiently. In this brief, a switching-frequency-modulation-based Sigma-Delta (ΣΔ) modulator is proposed for driving the PA with minimal hardware overhead. The control of the Class-D PA's output voltage at the fundamental tone is proved to be linear. Using a ΣΔ modulator, a WPT system is designed with a fast control loop for the voltage regulation of the receiver at the secondary coil. A transmitter chip and a receiver chip are fabricated using the AMS 0.35-μm CMOS process. The ΣΔ modulator is implemented in a field-programmable gate array. The measurement results show that, with the proposed ΣΔ modulator, the tuning range of the receiver output voltage could be improved by 50% compared with the conventional one. The load transient response time is only 10 μs.
Li et al. (2016) studied this question.