Wireless power transfer systems in commercial electronic devices often employ duty cycle control for enhanced load regulation capabilities. When the transmitter operates in deep duty cycle mode, e.g. under light load, power receivers based on synchronous rectifiers can exhibit significant voltage ringings on the ac nodes. For receiver-side in-band communication demodulation, e.g. frequency-shift keying as regulated in WPC Qi standards, these transient ringings can be particularly concerning as they can cause data decoding errors, resulting in frequent disconnections. While there is a clear need to better understand the ac ringing patterns, most existing literature provide sets of transcendental equations that have no easy analytical solutions. This paper introduces a modeling approach for ringings in duty cycle mode, together with the simplifying assumptions that allow derivations of the ringing waveform without overly complex timing analysis. Good matching is achieved between the analytical predictions, circuit simulations, and experimental results. The proposed method enables highlighting of likely problematic operating scenarios for enhanced in-band communication robustness.
