27.4 A 13.56MHz Wireless Power Transfer System with Hybrid Voltage-/Current-Mode Receiver and Global Digital-PWM Regulation Achieving 150% Transfer Range Extension and 72.3% End-to-End Efficiency

Wireless power transfer (WPT) shows extensive applications in sub-100mW batteryless biomedical implantable devices, where the robustness against variations in coupling and load conditions is still an unsolved and crucial challenge. Typically, a voltage-mode (VM) full-bridge rectifier (FBR) or voltage doubler (VD) is employed at the receiver (RX) side. Though they achieve remarkable power conversion efficiencies (PCE) with delay compensation techniques [1, 2], their limited voltage conversion ratios (VCR) are unfavorable in weak coupling cases. [3] presents a current-mode (CM) RX to build up $A C$ power before rectification, resulting in a higher VCR and a wider input range; however, it shows a degraded PCE and over-power risks in strong coupling cases. On the other hand, a WPT system necessitates not only voltage but also power regulations to achieve optimal end-to-end (E2E) performance. Many prior efforts focus solely on the regulations at RX with full-power emission from the transmitter (TX) [1], leading to low E2E efficiencies and undesirable overheating. Regulations at both TX and RX are achieved in [4–6]; however, their TX regulations rely on analog loops, giving rise to loop stability issues and slow transient recovery. They also demand a discrete $3^{\text {rd }}$ coil for demodulating load-shift-keying (LSK) signals from RX, increasing system cost/volume. [7] presents a fully integrated global hysteretic control achieving faster transient response; nonetheless, it employs dual LSK demodulation per TX regulation cycle, limiting adjustable power range and escalating LSK error rate. Further, a power-starving high-speed current sensor is required for the on-chip LSK demodulation.