12.6 A 64.4% Efficiency 5.8GHz RF Wireless Power Transfer Receiver with GaAs E-pHEMT Rectifier and 45.2µs MPPT Time SIDITO Buck-Boost Converter Using VOC Prediction Scheme

RF wireless power transfer (WPT) system enables long-distance power transfer of $1 \sim 10 \mathrm{~m}$, eliminating the need for power supply wiring to a huge number of sensors in the loT era. Recently, above $1 \mathrm{~W}$-class power receiver circuits (RX) are becoming required for short-time battery charging and high-performance sensor nodes. As shown in Fig. 12.6.1, the RX RF input power $P_{\text {IN }}$ varies significantly $(10 \sim 30 \mathrm{dBm})$ depending on the distance from the TX, and the output voltage of the rectifier widely fluctuates accordingly. Recently, rectifiers fabricated by GaAs enhancement-mode pseudomorphic high electron mobility transistor (E-pHEMT) monolithic microwave IC (MMIC) process that achieve maximum efficiency of $70 \sim 85 \%$ have been introduced [1]. However, the internal impedance of these rectifier is as high as several $100 \Omega$, and at $30 \mathrm{dBm} \mathrm{P}_{\mathrm{IN}}$, the output voltage reaches high voltage ($15 \mathrm{~V}$ in this work) at the maximum power point (MPP). This is near the breakdown voltage of the rectifier itself and since the open circuit voltage $\mathrm{V}_{\text {OC }}$ is nearly twice that $(\sim 30 \mathrm{~V})$, the rectifier cannot be opened. In addition, because this WPT system uses a beamforming technique to charge multiple RXs in a time-division manner, the system quickly switches the RX target on the order of few ms. The individual RX circuits have off-charging times of several $10 \mathrm{~ms}$ to several seconds, and $P_{\text {IN }}$ increases rapidly when the beam is directed. In addition, since the RX target may move, the DCDC converter connected to the rectifier must update the maximum power point tracking (MPPT) target voltage every few ms, thus the MPPT time must be fast enough (less than 100µs) to maintain system efficiency. However, existing reports of WPT RX with RF rectifiers and DC-DC converters are mostly aimed at energy harvesting, so the $P_{\text {IN }}$ level is low and the MPPT time is long [2]–[5]. In addition, the rectifier configuration is designed for frequencies in the 900MHz band or lower, which not only increases the antenna size, but also results in low efficiency at 5.8GHz due to substrate losses. There is a DC-DC converter with high input voltage and relatively short MPPT time [6], but it is specialized for PV cell applications and cannot be applied to RF rectifiers as a DC current sense FET is required between the PV cell cathode and ground. Therefore, a high-voltage input, fast $\mathrm{V}_{\text {OC }}$ prediction MPPT-controlled single-inductor dual-input triple-output (SIDITO) buck-boost (BB) converter is developed and a WPT RX system using this DCDC converter together with a GaAs E-pHEMT rectifier MMIC is demonstrated.