27.2 A 6.78-MHz 79.5%-Peak-Efficiency Wireless Power Transfer System using a Wireless Mode-Recognition Technique and a Fully-On/off Class-D Power Amplifier
Junfei Ge, Yu Lu, Ruoshu Yang, Dongfang Pan, Lin Cheng
Abstract
Wireless power transfer (WPT) systems are increasingly favored for applications such as implantable devices and portable electronics. In these systems, local voltage regulation on the receiver (RX) side ensures a well-regulated output voltage (V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</inf> ) under varied loading and coupling conditions. Meanwhile, global power regulation on the transmitter (TX) side improves end-to-end (E2E) efficiency, especially at light loads. For global power regulation, backscattering using load-shift keying (LSK) techniques is commonly adopted [1–6]. However, methods in [1, 2] necessitate an extra sensing coil and a data demodulator including multiple off-chip components to recover mode-switching signal containing full duty-cycle information of the RX in the TX, increasing costs and leading to poor transient responses. A constant-idle-time control is proposed in [3] to eliminate most of the off-chip components except the sensing coil by transmitting just one edge of the mode-switching signal to deactivate the TX for a fixed off-time. Hence, V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</inf> is only regulated by an upper boundary, resulting in poor load regulation with limited output power. A phase-locked loop is further introduced in [4] to ensure an adaptive off-time for a fixed mode-switching frequency, yet it worsens load regulation. In [5], a fully-integrated wireless hysteretic control sends the entire duty cycle back to the TX for tight regulation of V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</inf> . However, the light-load efficiency is compromised due to an always-active power amplifier (PA) in the TX and an inefficient linear current-sink voltage regulator in the RX. Additionally, the unpredictable V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</inf> noise spectrum limits its utility. Lastly, the fully-integrated wireless phase-shift control technique proposed in [6] relies on detecting a phase shift between coil current and voltage, preventing the full deactivation of the PA and thus also degrading the light-load efficiency.