A 40.68MHz Dual-Output Wireless Power Transfer System Achieving 149.7mW Maximum Power and 90.3%/51.2% RX/E2E Efficiency with 8mm-Diameter RX Coil

Wireless Power Transfer (WPT) is a non-invasive solution for powering miniaturized battery-less bio-implants, such as retinal implants and neural interfaces [1]. For these applications, a WPT system should be not only compact and efficient at the receiver (RX) side but also long-lasting when paired with a wearable battery-operated transmitter (TX). Additionally, bio-implants often result in a loose and variable wireless link, as illustrated in Fig. 1, necessitating couplingadaptive IC interfaces. Most reported bio-WPT systems operate at 6.78MHz or 13.56MHz [2]–[12]. Though they can achieve high efficiencies and reduced BoM using techniques like single-stage regulating rectifiers, global regulation, and integrated data telemetry, they are fundamentally limited by large RX coils (typically centimetric) due to the low carrier frequencies. 40.68MHz WPT solutions have been explored in cutting-edge bio-implant studies [13], [14] (Fig. 1); however, they use passive rectifiers and low-dropout regulators at RX while keeping TX always activated. This leads to an increased risk of overheating at RX and a short battery life at TX, hindering practical applications. Prior dedicated 40.68MHz WPT work mostly focused on RX designs [15]–[17], where [15] reported the first regulating rectifier design in 2023 (Fig. 1). However, the lack of global regulation leaves the end-to-end (E2E) (TX-to-RX) performance still inferior, and the low voltage conversion ratio (VCR) (<1) of the full-bridge structure casts doubt on its practical loosely coupled performance.