100 MHz Symmetric Current-Mode Class D Wireless Power Transfer

Power conversion for wireless power transfer (WPT) at 100 MHz is important for miniaturization and integration, which is critical for charging wearable consumer electronics and devices for the Internet of Things (IoT). Design and implementation at this frequency are challenging in the selection of components and circuit configurations for operation over a wide load range. The optimization of the tuned compensation networks together with a step-by-step guide for a systematic and rigorous optimal design is derived in this article from fundamental circuit models for symmetric current-mode class D (CMCD) WPT systems. Symmetric CMCD WPT systems can have inherently constant output current for a wide load range with small current ripple for the optimal design, which is useful in direct battery charging applications. Both uncontrolled and synchronous rectifiers are analyzed, demonstrated, and compared in hardware with the optimal designs. A 100MHz prototype with an output peak power of 6.9 W has a power circuit dc–dc efficiency of 81.9% despite only natural convection cooling of miniaturized GaN devices. The prototype maintains nearly constant power circuit efficiency above 80.5% with a peak of 83.2% over a wide power range that spans from 1.7 to 6.9 W.