Adaptive 6.78-MHz ISM band wireless charging for small form factor receivers

This paper presents a low cost wireless power transfer system tailored particularly for size limited insulation sealed wearable devices. The 5-V USB input system features a crystal oscillator as reference signal source, a class-E power stage, a transmit coil with tunable series resonance and a feedback path to tune the resonance for peak power transmission. Regardless of the receive coil, load, component variations or any external blockers, the system continuously targets peak power at the strict center frequency of 6.78 MHz for compliance with international frequency regulations. Design choices and experimental measurement results were presented along with detailed circuit diagrams. The design can transfer around 50mW power to a 15mm × 17mm × 1mm 40-turn receive coil in a charging pad area of as wide as 9cm × 9cm.