180-nm CMOS Wideband Capacitor-Free Inductively Coupled Power Receiver and Charger

Wireless microsystems like biomedical implants and embedded sensors derive energy from tiny in-package sources that, unfortunately, exhaust easily, which means that operational life is short. Periodically coupling power wirelessly is one way of replenishing onboard batteries, except that small receiver coils suffer from low coupling factors k_C and induce low electromotive-force voltages. Today, receivers store and resonate incoming energy between the receiving coil and an off-chip capacitor until the voltage rises sufficiently high for a diode-bridge rectifier to steer power into a battery. The capacitor, however, requires board space and constrains the source to a particular frequency. The 180-nm CMOS power receiver presented in this paper removes the diode bridge, which establishes a minimum voltage below which the system cannot derive power, so that neither tuning nor a resonating capacitor is necessary. Experimental measurements show that the system draws power from 30-mV signals when k_C is 0.0046 and coil separation is 11.35 mm, and this threshold voltage only changes 13.6 mV across 100-150 kHz, which is a 27.1% lower threshold voltage that is 36 × less sensitive than its resonating counterpart. The peak efficiency of the receiver when rectifying to 1.2 V is 82% at 224 μW and 125 kHz and average efficiency is 76% for 90-386-mV coil voltages.