A Resonant Current-Mode Wireless Power Receiver and Battery Charger With −32 dBm Sensitivity for Implantable Systems

TLDR

Wireless power transfer for implantable systems must harvest very low power levels due to limited incident power on human tissues and the small size of the receiver coil. This study proposes resonant current‑mode charging to lower the minimum harvestable input power and boost power efficiency at low input power levels. The approach uses an LC tank resonator that accumulates energy over multiple cycles and directly delivers inductor current to charge a battery, implemented in a 0.18 µm CMOS prototype without rectification or voltage regulation. The receiver achieves a minimum harvestable input power of 600 nW and a peak efficiency of 67.6 % at 4.2 µW, with negligible efficiency loss when transmitting through bovine tissue, indicating suitability for implantable applications.

Abstract

Wireless power transfer for implantable systems must harvest very low power levels due to low incident power on human tissues and a small receiver coil size. This work proposes resonant current-mode charging to reduce minimum harvestable input power and increase power efficiency at low input power levels. Avoiding rectification and voltage regulation from conventional voltage-mode methods, this work resonates an LC tank for multiple cycles to build up energy, then directly charges a battery with inductor current. A prototype is fabricated in 0.18 μm CMOS technology. Minimum harvestable input power is 600 nW and maximum power efficiency is 67.6% at 4.2 μW input power. Power transmission through bovine tissue is measured to have negligible efficiency loss, making this technique amenable to implantable applications.