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    $\mu \text{m}$    CMOS technology. Minimum harvestable input power is 600 nW and maximum power efficiency is 67.6% at 4.2    $\mu \text{W}$    input power. Power transmission through bovine tissue is measured to have negligible efficiency loss, making this technique amenable to implantable applications.
['Myungjoon Choi', 'Taekwang Jang', 'Junwon Jeong', 'Seokhyeon Jeong', 'David Blaauw', 'Dennis Sylvester']
A Resonant Current-Mode Wireless Power Receiver and Battery Charger With −32 dBm Sensitivity for Implantable Systems