A variable-capacitance energy harvester with miniaturized inductor targeting implantable devices

Many variable-capacitance energy harvesters employ a large inductor to improve their power efficiency by reducing conduction losses, which is sub-optimal in applications requiring a small form-factor, such as in implants. This paper describes a variable-capacitance harvester that performs optimally using miniaturized inductors. The impact of scaling the inductor on the generated energy of conventional semi-synchronous and fully-synchronous charge-constraint topologies is investigated analytically as well as experimentally. It is shown that the proposed harvester outperforms the semi-synchronous and fully-synchronous charge-constraint harvesters while using very small inductance values. Using two reservoir capacitors to generate energy without requiring large inductors, as well as utilizing a different switching scheme are the main factors contributing to this advantage. Since harvesting energy from slow moving mechanical sources, such as body movements, constitutes a major challenge, all three harvesters are implemented and tested with an actuating frequency as low as 0.5 Hz and for inductance values between 1 uH to 1 mH. The experimental results for sample designs corroborate the analytical expressions and show that to generate optimal harvested energy of the proposed harvester, the semi-synchronous harvester requires a 15.6 times larger inductor.