Recycling piezoelectric switch-inductor charger

Wireless microsystems that sense information about their surrounding environment can save money, energy, and lives. Piezoelectric transducers can harvest ambient vibration energy to reduce the cost and prolong lifetime of the microsystems. Outputting high power and maintaining maximum power-point is important for the charger to support functionality for the microsystems. The proposed recycling switched-inductor charger induces current at the highest voltage by keeping the piezoelectric voltage near the breakdown voltage of the CMOS charger to draw the highest power from the transducer. The charger senses when the piezoelectric voltage reaches breakdown voltage, and draws a portion of the energy to charge the battery. The direct inductor transfers allow the inductor to transfer more energy than it carries, saving Ohmic loss. With a single inductor and a single stage to charge the battery across its voltage range, the proposed recycling switched-inductor charger can deliver 0.1–91 µW from vibrating a 15 nF piezoelectric transducer at 120 Hz with 0.1 to 10.5 V open circuit voltage gain. The prototype can output net power at 80% smaller voltage than the state of the art, can output up to 12× higher power than the lossless bridge can draw from the same transducer, and can output 76% of the theoretical maximum power a transducer can produce.