Efficient Power Transfers in Piezoelectric Energy-Harvesting Switched-Inductor Chargers

Piezoelectric -harvesting transducers can draw power from motion that can be used to energize a host of wireless microsystems that sense, process, and share vital information across a network. Since these tiny devices draw little power, maximizing the power they transfer is critical. Synchronized-discharge circuits are popular in this space because they can collect all the charge from the transducer and increase drawn power. Pre-charging the transducer can draw even more power, but only as much as voltage-breakdown limits allow. This brief examines how synchronous discharge circuits can output the most power possible from tiny difficult-to-overdamp piezoelectric transducers. Measurements will show how pre-charge symmetry and energy-transfer schemes alter the power drawn from the transducer and the power lost in the system. The charges are tested with a 10×50×1-mm
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15-nF transducer generates up to 10 μA at 100 Hz and the breakdown voltage of the harvester circuit is 3 V. Indirect and direct transfers with symmetric pre-charges output 21% and 43% more power with the same inductor than the 4.2 μW that indirect transfers without pre-charge can generate. Generating the highest power possible is important because, with more power, tiny piezoelectric transducers can power wireless microsensors with greater functionality and longer lifetime.