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Energy scavenging is a technology that derives required microwatt level power to drive electronics from ambient heat,
light, radio, or vibration, that is otherwise wasted. In this study, we have investigated the circuit optimization strategy for
vibration-based piezoelectric energy scavenging systems that supply the electric power to wireless sensors and
electronics for ubiquitous sensor networks. In this paper, we assert that not only the mechanical to electrical energy
conversion efficiency, but also the speed of energy storage is important in designing and evaluation of energy
scavenging system, since the energy scavenging system with higher speed of energy storage has faster start up response
to the mechanical input and can supply required power more frequently. Particularly in case of piezoelectric micropower
generator, we show that the low reverse leakage current characteristic rather than the low forward voltage drop of the
rectifying diodes is much more beneficial to improve conversion efficiency and that the storage capacitor should be as
small as possible to reduce the time to store required amount of energy. Experimental and PSPICE results show that,
when ultra-low leakage current diodes are adopted for a bridge rectifier, the mechanical-to-electrical energy conversion
efficiency is doubled or more in some cases, and the charging speed is increased by 20%, compared to the circuit that
adopts Schottky diodes. It is also found that, compared to the circuit with larger storage capacitor, the circuit with
smaller storage capacitor requires shorter time to store the same amount of energy and can drive voltage regulator more
frequently. This study suggests that low reverse leakage diode and small storage capacitor should be used to build
efficient high performance piezoelectric energy scavenging systems for ubiquitous sensor networks.
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