Valeria Bottarel

Analog Front End for Efficient Energy Harvesting from Piezoelectric Transducer

This work describes an Analog Front End for Efficient Harvesting. In addition to the description of the energy harvesting frontend circuitry for piezoelectric transducer, some autonomous system’s peculiar functionalities are presented. The system includes: low quiescent current voltage regulator, transducer energy detector that keeps the front end from dissipating when no energy is available to be harvested, and lithium battery charger functionality, in addition to the storage capacitance charging circuit for battery-less systems.
For what concerns harvesting circuit, the base idea is that if the electronics applies a low frequency – dc condition to the piezoelectric material, it interferes on the deformation of the piezoelectric material, actuating it. The presented frontend circuit allows piezoelectric deformation in open circuit condition and detects the maximum deflection point. It operates during the piezoelectric voltage peak, by short-circuiting the piezoelectric material output capacitance C with an external inductance L. The energy transfer occurs as a resonant LC transfer at sufficiently high electronic frequency, such that the electronic behavior cannot actuate the piezoelectric material due to mechanical inertia. After the energy transfer, the piezoelectric cantilever is left with the maximum mechanical deformation, but without any charge on it. This condition is a sort of pre-charge that allows for doubled voltage peak in the subsequent deformation. Alternatively, it is possible to harvest both negative and positive peaks through a full bridge. When the energy transfer is completed, the front-end circuit opens the LC ring and the inductor current flows through a diode to a storage element. Thus, the current on the inductance is transformed again, and stored as charge on a capacitance.
The current-output of this step-up converter front-end also allows the harvested voltage to be higher than the peak on the transducer.