Microfabricated Bulk Piezoelectric Transformers

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Date

2017

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Abstract

Piezoelectric voltage transformers (PTs) can be used to transform an input

voltage into a different, required output voltage needed in electronic and electro-

mechanical systems, among other varied uses. On the macro scale, they have been

commercialized in electronics powering consumer laptop liquid crystal displays, and

compete with an older, more prevalent technology, inductive electromagnetic volt-

age transformers (EMTs). The present work investigates PTs on smaller size scales

that are currently in the academic research sphere, with an eye towards applications

including micro-robotics and other small-scale electronic and electromechanical sys-

tems. PTs and EMTs are compared on the basis of power and energy density, with

PTs trending towards higher values of power and energy density, comparatively,

indicating their suitability for small-scale systems. Among PT topologies, bulk

disc-type PTs, operating in their fundamental radial extension mode, and free-free

beam PTs, operating in their fundamental length extensional mode, are good can-

didates for microfabrication and are considered here. Analytical modeling based on

the Extended Hamilton Method is used to predict device performance and integrate

mechanical tethering as a boundary condition. This model differs from previous PT

models in that the electric enthalpy is used to derive constituent equations of motion

with Hamilton’s Method, and therefore this approach is also more generally applica-

ble to other piezoelectric systems outside of the present work. Prototype devices are

microfabricated using a two mask process consisting of traditional photolithography

combined with micropowder blasting, and are tested with various output electri-

cal loads. 4mm diameter tethered disc PTs on the order of .002cm^3 , two orders

smaller than the bulk PT literature, had the following performance: a prototype

with electrode area ratio (input area / output area) = 1 had peak gain of 2.3 (±

0.1), efficiency of 33 (± 0.1)% and output power density of 51.3 (± 4.0)W cm^-3 (for

output power of 80 (± 6)mW) at 1MΩ load, for an input voltage range of 3V-6V (±

one standard deviation). The gain results are similar to those of several much larger

bulk devices in the literature, but the efficiencies of the present devices are lower.

Rectangular topology, free-free beam devices were also microfabricated across 3 or-

ders of scale by volume, with the smallest device on the order of .00002cm^3 . These

devices exhibited higher quality factors and efficiencies, in some cases, compared to

circular devices, but lower peak gain (by roughly 1/2 ). Limitations of the microfab-

rication process are determined, and future work is proposed. Overall, the devices

fabricated in the present work show promise for integration into small-scale engi-

neered systems, but improvements can be made in efficiency, and potentially voltage

gain, depending on the application

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