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Piezoelectric Transformers
behoward writes "Tired of those big honking power adapters needed for all your portable toys? Me too! So far the best solution I've seen is the AC adapter for my G4 Powerbook, a sleek combo yoyo/flying saucer; but while it looks good on the desktop, it still makes too big of a bulge in my case when I travel. Now engineers at Penn State are working on a real solution. Hope Apple gets this technology in place in time for the G5 Powerbook."
I am doing some graduate studies in this field, so here's a quick breakdown.
Power electronic circuits have traditionally been based on magnetic technology, and until recently, have not been part of the tide of miniaturization and integration advances from which signal-processing integrated circuits have benefited. In an effort to miniaturize power components, acoustic rather than inductive coupling can be used as the basis for a transformer. Note that acoustic coupling can be achieved through piezoelectric *or* magnetostrictive means.
In a piezoelectric transformer, the direct and converse piezoelectric effects are used to acoustically transform power from one voltage and current level to another through a vibrating structure. The converse piezoelectric effect, in which an applied electric field produces a resulting strain in a body, is used to convert an oscillating electric field applied to the left half of a structure, such as a bar, into a vibrational mode of the entire bar. If driven at resonance, standing-wave distributions of large amplitudes of stress and strain result. The resonantly amplified strain in the right half of each bar is converted to a voltage across the output terminals by the direct piezoelectric effect. Depending upon geometry and materials parameters, you can obtain voltage amplifications of various magnitudes, with associated step-downs in current level.
The unique nature of the piezoelectric transformer offers the opportunity for innovative circuit design such as operating above resonance for inductive behavior to achieve soft-switching without compensating inductors.