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Piezoelectric Transformers - Technology Introduction

Piezoelectric Transformer

Resonant dc/dc and ac/dc converters have attracted a great deal of attention over the last decade. Their ability to operate efficiently at high frequencies allows for considerable miniaturization of power supplies. When considering isolated topologies, the use of high frequencies wire-wound and recently planar magnetic transformers are the solutions to provide voltage transformation and isolation between the primary and the secondary circuits. However, and in spite of the technological advancement in magnetic technology, the transformer continues to be the largest and bulkiest component in dc/dc and ac/dc isolated converters.

From a component perspective, the use of higher power density piezoelectric transformers (PTs) may allow a breakthrough to the existing state of the art of transformer leading to a more compact, lower profile converter designs.  

What is a Piezoelectric Transformer

Piezoelectric transformers, like magnetic devices, are basically energy converters. A magnetic transformer operates by converting electrical input to magnetic energy and then reconverting that magnetic energy back to electrical output. A PT has an analogous operating mechanism. It converts an electrical input into mechanical energy and subsequently reconverts this mechanical energy back to an electrical output. This mechanical conversion is achieved by a standing wave vibrating at a frequency equal to a multiple of the mechanical resonance frequency of the transformer body, which is typically in the range of 50 to 150 kHz. Recently, PTs operating at 1MHz and higher have also been proposed.

Historical Background

The first significant invention on piezoelectric transformers (PTs) has been traditionally associated with the patent of Charles A. Rosen et al., which was disclosed on January 4, 1954 and finally granted on April 8, 1958. Briefly after this first application, on September 17, 1956, H.Jaffe and Don A. Berlincourt, on behalf of the Clevite Companies, applied for the second patent on PT technology, which was granted on Jan. 24, 1961. Since then, the PT technology has been growing simultaneously with the progress in piezoelectric-ceramic technology as well as with electronics in general. You may find more interesting facts on the evolution of the piezoelectric transformer technology in the paper "50 years of Piezoelectric Transformer. Trends in the Technology", presented at the 2003 Fall Meeting Proceedings.

Evolution of the technology

In spite of its relatively early development, piezoelectric transformer technology and the multiple targeted applications considered during the 70s and 80s suffered from serious challenges related to: i) immature materials fabrication technology; (ii) mechanical reliability problems at the nodal point; (iii) under-developed driving circuits. These problems created a barrier for technological advancement till the early 90’s. The 90’s saw piezoelectric manufacturing companies amending a revision in the concept of PT taking advantage the improvements in novel piezoelectric materials, more reliable manufacturing technologies like multilayer co-fired processes, new concepts in integrated circuits and housing solutions for the PTs. Furthermore, the technology development was accelerated thanks to the market demand of alternative products to magnetic transformers to be use in the backlighting of CCFL (cold cathode fluorescent lamps) for color liquid crystal displays (LCD).

Currently, PTs are used in the high voltage backlighting inverter of laptop computers and flat-panel TV displays. This type of PTs are so-called, “Rosen-type” PTs, since their structure follow the design envisioned by C.A. Rosen back in the 50s. It is estimated that 20-30 millions of PTs are annually sold commercially for different applications, mainly of the Rosen-type PTs with an excellent demonstrated reliability. Given the technical and commercial success demonstrated by the piezoelectric technology, significant interest has been demonstrated in other fields of electronics, such as higher power applications, mainly in the area of power distribution systems (ex: AC-DC adapters, DC-DC power distribution converters, etc.). This interests and the fact that Rosen-type PTs has a limited capability to deliver higher powers, has motivated the industry to develop new PTs topologies that be better adapted to higher power. In the middle 90s, Face developed "Transoner®", a high power piezoelectric transformer that can be applied with high efficiency and power levels to step-down applications.

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