Years of Piezoelectric Transformers . Trends In The Technology

The initial concept of a piezoelectric transformer (PT) was proposed by C.A. Rosen, K. Fish, and H.C. Rothenberg and is described in the U.S. Patent 2,830,274, applied for in 1954. Fifty years later, this technology has become one of the most promising alternatives for replacing the magnetic transformers in a wide range of applications. Piezoelectric transformers convert electrical energy into electrical energy by using acoustic energy. These devices are typically manufactured using piezoelectric ceramic materials that vibrate in resonance. With appropriate designs it is possible to step-up and step-down the voltage between the input and output of the piezoelectric transformer, without making use of wires or any magnetic materials. This technology did not reach commercial success until early the 90s. During this period, several companies, mainly in Japan, decided to introduce PTs for applications requiring small size, high step-up voltages, and low electromagnetic interference (EMI) signature. These PTs were developed based on optimizations of the initial Rosen concept, and thus typically referred to as “Rosen-type PTs”. Today’s, PTs are used for backlighting LCD displays in notebook computers, PDAs, and other handheld devices. The PT yearly sales estimate was about over 20 millions in 2000 and industry sources report that production of piezoelectric transformers in Japan is growing steadily at a rate of 10% annually. The reliability achieved in LCD applications and the advances in the related technologies (materials, driving circuitry, housing and manufacturing) have currently spurred enormous interest and confidence in expanding this technology to other fields of application. This, consequently, is expanding the business opportunities for PTs. Currently, the industry trend is moving in two directions: low-cost product market and valueadded product market. Prices of PTs have been declining in recent years, and this trend is expected to continue. Soon (if not already), this technology will become a serious candidate for replacing the magnetic transformers in cost-sensitive applications. Currently, leading makers are reportedly focusing on more value-added products. Two of the key value-added areas are miniaturization and higher output power. Piezoelectric transformers for power applications require lower output impedances, high power capabilities and high efficiency under step-down conditions. Among the different PT designs proposed as alternatives to the classical Rosen configuration, Transoner laminated radial PT has been demonstrated as the most promising technology for achieving high power levels. Higher powers than 100W, with power densities in the range of 30-40 W/cm2 have been demonstrated. Micro-PTs are currently being developed with sizes of less than 5mm diameter and 1mm thickness allowing up to 0.5W power transfer and up to 50 times gain. Smaller sizes could be in the future integrated to power MEMs systems. This paper summarizes the state of the art on the PT technology and introduces the current trends of this industry. HISTORICAL INTRODUCTION It has been 50 years since the development of piezoelectric ceramic transformers began. The first 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 [1]. 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 [2]. Since then, the PT technology has been growing simultaneously with the progress in piezoceramic technology as well as with the electronics in general. Currently, it is estimated that 25-30 millions of PTs are annually sold commercially for different applications. Thus, the growth of the technology is promising and is expected to expand to many other areas as an alternative to magnetic transformers. In attempt to be historically accurate, it is required to mention that the first studies on PTs initially took place in the late 20s and early 30s. Based on the research of the author of this paper, Alexander McLean Nicolson has the honor of being the first researcher to consider the idea of a piezoelectric transformer. In his patent US1829234 titled “Piezo-electric crystal transformer” [3], Nicolson describes the first research in this field. The work of Nicolson on piezoelectric transformers, recognized in several other patents [4], was limited to the use of piezoelectric crystals with obvious limitations in performance, design and applicability as compared to the later developed piezoceramic materials. Piezoelectric transformers (from now on referred to as piezoelectric ceramic 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. Piezoelectric transformers were initially considered as high voltage transformer devices. Two different designs driving the initial steps in the development on these “conventional” PTs were, the so-called Rosen-type PT designs and the contour extensional mode uni-poled PTs. Until early in 90s, the technology evolution was based on improvements in these two basic designs. Although Rosen proposed several types of PT embodiments in his patents and publications, the name of “Rosen-type PT” currently refers to those PTs representing an evolution on the initial rectangular design idea proposed by C. Rosen in 1954, as shown in Figure 1.