# Piezoelectric Transformers: An Historical Review
## 1 Historical Introduction
### Beyond the 2000s: Power Piezolectric Transformers
However, it was found that their efficiency was low, in the high 80% range, and their integration in DC-DC converters was difficult due to limitations relating to efficient driving circuits (especially field-effect transistor, FETs) for such frequency and large output power. In 1997, NEC changed its focus to a different type of piezoelectric transformer, the longitudinal vibration mode transformer shown in Figure 14[71]. Figure 14: Piezoelectric transformer using longitudinal and transverse mode vibration [72]. Figure 13: Thickness mode piezoelectric transformer by NEC, 1992 [61; 62]. The longitudinal transformer provided better efficiency than the thickness mode transformer. In addition, the multilayer construction of both the input and output allowed better design flexibility for AC-DC adapter integration [72; 73; 74; 75; 76; 77]. Toward the mid-1990s, Face Electronics, Norfolk, VA, USA, invented a new concept of power PT, Transoner(r) (registered name by Face Electronics, Figure 15), based on a laminated construction [78]. The Transoner radial operation and a symmetric design was included in a second patent by [PERSON] [79]. The power density, design flexibility, manufacturing simplicity and toughness of the Transoner have resulted in this design becoming the reference for power applications in many research publications. In the late 1990s and early 2000s, the VPEC at Virginia Tech began a characterization study of Transoner piezoelectric transformer, funded in part by a grant from Virginia's Center for Innovative Technology (CIT), and also by Face Electronics [80; 81]. The objective of this collaboration led to intensive research in the area of fluorescent ballast using piezoelectric transformers. Perhaps the most relevant outcome of this research was the development of a new inductor-less concept to drive piezoelectric transformers (Figure 16), which eliminated the input series inductor used in previous driving approached [82; 83; 84]. Also, late in the 1990s, shortly after the invention of Transoner, NEC switched the research on power PTs to a different type of PT, using a similar approach as the Transoner Face's radial piezoelectric transformer. The NEC approach used a square geometry with an internal radial electrode [85; 86], operated in a contour-extensional vibration mode (Figure 17). Figure 16: Inductor-less Piezoelectric Transformer-based Linear Ballast with Power Factor Correction using a Transoner radial piezoelectric transformer (Courtesy of Micromechatronics). Figure 17: Contour-mode piezoelectric transformer [86]. Figure 15: Different configurations of Transoner radial-type piezoelectric transformer: (**a**) square-shape Transoner; (**b**) disc-shape Transoner; (**c**) commercial sample of non-isolated Transoner piezoelectric transformer [78; 79]. The use of Transoner PTs and the concept of inductor-less driving techniques was followed by several other research groups, including Ben-Gurion University (Beersheba, Israel) of the Negev, Beer-Sheva, Israel [87; 88; 89]. Several other research groups have investigated other structures for step-down conversion. The unipoled PT, originally developed by Berlincourt in late 1960s, is one of the topologies that have also been considered by several researchers for step-down applications. In Europe, Alcatel Spain, Ferroperm, Denmark, the Universidad Politecnica de Madrid, Spain, and the Universidad de Oviedo, Spain, collaborated in the ESPRIT IV project \"TRAMST\" for the development of a piezoelectric AC-DC converter for mobile phone battery chargers. The PT design was based on a ring-shaped element operating in thickness vibrational mode, where the primary and secondary sections are separated by an isolation layer [90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100]. In the mid-2000s, Face Electronics, Fraunhofer Institute, led by [PERSON], and Infineon, collaborated in several research projects on the application of piezoelectric transformers, including the development of a control IC [101; 102; 103; 104; 105].