# Piezoelectric Transformers: An Historical Review
## 1 Historical Introduction
### Beyond the 2000s: Power Piezolectric Transformers  
The evolution in the know-how of PT technology during the 1990s and first decade of 2000s inspired new developments beyond the CCFL backlighting in areas such as fluorescent ballasts, AC-DC battery adapters and, more recently, for AC-DC LED drivers. Compared to backlighting inverters, where the typical power level is 5 W and the output voltage is several hundred volts (step up), most DC-DC or AC-DC applications require several tenths to hundreds of watts with output voltage levels of just a few volts (step-down conversion). The Rosen PT topology is not suitable for those levels of power conversion since its design is prominent in high voltage generation. Thus, new types of PTs designs and revisions of some previously suggested were considered by researcher interested in power conversion applications. The effort was mainly led by two companies: Face Electronics in the U.S. and NEC in Japan. Other groups in Europe and Korea, with alternative proposals, followed these tendencies toward power applications. Strictly speaking, the first published research effort on power PTs was undertaken in 1966 by [PERSON] [60] of Sandia Laboratory (Albuquerque, NM, USA). [PERSON] analyzed the power capabilities of a PT consisting of two thin piezoelectric discs bonded to opposite sides of a metal wall and operating in the thickness mode (Figure 12). In spite of this earlier and mainly theoretical study, no other work on high-power PTs is mentioned until the 1990s. Early in the 1990s intense work was re-initiated in Japan by NEC on the same concept of thickness mode PTs (Figure 13) and its use for power applications. Some of the proposed thickness mode designs consisted of 1 to 2 mm-thick plates with input and output multilayer sections and operating at very high frequencies, such as 1 MHz or higher. [PERSON], [PERSON], [PERSON], among others at NEC, hold several of the patents for thickness mode PT [61; 62] designs. Figure 12: Thickness mode transformer suggested by [PERSON], Sandia Labs, in 1966 [60]. One of the main operational challenges of using a thickness mode is the large number of high order modes for the longitudinal or width extensional vibrations resulting in spurious modes near the thickness mode resonance frequency. The reason is related to the fact that PZT ceramics have high electromechanical coupling factor k\({}_{\text{t}}\) necessary for thickness operation, but also have large coupling factor k\({}_{31}\) for longitudinal and width vibrations. The k\({}_{31}\) large coupling factor results in a large number of high-order harmonics that couple with the thickness mode, leading to poor operational characteristics. To suppress the spurious resonances, based on the unstiffened piezoelectric effect, ceramic materials with large anisotropy were considered, in particular the PbTiO\({}_{3}\) ceramic material, where k\({}_{\text{t}}\) is larger than 50% and k\({}_{31}\) is less than 5% [63]. Simultaneously with the development and improvement in PT designs for power applications, intensive work, also by NEC, resulted in publications on circuit strategies to drive the PTs on power applications [64; 65; 66; 67]. During this period, activities were initiated at Virginia Power Electronic Center (VPEC) of Virginia Tech University, led by [PERSON], on converters using piezoelectric transformers [68; 69; 70]. These activities were in part the result of the collaboration between VPEC and NEC. In July 1993, [PERSON], a member of NEC team, joined as a visiting researcher the Virginia Power Electronic team at Virginia Tech [65]. He stayed for one year, until August 1994, and returned to Japan where he continued his work at NEC [71; 72] and published his doctoral dissertation in 1997 [73]. The evaluation of thickness expansion mode PTs by NEC demonstrated that these transformers can deliver higher net output power due to their higher driving frequency.