# Piezoelectric Transformers: An Historical Review
## 1 Historical Introduction
### 1990s-2000s: Commercial Introduction of PTs for CCFL Backlighting  
In the late 1980s, several Japanese companies, including NEC Corp. (Kawasaki, Japan), Tokin Corp. (Sendai, Japan), Tamura Corp. (Tokyo, Japan), Matsushita Electric Industrial Co. Ltd., Nihon Cement Kabushiki Kaisha (Tokyo, Japan) and others, reconsidered the use of PTs, taking advantage of improvements in novel piezoelectric materials, more reliable manufacturing technology, including multilayer co-firing processes, new concepts on integrated circuits, and housing solutions. The main application was to generate high voltage for backlighting the small cold cathode fluorescent lamps (CCFLs) used in liquid crystal displays (LCDs) of the increasingly popular portable cell phones and computers. In this application, the piezoelectric transformer enabled thinner, lighter, and more efficient CCFL backlighting modules, thus allowing the reduction of LCD screen thickness and improving the battery life of portable devices. Due to its small size, the cost of the piezoelectric transformers was relatively low and competitive for this application, rapidly enabling high-volume production. During this period, many technical publications and patents (Figure 8a) were issued for modifications of [PERSON]'s initial concept, including variations of the design, driving circuitry, mounting solutions, and novel materials to enhance the performance of these devices [48; 49; 50; 51; 52; 53; 54; 55]. During the 1990s, CCFL technology enjoyed a significant commercial boom for compact, portable devices. By early the 2000s, it was estimated that 25%-30% of the produced CCFL backlighting circuits were made using piezoelectric transformer technology (Figure 8b). Figure 8: Evolution of PT technology during late 1980s to early 2000s based on author sources. (**a**) Number of filled patents in U.S. and Japan; (**b**) Production of piezoelectric transformers for cold cathode fluorescent lamps (CCFL) applications compared to total CCFL inverter production. Figure 7: Some representative applications considered for piezoelectric transformers during the 1970s and 1980s. (**a**) Gate drivers for solid state switches; (**b**) High voltage power supply for TV cathode-ray tube; (**c**) Application in small engine ignition. Companies like Toshiba (Tokyo, Japan), NEC, Hitachi (Tokyo, Japan), Panasonic (Osaka, Japan) and Apple (Cupertino, CA, USA) adopted PT-based backlighting in their laptops. Figure 9 shows a commercial piezoelectric inverters for CCFLs integrated in the Apple MacBook Pro 15\" and 17\" LCD screen computers during the mid-2000s. In addition to the improvements in the manufacturing process of PTs, developments in the driving techniques for PTs were critical for their commercial success. Two relevant examples are the use of resonant converter topologies for driving the PTs and the use of integrated control circuits to provide stable operation to the PT under different operation conditions. Resonant converters were originally considered in the early 1990s as an extension of the general approach undertaken in power electronics to use high frequency switching circuits to minimize the size of DC-DC and AC-DC converters. Numerous efforts were undertaken in this area mainly with magnetic components [56]. In general, to drive a PT, either a sine wave or a square wave voltage can be used. A sine wave voltage is preferred for minimizing the circulating energy through the shunt input capacitance characteristic of the input of the PT. However, generating a sine wave requires more reactive components in the converter than generating a square wave. With resonant converters, the sine input waveform is generated by using a square wave voltage in combination with an inductive element in series with the transformer. The control of the ON/OFF of the switches of the converter is selected to minimize the switching losses. This technique is the so-called Zero Voltage Switching, ZVS [57].