Review

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Piezoelectric Transformers: An Historical Review Review Alfredo Vazquez Carazo

Piezoelectric Transformers: An Historical Review

Micromechatronics, Inc., 200 Innovation Blvd. Suite 155, State College, PA 16803, USA; [email protected]; Tel.: +1-814-861-5688; Fax: +1-814-861-1418 Alfredo Vazquez Carazo Micromechatronics, Inc.,Uchino 200 Innovation Blvd. Suite 155, State College, PA 16803, USA; [email protected]; Academic Editor: Kenji Tel.: +1-814-861-5688; Fax: +1-814-861-1418 Received: 15 November 2015; Accepted: 20 April 2016; Published: date Academic Editor: Kenji Uchino Abstract: Piezoelectric transformers (PTs) solid-state Received: 15 November 2015; Accepted: 20 Aprilare 2016; Published:devices 26 Aprilthat 2016transform electrical energy

into electrical energy by means of a mechanical vibration. These devices are manufactured using Abstract: Piezoelectric transformers (PTs) are solid-state devices that transform electrical energy piezoelectric materials that are driven at resonance. With appropriate design and circuitry, it is into electrical energy by means of a mechanical vibration. These devices are manufactured using possible to step up and step down the voltages between the input and output sections of the piezoelectric materials that are driven at resonance. With appropriate design and circuitry, it is piezoelectric transformer, without making use of magnetic materials and obtaining excellent possible to step up and step down the voltages between the input and output sections of the conversion efficiencies. The initial concept of a piezoelectric ceramic transformer was proposed by piezoelectric transformer, without making use of magnetic materials and obtaining excellent Charles A. Rosen in 1954. Since then, the evolution of piezoelectric transformers through history has conversion efficiencies. The initial concept of a piezoelectric ceramic transformer was proposed been linked to the relevant work of some excellent researchers as well as to the evolution in by Charles A. Rosen in 1954. Since then, the evolution of piezoelectric transformers through history materials, manufacturing processes, and driving circuit techniques. This paper summarizes the has been linked to the relevant work of some excellent researchers as well as to the evolution in historical evolution of the technology. materials, manufacturing processes, and driving circuit techniques. This paper summarizes the historical evolution of the technology. Keywords: piezoelectric transformer; piezoelectric-based power supply; non-magnetic transformers Keywords: piezoelectric transformer; piezoelectric-based power supply; non-magnetic transformers

1. Historical Introduction 1. Historical Introduction 1.1. Using Salt Salt Rochelle Rochelle Single Single Crystal Crystal 1.1. 1920s–1930s: 1920s–1930s: Early Early Studies Studies on on Piezoelectric Piezoelectric Transformers Transformers Using The first studies studies on on electrical electrical to to electrical electrical conversion conversion using using piezoelectric piezoelectric materials materials took The first took place place in in the late 1920s and early 1930s. Alexander McLean Nicolson has the honor of being the first researcher the late 1920s and early 1930s. Alexander McLean Nicolson has the honor of being the first researcher to investigate the idea of of aa piezoelectric piezoelectric transformer. transformer. Nicolson Nicolson considered considered two two single single crystal crystal blocks blocks to investigate the idea preloaded against against each each other other with with an an external external frame frame (Figure (Figure 1). 1). preloaded

Piezoelectric crystals

Representative sketch sketch of the work of Alexander Mclean Nicolson on piezoelectric crystal Figure 1. Representative transformers [1].

One of the crystal elements was driven with an input electric signal generating a vibration in www.mdpi.com/journal/actuators the crystal. The second crystal element was used to convert the vibration back to an electric energy.

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Actuators 2016, 5, 12crystal elements was driven with an input electric signal generating a vibration in 2 ofthe 22 One of the

crystal. The second crystal element was used to convert the vibration back to an electric energy. The ratio of input to output electric energy was measured to evaluate the energy conversion. The work of The ratio of input to output electric energy was measured to evaluate the energy conversion. The work Nicolson on piezoelectric transformers, disclosed in multiple patents [1–3], was limited to the use of of Nicolson on piezoelectric transformers, disclosed in multiple patents [1–3], was limited to the use of Salt Rochelle single crystals, the only extensively available material at the time. The use of this Salt Rochelle single crystals, the only extensively available material at the time. The use of this material material led to obvious limitations in performance, design and applicability as compared to the later led to obvious limitations in performance, design and applicability as compared to the later developed developed piezoceramic materials. piezoceramic materials. 1.2. 1940s Work at at General General Electric, Electric, Syracuse, Syracuse, NY, NY, with with BaTiO BaTiO3 1.2. 1940s to to Early Early 1960s: 1960s: Charles Charles A. A. Rosen’s Rosen’s Work 3 Ferroelectric Ceramics Ceramics Ferroelectric Prior to Prior to about about 1940, 1940, only only two two types types of of ferroelectrics ferroelectrics were were known, known, Salt Salt Rochelle Rochelle and and some some closely closely related tartrates, tartrates, and and potassium potassium dihydrogen dihydrogen phosphate phosphate and and its its isomorphs isomorphs [4]. [4]. That changed in in 1942 1942 related That changed with the announcement by Wainer and Salomon [5] of barium titanate (BaTiO 3 ) as a new ferroelectric with the announcement by Wainer and Salomon [5] of barium titanate (BaTiO3 ) as a new ferroelectric ceramic. This associated with: the electrical electrical poling poling process process ceramic. This discovery discovery came came to to be be associated with: (i) (i) the the discovery discovery of of the in polycrystaline polycrystaline ceramics Roberts [6,7]; (ii) the the development development of of the the systematics systematics of of the the in ceramics by by Gary Gary and and Roberts [6,7]; (ii) piezoelectric effect in polarized ceramics by Mason [8]; and (iii) the extraction of the first values for piezoelectric effect in polarized ceramics by Mason [8]; and (iii) the extraction of the first values for the the piezoelectric coefficients of BaTiO 3 by Hans Jaffe in 1948 [9]. piezoelectric coefficients of BaTiO 3 by Hans Jaffe in 1948 [9]. Ferroelectric ceramics were quickly significant advantages advantages for many Ferroelectric ceramics were quickly recognized recognized to to have have significant for many electromechanical applications, including ease of fabrication in desired shapes, ability to operate electromechanical applications, including ease of fabrication in desired shapes, ability to operate under high highhumidity, humidity, control of response through selection of polarization directions, a high under control of response through selection of polarization directions, a high dielectric dielectric constant, high electromechanical coupling, and potential low cost. Furthermore, the constant, high electromechanical coupling, and potential low cost. Furthermore, the physical properties physical properties the ceramics could be the use and of additive materials and of the ceramics couldofbe altered selectively byaltered the useselectively of additiveby materials by variations in firing by variations in to firing procedures so as to produce theparticular characteristics desiredDuring for particular procedures so as produce the characteristics desired for applications. the late applications. During late BaTiO 1940s 3and through the 1950s, available, BaTiO3 became commercially available, 1940s and through thethe 1950s, became commercially triggering the proliferation of triggering theapplications. proliferation of piezoelectric applications. piezoelectric During this this period, period,Charles CharlesAbraham AbrahamRosen Rosen(Figure (Figure a young researcher joined During 2),2), a young researcher thatthat hadhad justjust joined the the team of the Electronics Laboratory at General Electric Company in Syracuse, NY, USA, started team of the Electronics Laboratory at General Electric Company in Syracuse, NY, USA, started working working on his research doctoral on research on piezoelectric transformers newly BaTiO available BaTiO3 and on his doctoral piezoelectric transformers using the using newlythe available 3 and applying applying the theoretical work developed by Mason. The impact of Rosen’s work in piezoelectric the theoretical work developed by Mason. The impact of Rosen’s work in piezoelectric transformer transformerhas technology has been tremendous. work has been extensively technology been tremendous. Although hisAlthough work has his been extensively referenced, hisreferenced, figure has his figure has been rarely remembered. We want to use some lines of this paper as a tribute to his been rarely remembered. We want to use some lines of this paper as a tribute to his relevancy in relevancy in this field. this field.

Figure December 2002 2002 [10]. [10]. Figure 2. 2. Charles Charles Abraham Abraham Rosen, Rosen, 77 December December 1917–8 1917–8 December

Born in Canada, Charles Rosen [10,11] received a B.S. degree in Electrical Engineering at Cooper Born in Canada, Charles Rosen [10,11] received a B.S. degree in Electrical Engineering at Cooper Union, NY, USA, in 1940. He returned to Canada where he spent the period of World War II Union, NY, USA, in 1940. He returned to Canada where he spent the period of World War II supervising supervising a group testing Canadian fighter planes before they were sent to battle in Britain. After a group testing Canadian fighter planes before they were sent to battle in Britain. After the war, in 1950, the war, in 1950, he received his M. Eng. in Communications at McGill University in Montréal (QC, he received his M. Eng. in Communications at McGill University in Montréal (QC, Canada). Shortly Canada). Shortly after, he joined the General Electric Company (G.E.) team in Syracuse, NY, USA. after, he joined the General Electric Company (G.E.) team in Syracuse, NY, USA. While at G.E., Rosen

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While at at G.E., G.E., Rosen Rosen worked worked on on solid-state solid-state devices devices and and coauthored coauthored an an influential influential textbook textbook on on While transistors [12]. By that time, Rosen entered in the doctoral program at Syracuse University and transistors [12]. By that time,and Rosen entered an in influential the doctoral program at Syracuse[12]. University and worked on solid-state devices coauthored textbook on transistors By that time, developed an an extended extended work work that that established established the the foundation foundation on on piezoelectric piezoelectric ceramic ceramic transformer transformer developed Rosen entered in the doctoral program at Syracuse University and developed an extended work that technology. His His research research work on on this this area area is is gathered gathered in in several several U.S. U.S. Patents [13–15] [13–15] and and in in his his technology. established the foundationwork on piezoelectric ceramic transformer technology.Patents His research work on this doctoral work published in 1956 [16,17]. doctoral work published 1956 [16,17].[13–15] and in his doctoral work published in 1956 [16,17]. area is gathered in severalinU.S. Patents Unlike the the earlier earlier attempts attempts by by Nicolson Nicolson to make make piezoelectric piezoelectric transformers transformers using two two pieces of of Unlike Unlike the earlier attempts by Nicolson to to make piezoelectric transformers using using two pieces pieces of piezoelectric solid crystals bonded together and clamped with an external structure, Rosen's piezoelectric solid solidcrystals crystals bonded together and clamped with an external Rosen's piezoelectric bonded together and clamped with an external structure,structure, Rosen's approach approach involved involved the the use use of of aa single single rectangular rectangular block block of of BaTiO BaTiO33 polycrystalline polycrystalline ceramic ceramic comprising comprising approach involved the use of a single rectangular block of BaTiO3 polycrystalline ceramic comprising two two distinct distinct polarization polarization zones (Figure (Figure 3). 3). Rosen Rosen created the the two two zones zones by by applying applying separated separated two distinct polarization zones zones (Figure 3). Rosen created created the two zones by applying separated electrodes electrodes and different polarizing voltages to the halves of the block. Half of the block was polarized electrodes andpolarizing different polarizing voltages to the of theHalf block. of thewas block was polarized and different voltages to the halves ofhalves the block. of Half the block polarized in the in the longitudinal direction and the other half in the thickness direction. The total length of the the in the longitudinal direction thehalf other halfthickness in the thickness direction. The totaloflength of longitudinal direction and theand other in the direction. The total length the ceramic ceramic block block determines determines the the resonant frequency frequency of of the the transformer, transformer, while while the ratio ratio of of the the input input and and ceramic block determines the resonantresonant frequency of the transformer, while the ratiothe of the input and output output sections determines the voltage conversion. The concept was great but the results were poor, output determines the voltage conversion. The concept was great but thewere results were poor, sectionssections determines the voltage conversion. The concept was great but the results poor, because because it it was was very very difficult difficult to to make make aa bipolarized bipolarized block block that that would would not not break break into into pieces pieces when when the the because it was very difficult to make a bipolarized block that would not break into pieces when the alternating alternating current was applied. Rosen needed just one unit to prove his concept, but manufacturers alternating was applied. Rosen justtoone unithis to concept, prove hisbut concept, but manufacturers current wascurrent applied. Rosen needed justneeded one unit prove manufacturers had to be had to to be be able able to to produce produce them them reliably. reliably. had able to produce them reliably.

Figure 3. 3. Rectangular Rectangular piezoelectric piezoelectric transformer transformer presented presented by by Rosen Rosen in in 1954 1954 [13]. [13]. Figure Rectangular piezoelectric

The rectangular piezoelectric piezoelectric transformer of of Figure 33 is is traditionally known known as the the “Rosen-type” The The rectangular rectangular piezoelectric transformer transformer of Figure Figure 3 is traditionally traditionally known as as the“Rosen-type” “Rosen-type” transformer. Rosen presented several other PT configurations including a cylindrically-segmented transformer. other PT PT configurations configurations including including aa cylindrically-segmented cylindrically-segmented transformer. Rosen Rosen presented presented several several other PT, a radially radially polarized polarized disc disc PT, PT, and and aa ring-type ring-type PT PT with with double double polarization. polarization. This is is summarized in in PT, PT, aa radially polarized disc PT, and a ring-type PT with double polarization. This This is summarized summarized in Figure 4. However, none of these other embodiments have gained the attention of the rectangularFigure 4. However, However,none noneofofthese theseother other embodiments have gained attention of rectangular-type the rectangularFigure 4. embodiments have gained thethe attention of the type PT. PT. In In 1957, 1957, Charles Charles Rosen Rosen moved moved to to California California and and joined joined the the Stanford Stanford Research Research Institute Institute where where type PT. In 1957, Charles Rosen moved to California and joined the Stanford Research Institute where he he switched his research into artificial intelligence [10] and became a relevant figure in that field. he switched his research into artificial intelligence [10] and became a relevant figure in that field. switched his research into artificial intelligence [10] and became a relevant figure in that field. Rosen Rosen passed passed away away in in California California in in 2002, at at the age age of of 85 85 [10,11]. [10,11]. Rosen passed away in California in 2002, at2002, the agethe of 85 [10,11].

Figure 4. Other piezoelectric transformer embodiments presented by Rosen in 1954 [13]. Figure 4. 4. Other Other piezoelectric piezoelectric transformer transformer embodiments embodiments presented presented by by Rosen Rosen in in 1954 1954 [13]. [13]. Figure

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The work of Rosen was continued by Stephen W. Tehon at the University of Illinois (Urbana, IL, The work of Rosen was continued by Stephen W. Tehon at the University of Illinois (Urbana, IL, USA). Tehon, also a member of the Electronic Laboratory at General Electric Co. (Syracuse, NY, USA), USA). Tehon, also a member of the Electronic Laboratory at General Electric Co. (Syracuse, NY, USA), extended his work to new ferroelectric materials and magnetostrictive materials. His doctoral thesis extended his work to new ferroelectric materials and magnetostrictive materials. His doctoral thesis was published in 1958 [18]. From 1960 to 1962, the team of General Electric at Syracuse continued the was published in 1958 [18]. From 1960 to 1962, the team of General Electric at Syracuse continued the work of Rosen and Tehon on ceramic power transformers through a research contract granted by the work of Rosen and Tehon on ceramic power transformers through a research contract granted by the U.S. Navy [19]. U.S. Navy [19]. In 1959, H. W. Katz, also a member of the Electronic Laboratory at General Electric Co., edited In 1959, H. W. Katz, also a member of the Electronic Laboratory at General Electric Co., edited the book “Solid State Magnetic and Dielectric Devices” [20], with the collaboration of Rosen (by then the book “Solid State Magnetic and Dielectric Devices” [20], with the collaboration of Rosen (by then at Stanford Research Institute, Menlo Park, CA, USA) and Tehon, among others. This is the first at Stanford Research Institute, Menlo Park, CA, USA) and Tehon, among others. This is the first published book discussing piezoelectric transformer technology in detail and is still a key reference published book discussing piezoelectric transformer technology in detail and is still a key reference to to any researcher in this field. any researcher in this field. 1.3. 1.3. Late Late 1950s 1950s to to Late Late 1960s: 1960s: Clevite Clevite Corporation—Introduction Corporation—Introduction of of PZT PZT to to PT PT Technology Technology Most of the work of the General Electric team on piezoelectric transformers during the 1950s– Most of the work of the General Electric team on piezoelectric transformers during the 1950s–1960s 1960s was based on Barium Titanate, the commercially only commercially available ferroelectric material the was based on Barium Titanate, the only available ferroelectric material at theattime. time. This situation changed when, in 1954, Bernard Jaffe recognized the importance of the This situation changed when, in 1954, Bernard Jaffe recognized the importance of the morphotropic morphotropic phase boundary in the Lead Zirconate Lead Titanate (PZT) family of ceramics [21–23]. phase boundary in the Lead Zirconate Lead Titanate (PZT) family of ceramics [21–23]. Over the decade Over decade of the B. Jaffe was the head of the center ofdivision the piezoelectric of thethe 1960s, B. Jaffe was1960s, the head of the research center of research the piezoelectric at the thendivision Clevite at the then Clevite Corporation, Bedford, OH, USA, which later became known Vernitron Corporation, Bedford, OH, USA, which later became known as Vernitron Corporation,as and today is Corporation, and today is known as Morgan Ceramics. B. Jaffe’s studies were followed by more known as Morgan Ceramics. B. Jaffe’s studies were followed by more detailed studies by Hans Jaffe detailed studies Hans Jaffe (not R. a relative of B. Jaffe), William. R.(all Cooke, Jr., Don A. Berlincourt (not a relative of by B. Jaffe), William. Cooke, Jr., Don A. Berlincourt at Clevite Corporation) and (all at Clevite Corporation) and others, whichfamily led toof the of theOver entire others, which led to the evolution of the entire PZTevolution piezoceramics. thefamily decadeofofPZT the piezoceramics. Over the decade of the 1960s, the group led by B. Jaffe, at Clevite Corporation, were 1960s, the group led by B. Jaffe, at Clevite Corporation, were unquestionably world leaders in this unquestionably world leaders in this subject area [24–26]. subject area [24–26]. Briefly after publication of Rosen work, H. Jaffe D. and A. Berlincourt, on behalf of Clevite Briefly afterthe the publication of Rosen work, H.and Jaffe D. A. Berlincourt, onthe behalf of Companies, filled for the filled patentfor “Piezoelectric Ceramic Resonator” forResonator” use in voltage the Clevite Companies, the patent “Piezoelectric Ceramic for transformation use in voltage [27]. Strictly speaking, this speaking, patent was the field toofthe ceramic but the transformation [27]. Strictly thisaddressed patent wastoaddressed field ofresonators, ceramic resonators, configurations presented in this patent have also been used for piezoelectric transformers. The but the configurations presented in this patent have also been used for piezoelectric transformers. topology presented by H. and and Berlincourt consisted of a of disc or plate polarized onlyonly in the The topology presented byJaffe H. Jaffe Berlincourt consisted a disc or plate polarized in thickness direction and with two separated electrodes representing the input from the output the thickness direction and with two separated electrodes representing the input from the output as as illustrated illustrated in in Figure Figure 5. 5.

Figure 5. Unipoled Unipoled piezoelectric piezoelectric transformers transformers by Jaffe and Berlincourt [27]. Figure

This topologyhas has been extensively evaluated by other many other researchers, only for This topology been extensively evaluated by many researchers, not only fornot piezoelectric piezoelectric but more for extensively for ceramic filters and resonators. the most transformerstransformers but more extensively ceramic filters and resonators. Perhaps thePerhaps most appealing appealing characteristic of this topology is the single polarization required for the full ceramic, so the characteristic of this topology is the single polarization required for the full ceramic, so the electrodes electrodes shape can beby obtained partially the full electroded C. Munk shape can be obtained partiallybyetching theetching full electroded ceramic. Inceramic. 1965, E. In C.1965, MunkE.presented presented the equivalent circuit development for the radial modes of the unipoled disc-type the equivalent circuit development for the radial modes of the unipoled disc-type configuration [28]. configuration [28].

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In 1972, 1972,Berlincourt Berlincourtpatented patentedone oneofofthe thefirst firstspecific specific applications piezoelectric transformers In applications forfor piezoelectric transformers as as starter ballast gaseous discharge lamps [29]. In this patent, Berlincourt suggested a variety starter andand ballast forfor gaseous discharge lamps [29]. In this patent, Berlincourt suggested a variety of of configurations for piezoelectric transformers Figurethe6.different Amongconfigurations, the different configurations for piezoelectric transformers as shownasin shown Figure 6.inAmong configurations, of special relevancy configuration is the rectangular having two sections with of special relevancy is the rectangular havingconfiguration two sections with thickness polarization thickness (Figureof6a). The6b,c configurations Figure 6b,c correspond to the same (Figure 6a).polarization The configurations Figure correspond toofthe same topologies illustrated above in topologies Figure 5 asbyintroduced in Berlincourt. the 1960 patent by H. Jaffe and Berlincourt. Figure 5 as illustrated introducedabove in the in 1960 patent H. Jaffe and

Figure 6. 6. Configurations Configurations of of piezoelectric piezoelectric transformers transformers suggested suggested by by Berlincourt Berlincourt [29]. [29]. Figure

1.4. Late Late 1960s–1980s: 1960s–1980s: Period Period of of Proliferation Proliferation 1.4. Until the the mid-1960s, mid-1960s, piezoelectric piezoelectric transformers transformers were were not not practical practical due due to to the the lack lack of of materials materials Until able to operate with low mechanical losses at high amplitudes. This situation improved withwith the able to operate with low mechanical losses at high amplitudes. This situation improved introduction of hard-doped piezoelectric ceramic compositions, such as PZT8, showing high the introduction of hard-doped piezoelectric ceramic compositions, such as PZT8, showing high mechanical and and dielectric dielectric Q Q factors factors at at high high amplitudes. amplitudes. mechanical Several U.S. and Japanese companies, like Motorola, Inc. Inc. (Chicago, (Chicago, IL, Several U.S. and Japanese companies, like Motorola, IL, USA) USA) [30–32], [30–32], RCA RCA Corp. Corp. (Wilmington , DE , USA) [33,34], Denki Onkyo Co. Ltd. (Tokyo, Japan) [35–41], and Matsushita Electric (Wilmington, DE, USA) [33,34], Denki Onkyo Co. Ltd. (Tokyo, Japan) [35–41], and Matsushita Industrial Co. Ltd. Co. (Osaka, [42],Japan) focused their attention toattention PTs for generating high voltage Electric Industrial Ltd.Japan) (Osaka, [42], focused their to PTs forthe generating the required for the cathode-ray tubes in black and white television receivers (see Figure 7b forFigure a typical high voltage required for the cathode-ray tubes in black and white television receivers (see 7b example). Several attempts were also reported for using piezoelectric transformers as igniter in gasfor a typical example). Several attempts were also reported for using piezoelectric transformers as based stoves (Matsushita Industrial Ltd. [43]), small engine applications (Briggs & igniter in gas-based stoves Electric (Matsushita ElectricCo. Industrial Co.inLtd. [43]), in small engine applications Straton (Milwaukee, WI) [44], Figure 7a) and in automobiles (Nippon Soken, Nishio, Japan) In (Briggs & Straton (Milwaukee, WI) [44], Figure 7a) and in automobiles (Nippon Soken, [45]). Nishio, the 1980s, (Berlin, Germany) [46], Figure 7c, and General Electric (Schenectady, NY, USA) Japan) [45]).Siemens In the 1980s, Siemens (Berlin, Germany) [46], Figure 7c, and General Electric (Schenectady, [47] were among some companies working on the application of PTs for triggering power switch NY, USA) [47] were among some companies working on the application of PTs for triggering power gates such as triacs, thyristors, Mosfets, etc., with galvanic decoupling. switch gates such as triacs, thyristors, Mosfets, etc., with galvanic decoupling. None of of these success because trial production revealed the None these PT PTapplications applicationsreached reachedcommercial commercial success because trial production revealed piezoelectric transformer to be fragile and the overall piezoelectric-solution to be more expensive the piezoelectric transformer to be fragile and the overall piezoelectric-solution to be more expensive than the the electromagnetic ofof the technical problems were related to: than electromagnetic coil coil ititwas wasmeant meanttotoreplace. replace.Some Some the technical problems were related immature materials fabrication technology, lack of mechanical reliability, inadequate mounting, and to: immature materials fabrication technology, lack of mechanical reliability, inadequate mounting, bulky driving circuits. and bulky driving circuits.

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(a) (a)

(b) (b)

(c) (c)

Figure Some representative applications considered the 1970s Figure 7. Some representativeapplications applicationsconsidered considered for for piezoelectric piezoelectric transformers during the 1970s Figure 7. 7. Some representative for piezoelectrictransformers transformersduring during the 1970s and 1980s. (a) Gate drivers for solid state switches; (b) High voltage power supply for TV cathodeand 1980s. (a) Gate drivers for solid state switches; (b) High voltage power supply for TV cathodeand 1980s. (a) Gate drivers for solid state switches; (b) High voltage power supply for TV cathode-ray ray tube; (c) in engine ignition. ray(c) tube; (c) Application Application in small small engine ignition. tube; Application in small engine ignition.

120 120

Millionsof ofunits units Millions

JP850 850 JP US150 150 US

JP175 175 JP 1986-90 1986-90

US25 25 US

US75 75 US

US15 15 US

Numberof ofPatents PatentsFilled Filled Number

1.5. 1.5. 1990s–2000s: 1990s–2000s: Commercial Commercial Introduction Introduction of of PTs PTs for for CCFL CCFL Backlighting Backlighting 1.5. 1990s–2000s: Commercial Introduction of PTs for CCFL Backlighting In In the the late late 1980s, 1980s, several several Japanese Japanese companies, companies, including including NEC NEC Corp. Corp. (Kawasaki, (Kawasaki, Japan), Japan), Tokin Tokin In the late 1980s, several Japanese companies, including NEC Corp. (Kawasaki, Japan), Tokin Corp. Corp. (Sendai, Japan), Tamura Corp. (Tokyo, Japan), Matsushita Electric Industrial Co. Corp. (Sendai, Japan), Tamura Corp. (Tokyo, Japan), Matsushita Electric Industrial Co. Ltd., Ltd., Nihon Nihon (Sendai, Japan), Tamura Corp. (Tokyo, Japan), Electric Industrial Nihon Cement Cement Kabushiki Kaisha (Tokyo, Japan) and others, the PTs, taking advantage Cement Kabushiki Kaisha (Tokyo, Japan) andMatsushita others, reconsidered reconsidered the use use of ofCo. PTs,Ltd., taking advantage Kabushiki Kaisha (Tokyo, Japan) and others, reconsidered the use of PTs, taking advantage of of improvements improvements in in novel novel piezoelectric piezoelectric materials, materials, more more reliable reliable manufacturing manufacturing technology, technology, including including of improvements in novel piezoelectric materials, reliable manufacturing including multilayer processes, new on integrated circuits, and solutions. main multilayer co-firing co-firing processes, new concepts concepts on more integrated circuits, and housing housingtechnology, solutions. The The main 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 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 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 the transformer enabled thinner, lighter, more (CCFLs) used in crystal displays (LCDs) of the increasingly popular portable cell and phones and computers. In liquid this application, application, the piezoelectric piezoelectric transformer enabled thinner, lighter, and more efficient modules, thus the of screen thickness and computers. In this backlighting application, the piezoelectric transformer enabled lighter, more efficient efficient CCFL CCFL backlighting modules, thus allowing allowing the reduction reductionthinner, of LCD LCD screenand thickness and improving the life portable devices. Due small size, cost the improving the battery battery life of of portable devices. Due to to its its smallscreen size, the the cost of of and the piezoelectric piezoelectric CCFL backlighting modules, thus allowing the reduction of LCD thickness improving the transformers was relatively low and competitive for this application, rapidly enabling high-volume transformers was relatively lowDue andto competitive for this rapidly enabling high-volume battery life of portable devices. its small size, theapplication, cost of the piezoelectric transformers was production. production. relatively low and competitive for this application, rapidly enabling high-volume production. During this period, many technical publications Duringthis thisperiod, period,many many technical technical publications publications and and patents (Figure 8a) were issued forfor During and patents patents (Figure (Figure8a) 8a)were wereissued issuedfor modifications of Rosen’s initial concept, including variations of the design, driving circuitry, modifications of Rosen’s initial concept, including variations of the design, driving circuitry, modifications of Rosen’s initial concept, including variations of the design, driving circuitry, mounting mounting solutions, and materials to enhance the of devices [48–55]. During mounting solutions, and novel novel the performance performance of these these[48–55]. devices During [48–55]. the During solutions, and novel materials tomaterials enhance to theenhance performance of these devices 1990s, the 1990s, CCFL technology enjoyed a significant commercial boom for compact, portable devices. the 1990s, CCFL technology enjoyed a significant commercial boom for compact, portable devices. CCFL technology enjoyed a significant commercial boom for compact, portable devices. By early the By that 25%–30% By early early the the 2000s, 2000s, it it was was estimated 25%–30% of of the the produced produced CCFL CCFL backlighting backlighting circuits circuits were 2000s, it was estimated thatestimated 25%–30%that of the produced CCFL backlighting circuits were madewere using made made using using piezoelectric piezoelectric transformer transformer technology technology (Figure (Figure 8b). 8b). piezoelectric transformer technology (Figure 8b).

100 100 80 80

CCFL CCFL Inverters Inverters

60 60 40 40

Piezoelectric Piezoelectric Transformers Transformers

20 20 0 0

1991-95 1991-95

(a) (a)

1996-2002 1996-2002

1994 1994

1995 1995

1996 1996

1997 1997

1998 1998

1999 1999

2000 2000

(b) (b)

Figure 8. Evolution PT technology during 1980s to early 2000s based on author sources. Figure Evolutionofof ofPT PTtechnology technology during during late late sources. Figure 8. 8.Evolution late 1980s 1980s to to early early2000s 2000sbased basedononauthor author sources. (a) Number of filled patents in U.S. and Japan; (b) Production of piezoelectric transformers (a) Number of filled patents in U.S. and Japan; (b) Production of piezoelectric transformers for for cold cold (a) Number of filled patents in U.S. and Japan; (b) Production of piezoelectric transformers for cold cathode cathode fluorescent fluorescent lamps lamps (CCFL) (CCFL) applications applications compared compared to to total total CCFL CCFL inverter inverter production. production. cathode fluorescent lamps (CCFL) applications compared to total CCFL inverter production.

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Companies like Toshiba (Tokyo, Japan), NEC, Hitachi (Tokyo, Japan), Panasonic (Osaka, Japan) 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 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 commercial piezoelectric inverters for CCFLs integrated in the Apple MacBook Pro 15” and 17” LCD screen computers during the mid-2000s. screen computers during the mid-2000s.

(a)

(b)

Figure 9. 9. Piezoelectric Piezoelectric inverter inverter used used in in Apple Apple Powerbook Powerbook 15” 15” and and 17” 17” computer computer for for LCD LCD backligting backligting Figure (courtesy of Micromechatronics, Inc., State College, PA, USA). (a) 5W piezoelectric transformer; (courtesy of Micromechatronics, Inc., State College, PA, USA). (a) 5W piezoelectric transformer; (b)Assembled Assembledpiezoelectric piezoelectricinverter. inverter. (b)

In addition to the improvements in the manufacturing process of PTs, developments in the In addition to the improvements in the manufacturing process of PTs, developments in the driving driving techniques for PTs were critical for their commercial success. Two relevant examples are the techniques for PTs were critical for their commercial success. Two relevant examples are the use of use of resonant converter topologies for driving the PTs and the use of integrated control circuits to resonant converter topologies for driving the PTs and the use of integrated control circuits to provide provide stable operation to the PT under different operation conditions. stable operation to the PT under different operation conditions. Resonant converters were originally considered in the early 1990s as an extension of the general 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 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 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 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 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 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, 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 the sine input waveform is generated by using a square wave voltage in combination with an inductive inductive element in series with the transformer. The control of the ON/OFF of the switches of the element in series with the transformer. The control of the ON/OFF of the switches of the converter converter is selected to minimize the switching losses. This technique is the so-called Zero Voltage is selected to minimize the switching losses. This technique is the so-called Zero Voltage Switching, Switching, ZVS [57]. The combination of the ZVS technique and the PT can reduce the capacitive ZVS [57]. The combination of the ZVS technique and the PT can reduce the capacitive turn-on loss due turn-on loss due to the input capacitance of the PT, and achieve high efficiency. to the input capacitance of the PT, and achieve high efficiency. There are two main strategies to achieve ZVS in a PT-driven topology. First, and most common, There are two main strategies to achieve ZVS in a PT-driven topology. First, and most common, the PT should present an inductive characteristic at the operating frequency. In order to achieve this, the PT should present an inductive characteristic at the operating frequency. In order to achieve this, normally an inductor is connected in series with the PT. In this type of strategy, the PT can be driven normally an inductor is connected in series with the PT. In this type of strategy, the PT can be driven at a fixed duty cycle of 0.5. A second approach to achieve ZVS is to use the PT without an external at a fixed duty cycle of 0.5. A second approach to achieve ZVS is to use the PT without an external inductor (so-called “inductor-less” driving). In this case, a variable duty-cycle is required for the inductor (so-called “inductor-less” driving). In this case, a variable duty-cycle is required for the control signal to the Mosfets to achieve ZVS. control signal to the Mosfets to achieve ZVS. Figure 10 shows some of the standard topologies [54] used to drive PTs using a series inductor, Figure 10 shows some of the standard topologies [54] used to drive PTs using a series inductor, including push-pull [53], half-bridge and Class-E [57–59]. Inductor-less techniques is discussed in including push-pull [53], half-bridge and Class-E [57–59]. Inductor-less techniques is discussed in Section 1.6. Section 1.6.

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Vcc

Q1

L2

L1 Vcc

L

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L1

L

Q1

PT

Vcc

Q1

L

PT

Vcc

Q1 Q2

L

Vcc PT

Q Q

Q2

(a) (a)

PT

PT

Q2

Q2

PT

Vcc

(b) (b)

(c) (c)

Figure 10. 10. Commonly topologies: (a) Figure Commonly used used piezoelectric piezoelectric transformer transformer circuit circuit topologies: (a) push-pull; push-pull; (b) (b) half-bridge; half-bridge; Figure 10. Commonly used piezoelectric transformer circuit topologies: (a) push-pull; (b) half-bridge; (c) Cass-E. (c) Cass-E. (c) Cass-E.

In order to ensure the ZVS condition and to maintain the stability of the converter under load In order to ensure the ZVS condition and to maintain the stability of the converter under load ordervoltage to ensure the ZVS condition maintain the stability the converter load and In input fluctuation, strategies and for to controlling PTs wereofdeveloped andunder integrated and input voltage fluctuation, strategies for controlling the PTs were developed and integrated within and input fluctuation, strategies controlling thefrequency PTs wereoscillators developednear andthe integrated within ICs.voltage The first control circuits werefor designed as fixed resonant ICs. The first control circuits were designed as fixed frequency oscillators near the resonant frequency. within ICs. The first control circuits were designed as fixed frequency oscillators near the resonant frequency. However, due to the high mechanical quality factor (Q), of the PT, changes in the resonant However, due to the high mechanical quality factor (Q), of the PT, changes in the resonant frequency frequency. to the high mechanical quality (Q), oforthe PT, changes the resonant frequency However, associateddue with aging, temperature, load factor variation, input voltage,incould not be associated with aging, temperature, load variation, or input voltage, could not be compensated for, frequency associated with aging, temperature, load variation, compensated for, negatively affecting the performance of the PT. or input voltage, could not be negatively affecting the performance of the PT. compensated for, negatively performance of the PT. Progressively, differentaffecting control the schemes have been developed to ensure the stability of the Progressively, different control schemes have been developed to ensure the stability of the output Progressively, control have the been developed to ensure stability of the output voltage or different current, as well schemes as maximize circuit efficiency under the different operation voltage or current, as well as maximize the circuit efficiency under different operation conditions. output voltage or current, as well as maximize the circuit efficiency under different operation conditions. Since PTs are frequency-dependent devices, a way to control the operation of the PT is to Since PTs are frequency-dependent devices, a way to control the operation of the PT is to vary the conditions. Since PTsaccording are frequency-dependent devices,orato way to control operation of the PT is to vary the frequency to output load changes input voltagethe variations. frequency according to output load changes or to input voltage variations. vary the according to output load changes or to voltage variations. Onefrequency of the first control techniques introduced was theinput so-called “frequency control,” sometimes One of the first control techniques introduced was the so-called “frequency control,” sometimes One of the first control techniques introduced was the so-called “frequency control,” sometimes referred as PFM (pulse frequency modulation). Frequency control requires one or several feedback referred as PFM (pulse frequency modulation). Frequency control requires one or several feedback referred PFMThese (pulsefeedback frequencyloops modulation). one voltage or several control as loops. provide Frequency informationcontrol aboutrequires the output (orfeedback current) control loops. These feedback loops provide information about the output voltage (or current) control loops. These loops provide information the output (or current) variations so they canfeedback be appropriately regulated. Different about companies such asvoltage Rohm (Tokyo, Japan variations so they can be appropriately regulated. Different companies such as Rohm (Tokyo, variations so they can be appropriately regulated. Different companies such as Rohm (Tokyo, Japan (BA9785 [58], BA9802), Sanyo (Tokyo, Japan) (LA5663V), Texas Instrument (Dallas, TX, USA) Japan (BA9785 [58], BA9802), Sanyo (Tokyo, Japan) (LA5663V), Texas Instrument (Dallas,TX, TX, USA) (BA9785 [58], BA9802), Sanyo (Tokyo, (LA5663V), Texas (Dallas, (UCC3975 [59]), O2 Micro (Santa Clara,Japan) CA, USA) (OZ9913), andInstrument Maxim (Sunnyvale, CA,USA) USA) (UCC3975 [59]), [59]), O2 O2 Micro Micro (Santa (Santa Clara, CA, CA, USA) (OZ9913), (OZ9913), and Maxim Maxim (Sunnyvale, (Sunnyvale,CA, CA,USA) USA) (UCC3975 (MAX8785), among others, have Clara, developed USA) ICs integrating aand combination of these strategies to (MAX8785), among others, have developed ICsICs integrating a combination of these strategies to provide (MAX8785), among others, have developed integrating a combination of these strategies to provide full control of the PTs. There are several possibilities that have been proposed regarding the full control the PTs. areThere several that have been proposed parameters provide fullof control of There the PTs. arepossibilities several possibilities thatincluding: have been regarding proposed the regarding the parameters to be measured to determine the frequency control, to be measured to determine the frequency control, including: parameters to be measured to determine the frequency control, including: (i) Frequency control by measuring the output voltage. (i) Frequencycontrol control by measuring measuring the output output voltage. (i) (ii) Frequency Frequency controlby by measuringthe the outputvoltage. current (Figure 11a). (ii) Frequency control by measuring the output current (Figure 11a). (ii) Frequency control by measuring the output current (Figure 11a). (iii) Frequency control by measuring the phase difference between input voltage and current. (iii) Frequency control by measuring the phase difference between input voltage current. (iii) between input (vi) Frequency Frequencycontrol controlby bymeasuring measuringthe thephase input difference to output phase difference (Figureand 11b). (iv) Frequency Frequencycontrol control by by measuring measuring the the input input to to output output phase phase difference difference (Figure 11b). (vi)

VC

+ +-

VC

ERROR AMPLIFIER ERROR AMPLIFIER

(a) (a)

REF REF

ILOAD ILOAD

VOLTAGE CONTROLLED VOLTAGE OSCILLATOR CONTROLLED OSCILLATOR

PT PT

DC INPUT VOLTAGE DC INPUT VOLTAGE PWM RESONANT CONTROL DRIVER PWM RESONANT CIRCUIT CONTROL DRIVER CIRCUIT INTEGRATING CIRCUIT INTEGRATING CIRCUIT VCO VCO

PT PT Current Detection Current Circuit Detection Circuit

ILOAD ILOAD

DC INPUT VOLTAGE DC INPUT VOLTAGE RESONANT POWER RESONANT STAGE POWER STAGE

PHASE DIFFERENCE PHASE DETECTION DIFFERENCE CIRCUIT DETECTION CIRCUIT

(b) (b)

Figure 11. Different tracking control schemes for piezoelectric transformers: (a) based on measuring Figure 11. for transformers: the output current; tracking (b) basedcontrol on the schemes input to output phase difference. Figure 11.Different Different tracking control schemes forpiezoelectric piezoelectric transformers:(a) (a)based basedononmeasuring measuring the output current; (b) based on the input to output phase difference. the output current; (b) based on the input to output phase difference.

In some applications, the use of a single frequency loop cannot ensure the complete control of In the use of ainput singlevoltage frequency loop ensure complete control the of the PT some underapplications, large variations of the and/or thecannot output load. the When this happens, the PT under large variations of the input voltage and/or the output load. When this happens, the

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9 of 22 the use of a single frequency loop cannot ensure the complete control of the PT under large variations of the input voltage and/or the output load. When this happens, frequency shift required in the transformer toto adjust the frequency shift required in the transformer adjustthe theoutput outputconditions conditionsmay maybe beso so large large that that the the efficiency of the PT will be significantly affected or/and the correct transformation ratio to achieve efficiency of the PT will be significantly affected or/and the correct transformation ratio to achieve the the expected values is possible. not possible. In order to overcome limitations, a second voltage feedback expected values is not In order to overcome thesethese limitations, a second voltage feedback loop loop control is sometimes considered to extend the regulation range of the converter. control is sometimes considered to extend the regulation range of the converter. The voltage voltage feedback feedback has been implemented implemented in basic ways. ways. One use The has been in two two basic One proposed proposed option option is is to to use pulse modulation technique to control the driving circuit signal of the transistor gates (PWM pulse modulation technique to control the driving circuit signal of the transistor gates (PWM technique). technique). The secondincludes alternative includes the input control the input DC voltage by circuit using acontrolled chopper The second alternative the control of the DCofvoltage by using a chopper circuit controlled at much lower frequency than the PT (for instance, 100–1000 Hz), which switches at much lower frequency than the PT (for instance, 100–1000 Hz), which switches the input voltage to the input voltage to the switching transistor. This sometimes referred as “dimming technique”. the switching transistor. This sometimes referred as “dimming technique”. By the late 2000s and early 2010s, the CCFL backlighting technology used for LCD started to be By the late 2000s and early 2010s, the CCFL backlighting technology used for LCD started to replaced by light-emitting diodes (LEDs). This resulted in a production decline of high voltage be replaced by light-emitting diodes (LEDs). This resulted in a production decline of high voltage piezoelectric transformers and eventually stopped the high-volume production of these components piezoelectric transformers and eventually stopped the high-volume production of these components by most of the leading suppliers. This also impacted the production of IC and many of the specifically by most of the leading suppliers. This also impacted the production of IC and many of the specifically designed ICs for the CCFL business are currently discontinued. designed ICs for the CCFL business are currently discontinued.

1.6. Beyond 1.6. Beyond the the 2000s: 2000s: Power Power Piezoelectric Piezoelectric Transformers Transformers The evolution evolution in the know-how know-how of PT technology technology during first decade decade of of 2000s 2000s The in the of PT during the the 1990s 1990s and and first inspired new developments beyond the CCFL backlighting in areas such as fluorescent ballasts, ACinspired new developments beyond the CCFL backlighting in areas such as fluorescent ballasts, AC-DC DC battery adapters and, more recently, for AC-DC LED drivers. Compared to backlighting inverters, battery adapters and, more recently, for AC-DC LED drivers. Compared to backlighting inverters, where the the typical typical power power level level is is 55 W W and and the the output output voltage voltage is is several several hundred hundred volts volts (step (step up), up), most most where DC-DC or AC-DC applications require several tenths to hundreds of watts with output voltage levels DC-DC or AC-DC applications require several tenths to hundreds of watts with output voltage levels of just just aa few few volts volts (step-down (step-down conversion). conversion). The not suitable suitable for for those those levels levels of of of The Rosen Rosen PT PT topology topology is is not power conversion conversion since since its its design design is is prominent prominent in in high high voltage voltage generation. generation. Thus, power Thus, new new types types of of PTs PTs designs and revisions of some previously suggested were considered by researcher interested in designs and revisions of some previously suggested were considered by researcher interested in power power conversion applications. The effort was mainly bycompanies: two companies: Electronics inU.S. the conversion applications. The effort was mainly led byled two Face Face Electronics in the U.S. and NEC in Japan. Other groups in Europe and Korea, with alternative proposals, followed these and NEC in Japan. Other groups in Europe and Korea, with alternative proposals, followed these tendencies toward toward power power applications. applications. tendencies Strictly speaking, was undertaken undertaken in Strictly speaking, the the first first published published research research effort effort on on power power PTs PTs was in 1966 1966 by by Othmar M. Stuetzer [60] of Sandia Laboratory (Albuquerque, NM, USA). Stuetzer analyzed the Othmar M. Stuetzer [60] of Sandia Laboratory (Albuquerque, NM, USA). Stuetzer analyzed the power power capabilities of a PT consisting of two thin piezoelectric discstobonded opposite sides wall of a capabilities of a PT consisting of two thin piezoelectric discs bonded oppositetosides of a metal metal wall andin operating in the thickness In spiteand of mainly this earlier and mainly and operating the thickness mode (Figure mode 12). In (Figure spite of 12). this earlier theoretical study, theoretical study, no other work on high-power PTs is mentioned until the 1990s. no other work on high-power PTs is mentioned until the 1990s.

Figure 12. Thickness Thickness mode mode transformer suggested by Othmar M. Stuetzer, Sandia Labs, in 1966 [60].

Early in inthe the1990s 1990s intense work re-initiated in Japan byon NEC on the same ofconcept of Early intense work waswas re-initiated in Japan by NEC the same concept thickness thickness mode PTs (Figure 13) and its use for power applications. Some of the proposed thickness mode PTs (Figure 13) and its use for power applications. Some of the proposed thickness mode designs mode designs consisted of 1plates to 2 mm-thick with input and output multilayer sections and consisted of 1 to 2 mm-thick with inputplates and output multilayer sections and operating at very operating at very such high as frequencies, such asT.1 Inoue, MHz or T. Y. Inoue, O.among Ohnishi, Y. Sasaki, among high frequencies, 1 MHz or higher. O.higher. Ohnishi, Sasaki, others at NEC, hold others atofNEC, hold several of the patents for[61,62] thickness mode PT [61,62] designs. several the patents for thickness mode PT designs. 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 kt necessary for thickness operation, but also have large coupling factor k31 for longitudinal and width vibrations. The k31 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 PbTiO3 ceramic material, Actuators 12 10 of 22 where2016, kt is5,larger than 50% and k31 is less than 5% [63].

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electromechanical coupling factor kt necessary for thickness operation, but also have large coupling factor k31 for longitudinal and width vibrations. The k31 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 PbTiO3 ceramic material, where kt is larger than 50% and k31 is less than 5% [63].

Figure piezoelectrictransformer transformerby byNEC, NEC,1992 1992 [61,62]. Figure13. 13.Thickness Thickness mode mode piezoelectric [61,62].

Simultaneously with the development in PTmode designs for power applications, One of the main operational challengesand of improvement using a thickness is the large number of high intensive also by NEC, resulted in publications circuit strategies the PTs on power order modeswork, for the longitudinal or width extensional on vibrations resultingtoindrive spurious modes near the applications [64–67]. During this period, activities initiated at Virginia Electronichave Center thickness mode resonance frequency. The reason were is related to the fact thatPower PZT ceramics high (VPEC) of Virginia Tech University, led by Fred Lee, on converters using piezoelectric transformers electromechanical coupling factor kt necessary for thickness operation, but also have large coupling [68–70]. activities were in width part thevibrations. result of theThe collaboration NEC.in In aJuly factor k31 These for longitudinal and k31 large between couplingVPEC factorand results large 1993, T. Zaitsu, a member of NEC team, joined as a visiting researcher the Virginia Power Electronic number of high-order harmonics that couple with the thickness mode, leading to poor operational team at Virginia Tech [65]. He stayed for one year, until August 1994, and returned to Japan where characteristics. To suppress the spurious resonances, based on the unstiffened piezoelectric effect, he continued his work at NEC [71,72] and published his doctoral dissertation in 1997 [73]. ceramic materials Figure with large anisotropy were considered, in particular the PbTiO 3 ceramic material, 13. Thickness mode piezoelectric by NEC, 1992that [61,62]. The evaluation of thickness expansion mode PTs transformer by NEC demonstrated these transformers where kt is larger than 50% and k31 is less than 5% [63]. can deliver higher net output power due to their higher driving frequency. However, it was found the and improvement inPT PTdesigns designs forpower power applications, Simultaneously with thedevelopment development improvement in for applications, thatSimultaneously their efficiency with was low, in the high 80% range, and their integration in DC-DC converters was intensive work, also by NEC, resulted in publications on circuit strategies to drive the PTs intensive work, also by NEC, resulted in publications on circuit strategies to drive the PTs on power difficult due to limitations relating to efficient driving circuits (especially field-effect transistor, FETs) on power applications [64–67]. During this period, were initiated at Virginia Power Electronic applications [64–67]. During this period, activities wereNEC initiated at Virginia Power Center for such frequency and large output power. In activities 1997, changed its focus to a Electronic different type of (VPEC) of Virginia Tech University, led by Fred Lee, on converters using piezoelectric transformers Center (VPEC) of Virginia Tech University, led by Fred Lee, on converters using piezoelectric piezoelectric transformer, the longitudinal vibration mode transformer shown in Figure 14 [71]. [68–70]. These activities were in part the result thethe collaboration between VPEC andbetween NEC. In July transformers [68–70]. These activities were in of part result of the collaboration VPEC 1993, T. Zaitsu, member of NEC team, joined as a visiting researcher the Virginia Power Electronic and NEC. In July a1993, T. Zaitsu, a member of NEC team, joined as a visiting researcher the Virginia teamElectronic at Virginiateam Techat[65]. He stayed for one until 1994, returned Japan where to Power Virginia Tech [65]. He year, stayed for August one year, untiland August 1994,toand returned he continued his work at NEC [71,72] and published his doctoral dissertation in 1997 [73]. Japan where he continued his work at NEC [71,72] and published his doctoral dissertation in 1997 [73]. The evaluationofofthickness thicknessexpansion expansion mode mode PTs transformers The evaluation PTs by by NEC NECdemonstrated demonstratedthat thatthese these transformers can deliver higher net output power due to their higher driving frequency. However, it was found can deliver higher net output power due to their higher driving frequency. However, it was found that that their efficiency was low, in the high 80% range, and their integration in DC-DC converters was their efficiency was low, in the high 80% range, and their integration in DC-DC converters was difficult difficult due to limitations relating to efficient driving circuits (especially field-effect transistor, FETs) due to limitations relating to efficient driving circuits (especially field-effect transistor, FETs) for such for such frequency and large output power. In 1997, NEC changed its focus to a different type of frequency and large output power. In 1997, NEC changed its focus to a different type of piezoelectric piezoelectric transformer, the longitudinal vibration mode transformer shown in Figure 14 [71]. transformer, the longitudinal vibration mode transformer shown in Figure 14 [71].

Figure 14. Piezoelectric transformer using longitudinal and transverse mode vibration [72].

Figure 14. Piezoelectric transformer using longitudinal and transverse mode vibration [72]. Figure 14. Piezoelectric transformer using longitudinal and transverse mode vibration [72].

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transformer provided better efficiency than the thickness mode The longitudinal transformer provided better efficiency than the thickness mode transformer. In In addition, the multilayer construction of both the input and output allowed better design flexibility The longitudinal transformer provided efficiency than theallowed thickness mode transformer. In addition, the multilayer construction of bothbetter the input and output better design flexibility The longitudinal transformer provided better efficiency than the thickness mode transformer. In foraddition, AC-DC adapter integration [72–77]. multilayer construction for AC-DCthe adapter integration [72–77].of both the input and output allowed better design flexibility addition, the multilayer construction of bothNorfolk, the input and output allowed better designofflexibility the mid-1990s, Face[72–77]. Electronics, VA, USA, invented a new concept power PT, forToward AC-DC adapter integration Toward the mid-1990s, Face Electronics, Norfolk, VA, USA, invented a new concept of power ® (registered for AC-DC adapter integration [72–77]. Transoner name by Face Electronics, Figure 15), based on a laminated construction [78]. ® (registered Toward the mid-1990s,name Face by Electronics, Norfolk,Figure VA, USA, invented new concept of power PT, Transoner Face Electronics, 15), based on aa laminated construction Toward the mid-1990s, Face Electronics, Norfolk, VA, USA, invented a new concept of power ® The radial operation and a symmetric design was included in aa second second patent PT,Transoner Transoner (registered name by Face Electronics, Figure 15), based on a laminated construction [78]. The Transoner radial operation and a symmetric design was included in patent by by ® (registered name by Face Electronics, Figure 15), based on a laminated construction PT, Transoner A. A. V. Carazo [79]. The power density, design flexibility, manufacturing simplicity and toughness [78]. Transoner radial operation a symmetric was included in a second patent by V.The Carazo [79]. The power density,and design flexibility,design manufacturing simplicity and toughness of [78].Transoner The Transoner radial operation and a becoming symmetric the design was included in applications a second patent by of the the have resulted in thisdesign design reference power many A. V. Carazo [79]. The powerin density, design flexibility, simplicity and toughness of Transoner have resulted this becoming themanufacturing reference forfor power applications in in many A. V. Carazo [79]. The power density, design flexibility, manufacturing simplicity and toughness of research publications. the Transoner have resulted in this design becoming the reference for power applications in many research publications. the Transoner have resulted in this design becoming the reference for power applications in many research publications. research publications.

(a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (a) Figure Different configurations of Transoner radial-type piezoelectric transformer: squareFigure 15.15. Different configurations of Transoner radial-type piezoelectric transformer: square-shape

Figure Transoner; 15. Different(b)configurations of Transoner piezoelectric (a) squareshape disc-shape (c) radial-type commercial sample of transformer: non-isolated Transoner Transoner; (b)Different disc-shape Transoner;Transoner; (c) commercial sample ofpiezoelectric non-isolated Transoner piezoelectric Figure 15. configurations of Transoner radial-type transformer: (a) squareshape Transoner; (b) disc-shape Transoner; (c) commercial sample of non-isolated Transoner piezoelectric transformer [78,79]. transformer [78,79]. (b) disc-shape Transoner; (c) commercial sample of non-isolated Transoner shape Transoner; piezoelectric transformer [78,79]. piezoelectric transformer [78,79].

In the late 1990s and early 2000s, the VPEC at Virginia Tech began a characterization study of In thethe late early the at Virginia Virginia Tech began characterization study In late1990s 1990sand and early2000s, 2000s, the VPEC VPEC at Tech began aacharacterization study of of Transoner piezoelectric transformer, funded in part by a grant from Virginia’s Center for Innovative In the late 1990s and early 2000s, the VPEC at Virginia Tech began a characterization study of Transoner piezoelectric transformer, funded in part a grant from Virginia’s Center for Innovative Transoner piezoelectric transformer, funded in 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 Transoner(CIT), piezoelectric transformer, funded in part by a grant from Virginia’s Center for Innovative Technology also The objective objective this collaboration led Technology (CIT),and also byFace FaceElectronics Electronics [80,81]. The ofofthis collaboration to to intensive research inand the areaby of fluorescent ballast[80,81]. using piezoelectric transformers. Perhaps theled most Technology (CIT), and also by Face Electronics [80,81]. The objective of this collaboration led to intensive research area fluorescent ballast using transformers. intensive researchin in the ofoffluorescent using piezoelectric Perhaps the mostthe relevant outcome ofthe thisarea research was theballast development ofpiezoelectric a new transformers. inductor-less conceptPerhaps to drive intensive research in the area of fluorescent ballast using piezoelectric transformers. Perhaps the most relevant outcome of this research waswas thethe development of input aofnew inductor-less concept to to drive most relevant outcome of this research development a new inductor-less concept drive piezoelectric transformers (Figure 16), which eliminated the series inductor used in previous 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 piezoelectric transformers (Figure 16), which eliminated the input series inductor used in previous driving approached [82–84]. piezoelectric transformers (Figure 16), which eliminated the input series inductor used in previous driving approached[82–84]. [82–84]. driving approached driving approached [82–84].

Figure 16. Inductor-less Piezoelectric Transformer-based Linear Ballast with Power Factor Correction Figure Inductor-less Piezoelectric Transformer-based Linear Ballast Factor Correction Figure 16. Inductor-less Piezoelectric Linear Ballastwith withPower Power Factor Correction using a16. Transoner radial piezoelectricTransformer-based transformer (Courtesy of Micromechatronics). Figure 16. Inductor-less Piezoelectric Transformer-based Linear Ballast with Power Factor Correction using a Transoner radial piezoelectric transformer (Courtesy of Micromechatronics). using a Transoner radial piezoelectric transformer (Courtesy of Micromechatronics). using a Transoner radial piezoelectric transformer (Courtesy of Micromechatronics).

Also, late in the 1990s, shortly after the invention of Transoner, NEC switched the research on Also, late in the 1990s,type shortly the invention of approach Transoner,asNEC theFace’s research on power PTs to athe different of after PT, using a similar the switched Transoner radial Also, late inin 1990s, of Transoner, Transoner, NEC NEC switchedthe theresearch research Also, late the 1990s,shortly shortlyafter afterthe the invention invention of switched onon 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 power PTsPTs to a to different type of PT, of using similara approach as the Transoner Face’s radial piezoelectric power a different type PT,ausing 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). transformer. The NEC approach used approach a square geometry with geometry an internalwith radial [85,86], piezoelectric transformer. The NEC used a square an electrode internal radial electrode [85,86], operated in a contour-extensional vibration mode (Figure 17). electrode operated in a contour-extensional vibration operated in a[85,86], contour-extensional vibration mode (Figure 17).mode (Figure 17).

Figure 17. Contour-mode piezoelectric transformer [86]. Figure 17. Contour-mode piezoelectric transformer [86]. Figure17. 17.Contour-mode Contour-mode piezoelectric piezoelectric transformer Figure transformer[86]. [86].

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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, Actuators 2016, 5, 12 12 of 22 Beer-Sheva, Israel [87–89]. Several other research groups have investigated other structures for step-down conversion. The The use of Transoner PTs and the concept of inductor-less driving techniques was followed unipoledbyPT, originally developed byincluding Berlincourt in lateUniversity 1960s, is one of the Israel) topologies that have also several other research groups, Ben-Gurion (Beersheba, of the Negev, been considered byIsrael several researchers for step-down applications. Beer-Sheva, [87–89]. Several other Spain, researchFerroperm, groups haveDenmark, investigatedthe other structures for step-down de conversion. In Europe, Alcatel Universidad Politécnica Madrid, Spain, The unipoled PT, originally developed by Berlincourt in late 1960s, is one of the topologies that have for the and the Universidad de Oviedo, Spain, collaborated in the ESPRIT IV project “TRAMST” also been considered by several researchers for step-down applications. development of a piezoelectric AC-DC converter for mobile phone battery chargers. The PT design In Europe, Alcatel Spain, Ferroperm, Denmark, the Universidad Politécnica de Madrid, Spain, was based on a ring-shaped in thickness mode,“TRAMST” where theforprimary and and the Universidad deelement Oviedo, operating Spain, collaborated in the vibrational ESPRIT IV project the secondary sections are an isolation [90–100]. development of aseparated piezoelectricby AC-DC converterlayer for mobile phone battery chargers. The PT design was based on a ring-shaped element operating in thickness vibrational where the primary andInfineon, In the mid-2000s, Face Electronics, Fraunhofer Institute, led by mode, Matthias Radecker, and secondary sectionsresearch are separated by an isolation layer [90–100].of piezoelectric transformers, including collaborated in several projects on the application In the mid-2000s, Face Electronics, Fraunhofer Institute, led by Matthias Radecker, and Infineon, the development of a control IC [101–105]. One of the outcomes of this work was the development of collaborated in several research projects on the application of piezoelectric transformers, including a duty cycle control strategy implemented byOne a mathematical thatwas setthe thedevelopment duty cycle to meet the development of a control IC [101–105]. of the outcomesfunction of this work ZVS depending on the frequency [106]. Figure shows afunction 40 W that isolated piezoelectric of a duty cycle control strategy implemented by a 18 mathematical set the radial duty cycle to meetdesigned ZVS depending on the frequency [106]. Figure battery 18 showscharger a 40 W isolated radial piezoelectric transformer for a 24 V piezoelectric AC-DC for universal input applications transformer designed for a 24 V piezoelectric AC-DC battery charger for universal input applications (85–265 V), which was the result of this development collaboration. (85–265 V), which was the result of this development collaboration.

Figure 18. Isolated piezoelectric radial transformer for use in AC-DC converter for universal input

Figure 18. Isolated piezoelectric radial transformer for use in AC-DC converter for universal input voltage operation (85–265 W) and 24V, 40 W output load (Courtesy of Micromechatronics, Inc.). voltage operation (85–265 W) and 24V, 40 W output load (Courtesy of Micromechatronics, Inc.). Late in the 2000s and the early 2010s, work was done at the University of Sheffield, UK,

LatebyinE.the 2000s and the early 2010s, work was donetoatinductor-less the University of Sheffield, L. Horsley on Transoner radial PT and its application converters [107–109].England, by a research line on transformers been established at the Department E. L. HorsleyRecently, on Transoner radial PTpiezoelectric and its application tohas inductor-less converters [107–109]. of Electrical Engineering of the Technical University of Denmark, led by Michael A. E. Andersen. Recently, a research line on piezoelectric transformers has been established at the Department The main focus of this research work is on inductor-less and bidirectional piezoelectric of Electrical Engineering of the Technical University of Denmark, led by Michael A. E. Andersen. The transformer-based converters [110–114]. main focus ofCurrently, this research work is onproduction inductor-less andof bidirectional piezoelectric transformer-based the development, and sale Transoner radial transformers is done at converters [110–114]. Micromechatronics, Inc., State College, Pennsylvania, USA, under the same technical team that started the development of the technology at Face. and sale of Transoner radial transformers is done at Currently, the development, production Figure 19 illustrates a summary of the most relevant power that has Micromechatronics, Inc., State College, Pennsylvania, USA,transformers under theconfigurations same technical team that been considered through the state of the art. started the development of the technology at Face. Figure 19 illustrates a summary of the most relevant power transformers configurations that has been considered through the state of the art.

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75 W Step-up Low Power

Step-down High Power 40W/cm3 97%

30 W

20W/cm3 95%

20 W 5W/cm3 20W/cm3 85-87%

5W

1954 Rosen Longitudinal

20W/cm3 89%

15W/cm3 92%

20W/cm3 89-92%

1992 Thickness 1996 Longitud. Mode PT Mode PT

1996 Radial Mode PT

1997 Contour Mode PT

1997 Contour Mode PT

1997 Thickness Mode PT

Figure The stateofofthe theart artofofpower powerpiezoelectric piezoelectric transformers transformers (PTs) Micromechatronics, Inc.). Figure 19.19. The state (PTs)technology technology(courtesy (courtesyofof Micromechatronics, Inc.).

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2. Space, Defense and Security Applications of Piezoelectric Transformers 2. Space, Defense and NASA Security Applications of Piezoelectric Transformers During the 2000s, granted several Small Business Innovation Research (SBIR) projects to integrate piezoelectric transformers into space applications [115]. During the 2000s, NASA granted several Small Business Innovation Research (SBIR) projects to integrate piezoelectric transformers into space applications [115]. 2.1. High Voltage Piezoelectric-Based Power Supply to Drive the Main Payload of Satellite Communication Systems: Traveling Wave Tube Power Supply to Drive the Main Payload of Satellite Communication 2.1. High The Voltage Piezoelectric-Based Systems: The Traveling Wave Tube This NASA SBIR-founded research program, led by Dr. Carazo, developed a piezoelectric-based This NASA research program, led bypower Dr. Carazo, developed electronic powerSBIR-founded conditioner (EPC) module to supply to traveling wavea piezoelectric-based tubes (TWTs) used electronic power conditioner (EPC) module to supply power to traveling wave tubes (TWTs) for space-earth long-distance wireless communication [116–118]. The developed design eliminated used for space-earth long-distance wireless communication [116–118]. The developed design conventional electromagnetic transformers with a compact and highly efficient piezoelectric eliminated conventional with a compact and highly efficient piezoelectric transformer driver. The electromagnetic developed EPCtransformers unit consisted of several modules providing different levels transformer driver. The developed EPC unit consisted of several modules providing different levels of high voltage and power to the different circuits of a TWT. Figure 20 illustrates the concept of the of high (Figure voltage 20a) and and power circuits of a TWT.used Figure 20 illustrates project onetoofthe thedifferent developed EPC modules to drive a 4 kV/10the W concept cathode of of the the project TWT. (Figure 20a) and one of the developed EPC modules used to drive a 4 kV/10 W cathode of the TWT.

SWITCHING REGULATOR FOR BUS VOLTAGE

FEEDBACK LOOP

PROTECTION CIRCUIT OUTPUT MULTIPLIER

SWITCHING HALF-BRIDGE INVERTER

(a)

SPACE FOR THE PIEZOELECTRIC TRANSFORMER

(b)

Figure 20. 20. (a) (a) Modular Modular concept concept using using Transoner Transoner technology technology applied applied to to aa traveling traveling wave wave tube tube (TWT); (TWT). Figure (b) Detail of a 4 kV/10 W cathode power supply using piezoelectric transformer. (b) Detail of a 4 kV/10 W cathode power supply using piezoelectric transformer.

2.2. Development of New Integrated Ignition Systems for Small Satellite Thruster by Using Piezoelectric 2.2. Development of New Integrated Ignition Systems for Small Satellite Thruster by Using Piezoelectric TransformerTechnology Technology Transformer This NASA NASA SBIR-founded SBIR-founded research research program program developed developed aa new new discharge discharge initiation initiation (DI) (DI) system system This using a piezoelectric transformer to initiate the discharge of Pulsed Plasma Thrusters used in small using a piezoelectric transformer to initiate the discharge of Pulsed Plasma Thrusters used in small satellites using solid-state spark-plugs [119–122]. The program took as a reference the design satellites using solid-state spark-plugs [119–122]. The program took as a reference the design specifications of of the (EO-1) launched in specifications the PPT PPT development developmentfor forthe theEarth EarthOrbit Orbitexperimental experimentalsatellite satellite (EO-1) launched November 2000. The complete prototype of the system (called IGNIT-SONER) was tested in May in November 2000. The complete prototype of the system (called IGNIT-SONER) was tested in May −6 to −7 torr) ´10 6 to 2005 at at NASA vacuum conditions (10(10 over 2005 NASA Glenn GlennResearch ResearchCenter Centerunder underhigh high vacuum conditions 10´7for torr) for50,000 over accumulated cycles. Moreover, the unit was tested for over 500,000 accumulated cycles under 50,000 accumulated cycles. Moreover, the unit was tested for over 500,000 accumulated cycles under atmospheric lab lab conditions conditions under under maximum maximum firing firing rates rates of of 55 Hz. Hz. The The developed developed transformer transformer is is an an atmospheric input to to output output 33 kV-isolated kV-isolated PT, PT, and and able able to to step step up up the the input input voltage voltage to to over over 2.5 2.5 kV. kV. The The physics physics input associated with semiconductor-type spark plugs used for ignition under vacuum conditions led to to associated with semiconductor-type spark plugs used for ignition under vacuum conditions led the development of a piezoelectric-to-capacitive discharge topology. This topology allows the storing the development of a piezoelectric-to-capacitive discharge topology. This topology allows the storing the high high voltage voltage energy energy delivery delivery by by the the piezoelectric piezoelectric transformer transformer in in aa capacitor, capacitor, and and then then suddenly suddenly the releasing it to the spark-plug. This sudden release makes it possible to achieve very high releasing it to the spark-plug. This sudden release makes it possible to achieve very high currentcurrent levels levels over A formicroseconds, few microseconds, as required to generate the spark. Figure 21a shows of of overof100 A 100 for few as required to generate the spark. Figure 21a shows one one of the

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IGNIT-SONER developed prototypes. Figure 21b shows the vacuum chamber at NASA Glenn where the IGNIT-SONER developed prototypes. Figure 21b shows the vacuum chamber at NASA Glenn the IGNIT-SONER developed prototypes. Figure 21b shows the vacuum chamber at NASA Glenn the prototype was tested. where the prototype was tested. where the prototype was tested. TEFLON PROPELLANT TEFLON BAR PROPELLANT BAR

PPT ELECTRODES PPT ELECTRODES

D.I. SPARK D.I.PLUG SPARK PLUG

(a) (a)

COAXIAL CABLE COAXIAL CABLE

(b) (b)

−3 Figure 21. 21. 15kV kV initiationspark spark generated with IGNIT-SONER prototype system Figure generated with thethe IGNIT-SONER prototype system underunder 1.10´31.10 torr −3 Figure 21. 15 15 kVinitiation initiation spark generated with the IGNIT-SONER prototype system under 1.10 torr vacuum conditions. (a) Prototype of the IGNIT-SONER, (b) Vacuum chamber with a Pulsed vacuum conditions. (a) Prototype of the IGNIT-SONER; (b) Vacuum chamber with a Pulsed Plasma torr vacuum conditions. (a) Prototype of the IGNIT-SONER, (b) Vacuum chamber with a Pulsed Plasma Thruster during tests atGlenn NASA Glenn Research Thruster during the teststhe at NASA Center. Center. Plasma Thruster during the tests at NASAResearch Glenn Research Center.

2.3. High Voltage Power Supplies for Compact Neutron Generators 2.3. High Voltage Power Supplies for Compact Neutron Neutron Generators Generators This project developed a very high voltage, (1 MV) and high power, (40 W) piezoelectric-driven This project developed a very high voltage, (1 MV) and high power, (40 W) piezoelectric-driven power supply to drive compact neutron generators. The power supply is based on a modular power supply The power power supply supply is based on a modular supply to to drive drive compact compact neutron neutron generators. generators. The stackable system which combines piezoelectric transformers and Cockroft-Walton voltage multiplier stackable system which combines piezoelectric transformers and Cockroft-Walton voltage multiplier circuits [123,124]. Figure 22 illustrates a 200 kV stage using the high voltage/high power piezoelectric circuits [123,124]. Figure Figure 22 22 illustrates illustrates aa 200 200 kV kV stage stage using using the the high high voltage/high voltage/high power piezoelectric transformer driving a multi-stage Cockroft-Walton circuit. transformer driving aa multi-stage multi-stage Cockroft-Walton Cockroft-Waltoncircuit. circuit.

Figure 22. Connection of two modules able to generate 200 kV. Figure to generate generate 200 200 kV. kV. Figure 22. 22. Connection Connection of of two two modules modules able able to

3. Other Applications of Piezoelectric Transformers 3. Other Applications of Piezoelectric Transformers 3. Other Applications of Piezoelectric Transformers 3.1. High Voltage Non-Resonant Piezoelectric Transformer for Monitoring High Voltage Networks 3.1. High High Voltage Voltage Non-Resonant Non-Resonant Piezoelectric Piezoelectric Transformer Transformer for for Monitoring MonitoringHigh HighVoltage VoltageNetworks Networks 3.1. One of the less usual fields of applications was proposed in the late 1990s by the University One of of the the less less usual usual fields fields of of applications applications was was proposed proposed in in the the late late 1990s 1990s by by the University University One Politecnica de Catalunya, Spain, by A. V. Carazo in his doctoral thesis research [125]. the In this work, a Politecnica de Catalunya, Spain, by A. V. Carazo in his doctoral thesis research [125]. In this work, a Politecnica depiezoelectric Catalunya, Spain, by A. V. Carazo in his thesis [125].measurement In this work, non-resonant transformer construction wasdoctoral evaluated as aresearch high voltage non-resonant piezoelectric transformer construction was evaluated as voltage measurement atransducer non-resonant piezoelectric transformer construction as aa high highinside voltage for high voltage networks. Figure 23a showswas oneevaluated of the prototypes of ameasurement high voltage transducer for high voltage networks. Figure 23a shows one of the prototypes inside of a high voltage transducer for high voltage networks. Figure 23a shows one of the prototypes inside of a high voltage testing chamber. The figure shows the external epoxy isolating housing inside of which there is a testing chamber. chamber. The figure shows the external epoxy epoxy isolating isolating housing inside of which there is is aa testing The figure shows the external housing inside of which there large piezoelectric actuating unit connected to a high voltage, and a small sensor unit connected that large piezoelectric actuating unit connected to a high voltage, and a small sensor unit connected that large piezoelectric actuating unit connected to a high voltage, and small sensor unit connected provides low voltage. Both units are mechanically connected but aelectrically isolated, as shownthat the provides low low voltage. voltage. Both Both units units are are mechanically mechanically connected connected but but electrically electrically isolated, isolated, as as shown shown the the provides sketch of Figure 23b. The unit was designed for 36 kV. sketch of of Figure Figure 23b. 23b. The The unit unit was was designed designed for for 36 36 kV. kV. sketch

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Vout Vout

Vin Vin

(a) (a)

(b) (b)

Figure23. 23.Non-resonant Non-resonantpiezoelectric piezoelectrictransformer transformerfor formeasuring measuringhigh highvoltages voltagesininpower powerdistribution distribution Figure Figure 23. Non-resonant piezoelectric transformer for measuring high voltages in power networks.(a) (a)Piezoelectric Piezoelectricmeasuring measuringtransformer transformerinside insideofofaahigh highvoltage voltagetesting testingchamber; chamber,distribution (b)Sketch Sketch networks. (b) networks. (a) Piezoelectric measuring transformer inside of a high voltage testing chamber, (b) Sketch illustrating the principle of operation of the high voltage measuring transformer. illustrating the principle of operation of the high voltage measuring transformer. illustrating the principle of operation of the high voltage measuring transformer.

3.2.Piezoelectric PiezoelectricTransformers Transformersfor forIsolation IsolationFeedback Feedbackand andGate GateDriver DriverApplications Applications 3.2. 3.2. Piezoelectric Transformers for Isolation Feedback and Gate Driver Applications Therehave havebeen beenseveral severalpublications publicationsrecently recentlyon onthe theuse useofofpiezoelectric piezoelectrictransformers transformerstotoisolate isolate There Therecircuits have been severalas publications recently on the useof of Mosfet piezoelectric transformers to isolate feedback [126–128], well as for driving the gate transistors while providing feedback circuits [126–128], as well as for driving the gate of Mosfet transistors while providing feedback This circuits [126–128], as well as for driving the gate of Mosfet transistors while providing isolation. topic wasinitially initially considered inthe the1980s 1980s [46,47] andthere there nowrenewed renewed interest isolation. This topic was considered in [46,47] and isisnow interest isolation. This topic was initiallyPiezoelectric considered in the 1980s [46,47] andan there is now isolation renewed interest with innovative technologies. transformers provide excellent means, with innovative technologies. Piezoelectric transformers provide an excellent isolation means, with innovative technologies. Piezoelectric transformers provide an excellent isolation means, simultaneously providing providing power power and andsignal signaltransfer transfertotodrive drivethe thegate gateofoftransistors. transistors. Figure Figure 24 simultaneously 24 simultaneously providing power and signal transfer to drive the gate of transistors. Figure 24 exemplifies the use of PTs for control of the gate of a Mosfet. In this case, the control gate signal exemplifies the use of PTs for control of the gate of a Mosfet. In this case, the control gate signalis exemplifies to the useresonance of PTs for control of required the gate of a Mosfet. In this case, the control gate signal is ismodulated modulated tothe the resonancefrequency frequency requiredforfordriving drivingthe thePT. PT.InInthe theoutput outputofofthe thePT, PT,a modulated to the resonance frequency required for driving the PT. In the output of the PT, a may consist of aofrectifier circuit, converts the signal backback into the ademodulator demodulatorcircuit, circuit,which which may consist a rectifier circuit, converts the signal intoinput the demodulator circuit, which may consist of a rectifier circuit, converts the signal back into the input control signal. input control signal. control signal. OSCILLATOR OSCILLATOR GATE DRIVING GATE SIGNAL DRIVING

MODULATOR

DEMODULATOR

MODULATOR

DEMODULATOR

SIGNAL

Figure 24. Piezoelectric-based isolated circuit for feedback or gate driver applications. Figure 24. Piezoelectric-based isolated circuit for feedback or gate driver applications. Figure 24. Piezoelectric-based isolated circuit for feedback or gate driver applications.

4. Conclusions 4.4. Conclusions Conclusions During the mid-1990s and through the first decade of 2000s, piezoelectric transformers spread During the and through first decade of 2000s, transformers spread During themid-1990s mid-1990s throughthe the 2000s,piezoelectric piezoelectric transformers spread widely to a diverse range of and applications. Thefirst maindecade area ofofapplication was high voltage piezoelectric widely to a diverse range of applications. The main area of application was high voltage piezoelectric widely tofor a diverse range of applications. The main area of application highapplications voltage piezoelectric inverters CCFL backlighting used in laptop computers LCD panels.was Other included inverters for CCFL backlighting backlightingused used in laptop computers LCD panels. Other applications inverters for CCFL in laptop computers LCD panels. Other applications included ozone generators for medical, sanitary and beauty related applications and air cleaners. The nonincluded ozone generators for medical, sanitary andrelated beautyapplications related applications and air cleaners. ozone generators for medical, sanitary and beauty and air cleaners. The nonmagnetic characteristic of PTs, their compact size, their ability to generate high voltages, and their The non-magnetic characteristic of PTs, their compact size, their ability to generate high voltages, magnetic characteristic of PTs, their match compact their ability to generate highthis voltages, and high power density was a perfect forsize, portable applications. During period, it their was and their high density power density was a perfect match for portable applications.During Duringthis this period, period, ititwas high power was a perfect match for portable applications. estimated that the total sales per year exceeded 25–30 million with the main production locatedwas in estimated thethe total sales perper yearyear exceeded 25–30 million with the main locatedlocated in Japan. estimatedthat that total sales exceeded 25–30 million with the production main production in Japan. Toward the end of the 2000s, the LCD backlighting technology transitioned from the use of CCFLs Japan. Toward the end of the 2000s, the LCD backlighting technology transitioned from the use of to LEDs. This has in a2000s, progressiveLCD decline in sales oftechnology high voltage piezoelectric transformers. Toward the resulted end of backlighting transitioned from the use of CCFLs to LEDs. This hasthe resulted the in a progressive decline in sales of high voltage piezoelectric Most of the manufacturers of PTs for CCFL backlighting have now discontinued their production. CCFLs to LEDs. This hasmanufacturers resulted in a of progressive decline in sales of high voltage piezoelectric transformers. Most of the PTs for CCFL backlighting have now discontinued their Some production stillofremains for some ionizing products using single layer-type PTs. transformers. Most the manufacturers of PTs for CCFL backlighting have now discontinued their production. Some production still remains for some ionizing products using single layer-type PTs. Significant research has been done on the applicability of PTs for other commercial uses, such as production. Some production still remains for some ionizing products using single layer-type PTs. Significant research has been done on the applicability of PTs for other commercial uses, such as powerSignificant convertersresearch for AC-DC and DC-DC applications. For these the required PTsas has been on applications. the applicability PTs applications, forapplications, other commercial uses,PTs such power converters for AC-DC and done DC-DC For of these the required are are typically the step-down type, isolated, and with higher power requirements. The cost and power converters for AC-DC and DC-DC applications. For these applications, the required PTs are typically the step-down type, isolated, and with higher power requirements. The cost and limitation limitation of readily available products are one of the mainpower challenges in extending the applicability of typically the step-down type, isolated, and with higher requirements. The cost and limitation of readily available products are one of the main challenges in extending the applicability of power power transformers. of readily available products are one of the main challenges in extending the applicability of power transformers. transformers.

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Currently, the main area of interest of piezoelectric transformers is in medical, defense, security and sensitive measurement equipment, where the PT can offer a differential value added compared to magnetic transformers, even at a higher cost. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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