Fabrication of High-Power Piezoelectric Transformers ... - IEEE Xplore

1 downloads 0 Views 660KB Size Report
In the past few decades, piezoelectric transformers (PTs) have been extensively investigated because of their high power density, small size and low weight, and ...
408

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

vol. 60, no. 2,

February

2013

Fabrication of High-Power Piezoelectric Transformers Using Lead-Free Ceramics for Application in Electronic Ballasts Song-Ling Yang, Shih-Ming Chen, Cheng-Che Tsai, Cheng-Shong Hong, and Sheng-Yuan Chu Abstract—CuO is doped into (Na0.5K0.5)NbO3 (NKN) ceramics to improve the piezoelectric properties and thus obtain a piezoelectric transformer (PT) with high output power. In X-ray diffraction patterns, the diffraction angles of the CuOdoped NKN ceramics shift to lower values because of an expansion of the lattice volume, thus inducing oxygen vacancies and enhancing the mechanical quality factor. A homogeneous microstructure is obtained when NKN is subjected to CuO doping, leading to improved electrical properties. PTs with different electrode areas are fabricated using the CuO-doped NKN ceramics. Considering the efficiency, voltage gain, and temperature rise of PTs at a load resistance of 1 kΩ, PTs with an electrode with an inner diameter of 15 mm are combined with the circuit design for driving a 13-W T5 fluorescent lamp. A temperature rise of 6°C and a total efficiency of 82.4% (PT and circuit) are obtained using the present PTs.

I. Introduction

I

n the past few decades, piezoelectric transformers (PTs) have been extensively investigated because of their high power density, small size and low weight, and high efficiency [1]. PTs are used in practical applications such as battery chargers for mobile phones, ac/dc adapters for mobile computers, electronic ballasts for fluorescent lamps, gate drivers of MOSFETs, and insulated-gate bipolar transistors (IGBTs) [2]–[9]. PTs were first proposed in 1954, and were patented in 1958 by Rosen et al. [10]. However, Rosen-type PTs have some disadvantages: 1) the different polarization directions of the input and output sections create internal stress at the interface between transversal and longitudinal polarizations; and 2) the power capabilities of Rosen-type PTs cannot be enhanced [11]. Berlincourt proposed a disk-type PT structure whose input and output sections had the same polarization direction, which eliminated the internal stress at the interface [12]. Moreover, the energy conversion efficiency and output power can be enhanced because the electromechanical coupling

Manuscript received April 24, 2012; accepted November 6, 2012. The authors acknowledge financial support from the Bureau of Energy, Ministry of Economic Affairs, Taiwan (grants NSC 99-ET-006-008-ET and 100-ET-E-006-003-ET). S.-L. Yang, S.-M. Chen, and S.-Y. Chu are with the Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C. (e-mail: [email protected]). C.-C. Tsai is with the Department of Electronics Engineering and Computer Science, Tung-Fang Design University, Kaohsiung, Taiwan, R.O.C. C.-S. Hong is with the Department of Electronic Engineering, National Kaohsiung Normal University, Kaohsiung County, Taiwan, R.O.C. S.-Y. Chu is with the Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan, R.O.C. DOI http://dx.doi.org/10.1109/TUFFC.2013.2577 0885–3010/$25.00

coefficient, kp, in the planar mode is higher than the coefficient k31 [13]. Pb(Zr,Ti)O3 (PZT)-based ceramics have been shown to be suitable for fabricating PTs because of their excellent piezoelectric properties, which include high electromechanical coupling coefficients (kp, kt), a high mechanical quality factor (Qm), and a low resonant impedance. A high Qm and a low resonant impedance can improve the temperature stability and output power of PTs. Yoo et al. reported that a 28-W fluorescent lamp can be lit by PTs fabricated using PZT-based ceramics [14]. However, the high volatility of PbO is a concern for the environment and human health. (Na,K)NbO3 (NKN) is commonly used for preparing lead-free piezoelectric ceramics because of its high electromechanical coupling coefficient. Pure NKN ceramics are difficult to synthesize as a dense microstructure and have a low Qm value. Consequently, CuO or copper oxide compounds are often doped into NKN ceramics to improve densification and enhance the Qm value [15], [16]. In our previous study, the Qm value of CuO-doped NKN ceramics prepared using the two-step calcination method [the two-step calcination (BO) method] was increased to more than 2100 [17]–[19]. The temperature coefficient of frequency (TCF), temperature coefficient of capacitance (TCC), dielectric constant (εT33/ε 0), and dielectric loss of CuO-doped NKN ceramics were also improved using the BO method. Few studies have focused on lead-free material for PTs because of their poor material properties compared with those of lead-based ceramics. Lin et al. fabricated contour-vibration-mode PTs using NKN + K5.4Cu1.3Ta10O29 + MnO2 ceramics (kp: 40%, Qm: 1900) that exhibited an output power of 8 W and a temperature rise of 20°C under an input voltage of 170 V(pp) [20]. Piezoelectric transformers with a high power density of 25 W/cm3 were obtained by Gurdal et al. [21]. In the present study, lead-free PTs with high output power and power density were fabricated using CuO-doped NKN ceramics. The PTs were combined with a circuit design to drive a 13-W T5 fluorescent lamp. This study is the first to fabricate electronic ballasts using lead-free PTs. II. Experimental Procedure A. Material Synthesis Our previous study showed that CuO-doped NKN ceramics have excellent piezoelectric properties when

© 2013 IEEE

yang et al.: fabrication of high-power piezoelectric transformers using lead-free ceramics

0.75 mol% CuO dopants were doped into NKN ceramics [17]. Therefore, NKN ceramics with 0.75 mol% CuO were used as the base material. A two-step calcination method (BO method) was used to prepare the ceramics because it effectively improves the temperature stability and sintering behavior of the samples. In the BO method, (Na0.5K0.5)NbO3 + 0.75 mol% CuO ceramics were prepared with pure oxides of Na2CO3, K2CO3, Nb2O5, and CuO powders (>99% purity) used as starting materials. Nb2O5 and CuO powders were ball-milled in a polyethylene jar for 24 h with ZrO2 balls using ethanol as the medium. After drying, the powders were first calcined at 1000°C for 5 h (first calcination). Then, Na2CO3 and K2CO3 were added into (Nb,Cu) precursors according to the stoichiometric formula of NKN + 0.75 mol% CuO, and ball-milled together for 24 h. After drying, the CuOdoped NKN powders were calcined at 900°C for 5 h in air (secondary calcination). After the calcination, the powders were mixed with 10 wt.% polyvinyl alcohol (PVA) aqueous solution and then pressed into a disk by cold isostatic pressing (CIP) at 100 MPa. Then, the disk samples underwent binder burnout at 650°C for 1 h. All of the specimens were sintered at 1100°C for 2 h at a heating rate of 5°C/min in air. The crystalline profile was determined by X-ray diffraction (XRD) with CuKα (λ = 0.15406 nm) radiation using a MultiFlex X-ray diffractometer (Rigaku Corp., Tokyo, Japan). To confirm the exact diffraction angles, silicon powders were used for calibration. The microstructure was observed using an S-4100 (Hitachi Ltd., Tokyo, Japan) scanning electron microscope (SEM). Bulk densities were measured using the Archimedes method. Silver paste was painted on both sides of the samples as electrodes. The samples were then heated at 800°C for 30 min and then poled by a dc field (30 kV/cm) at 125°C for 30 min in a silicone oil bath. The piezoelectric properties were measured by an HP 4294A precision impedance analyzer (Agilent Technologies Inc., Santa Clara, CA) and an APC 90–2031 d33 meter (APC International Ltd., Mackeyville, PA). The electromechanical coupling factor in planar (kp) and thickness (kt) modes and the mechanical quality factor (Qm) were calculated using the resonance–antiresonance method.

409

Fig. 1. Schematic diagram of disk-shaped piezoelectric transformers.

measured using a TCP A300 current probe (Tektronix Inc., Beaverton, OR) and a Tektronix DPO 2024 digital oscilloscope. The temperature rise of the PTs was measured by an infrared thermometer and the measured location is about the center of the PTs. III. Results and Discussion A. Material and Electrical Properties XRD patterns of NKN and CuO-doped NKN ceramics are shown in Fig. 3. Both samples have a perovskite structure without any secondary phases, indicating that CuO doping did not significantly change the structure of the NKN ceramics. However, the diffraction angles of CuOdoped NKN ceramics shifted to lower values because Nb5+ ions were replaced by Cu2+ ions (inset of Fig. 3), which caused the lattice volume to expand [the ionic radius of Cu2+ (0.73 Å) is greater than that of Nb5+ (0.64 Å)]. Fig. 4 shows SEM images of NKN and CuO-doped NKN ceramics. The pure NKN ceramics had some pores [Fig. 4(a)]. With CuO doping [Fig. 4(b)], the densification of the samples improved because of the effect of CuO as a sintering aid. Moreover, the microstructure of the samples became homogeneous (see our previous study for an explanation [17], [18]). Table I lists the electrical properties of NKN and CuO-doped NKN ceramics. The bulk density

B. Piezoelectric Transformer Fabrication and Measurement Fig. 1 shows a schematic diagram of a disk-shaped PT. The dimensions of the proposed PTs were an outer diameter (2a) of 25 mm; inner diameters (2c) of 9, 11, 13, and 15 mm; a ring gap width (b − c) of 1 mm; and a thickness (t) of 1 mm. The performances of the PTs under load resistances (RL) of 500 and 1 kΩ were investigated using the experiment setup shown in Fig. 2. The PTs were driven by an FG-708S function generator (Motech Industries Inc., Taipei, Taiwan) and a BA4825 high-speed bipolar amplifier (NF Corporation, Yokohama, Japan). The voltage, current, and power at the input and output sections were

Fig. 2. Experimental setup for measuring piezoelectric transformer performance.

410

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

vol. 60, no. 2,

February

2013

Fig. 5. Schematic diagram of the equivalent circuit of the piezoelectric transformers.

Fig. 3. X-ray diffraction patterns of (a) (Na0.5K0.5)NbO3 (NKN) and (b) CuO-doped NKN ceramics (star: Si powder). The inset shows the diffraction angles of NKN and CuO-doped NKN ceramics between 31.6° and 32°.

increased from 4.23 to 4.49 g/cm3 after CuO doping. Many researchers have reported that a liquid phase forms because of the effect of CuO as a sintering aid, thus enhancing densification [15], [22]. In general, samples with higher Qm have lower dielectric constants (εT33/ε 0) and piezoelectric coefficients (d33). However, the εT33/ε 0 and d33 values of CuO-doped NKN ceramics are higher than those of NKN ceramics because the densification of CuO-doped NKN ceramics is higher than that of NKN ceramic (see in Table I). The densification and microstructure of the samples improved the major electrical properties. Of note, the Qm value and resonant impedance increased from 67 to 2500 and decreased from 300 to 3 Ω, respectively, after CuO doping. A high Qm is commonly accompanied by a low resonant impedance, which helps to enhance the output power of PTs. A minimum resonant impedance of 3 Ω was obtained for CuO-doped NKN ceramics prepared using the BO method. From the XRD patterns, the increase in the lattice constant is caused by the Nb5+ ions being replaced by Cu2+ ions, resulting in the formation of oxygen vacancies. The formation of oxygen vacancies prevents the domain walls from moving, and thus a high Qm can be obtained [23]–[25]. B. Electrode Area Design of PTs for Application in Electronic Ballasts For a 13-W T5 fluorescent lamp, the equivalent resistance is approximately 1 kΩ. Therefore, PTs in electronic

ballasts must have a high efficiency at a load resistance of 1 kΩ. A PT can be simulated by the equivalent circuit shown in Fig. 5. The parameters of the equivalent circuit, including the turn ratio N, equivalent resistance R, equivalent inductance L, equivalent capacitance C, and damped capacitances Cd1 and Cd2 of the input and output, respectively, can be obtained using the built-in function of an impedance analyzer. The efficiency of PTs was significantly affected by the load resistance which related to the equivalent capacitance, Cd2. Theoretical efficiency (η) can be calculated from the equivalent circuit as [26]

η=

1 + N RR L(ω 0C d2)2 +

N 2R RL

.

The experimental and simulation efficiency of PTs as a function of the inner diameter of the electrode is shown in Fig. 6(a) (the PTs were measured under an input voltage of 20 Vpp). The efficiency of PTs increased with increasing inner diameter at load resistances of 500 and 1 kΩ because the equivalent capacitance also increased. The theoretical efficiency of PTs as a function of the inner diameter of the electrode is shown in Fig. 6(a) (dashed line). The trends of the experimental results are similar to those of the theoretical data. PTs with inner diameters of 13 and 15 mm have efficiencies of more than 90% at a load resistance of 1 kΩ. The voltage gain of PTs as a function of the inner diameter of the electrode is shown in Fig. 6(b). The voltage gain of PTs increases with decreasing ratio of the electrode area of output, which is related to the equivalent inductances of the input and output sections. For electronic ballasts, a voltage gain of 0.8 to 1 is enough to drive a 13-W T5 fluorescent lamp (when the lamp is drivTABLE I. Electrical Properties of (Na0.5K0.5)NbO3 (NKN) and CuO-Doped NKN Ceramics. Property

Fig. 4. Scanning electron micrograph images of (a) (Na0.5K0.5)NbO3 (NKN) and (b) CuO-doped NKN ceramics.

1 2

Density (g/cm3) kp (%) kt (%) k33 (%) Qm Rz (Ω) εT33/ε 0 (at 1 kHz) tan δ (at 1 kHz) d33 (pC/N) TCF (%/°C) Tc (°C)

CuO-doped NKN

Pure NKN

4.49 41.5 50.8 62.1 2500 3 280 0.0015 95 −0.021 416

4.23 36.8 45.6 56.1 67 300 266 0.0235 80 −0.037 424

yang et al.: fabrication of high-power piezoelectric transformers using lead-free ceramics

Fig. 6. (a) Efficiency and (b) voltage gain of piezoelectric transformers as functions of inner diameter at load resistances of 500 and 1 kΩ. The dashed line in Fig. 6(a) is the theoretical data of piezoelectric transformers.

ing). Fig. 7 shows the input voltage and temperature rise versus the output power for PTs with 3 different electrode areas. For a given output power, the PT with the largest inner electrode had the highest required input voltage [see Fig. 7(a)]. This is because a larger inner electrode in the PTs has a smaller electrode area of input section which decreased the input power, leading to a lower output power. Therefore, PTs with a larger input/output ratio have higher output power. However, higher output power is accompanied by a higher temperature rise [see Fig. 7(b)], which causes device instability. The temperature rise is inversely proportional to the electrode area of the output section for a given output power. Compared with other lead-free PTs, those fabricated using CuO-doped NKN ceramics exhibit better temperature stability [20]. In this study, the CuO-doped NKN ceramics showed a high Qm value of 2500 and a low resonant impedance (RZ) because

411

Fig. 7. (a) Input voltage and (b) temperature rise of piezoelectric transformers with three different electrode areas as functions of output power at a load resistance of 1 kΩ.

the microstructure densification of the samples was improved by a modified-process method. Low resonant impedance reduces the power loss in PTs and thus enhances power density. Therefore, a low temperature rise of the lead-free PTs was obtained. C. Circuit Design for Driving a 13-W T5 Fluorescent Lamp A PT is a good driver for T5 fluorescent lamps because the starting voltage of T5 fluorescent lamps is more than 800 Vpp and drops to be about 320 Vpp when the T5 fluorescent lamp is driven. The PTs fabricated in this study have a suitable turn ratio (the voltage gain is more than 40 V/V for an open load) and a high efficiency. To drive a 13-W T5 fluorescent lamp, the PTs were combined with a circuit design. A diagram of the circuit design is shown in Fig. 8. IC TL494 was chosen to produce a square wave

412

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

vol. 60, no. 2,

February

2013

Fig. 8. Schematic diagram of circuit design for piezoelectric transformers for driving a T5 fluorescent lamp.

with a frequency of 100 to 200 kHz. This signal was input to an IC IR2111 half-bridge driver. Then, the IC IR2111 driver produces signals of Hi and Lo to control the MOSFET (IRF740). To avoid distorting the output waveforms, a resonant tank (L1 and C1) was added in front of the PTs. A sine wave is produced to drive the PTs. The design of the resonant tank amplifies the input voltage (the gain reached 3 V/V). Therefore, an input voltage (VD) of 64 V is enough to drive a 13-W T5 fluorescent lamp. The voltage into the PTs reaches 384 Vpp and then steps-down to about 300 Vpp when the T5 fluorescent lamp is driven. Fig. 9 shows the output waveforms of the PTs when the 13-W T5 fluorescent lamp is driven. The output voltage and output current are in phase because the fluorescent lamp is a pure load resistance when the T5 fluorescent lamp is driven. Fig. 10 shows that the 13-W T5 fluorescent lamp was lit using the present PTs. A total efficiency (PTs and circuit) of 82.4% and the temperature rise of 6°C were obtained when the 13-W T5 fluorescent lamp is driven. The output power of 13 W of the lead-free PTs is the best data reported so far. IV. Conclusion CuO was doped in NKN ceramics to improve the piezoelectric properties for PT application. XRD patterns showed that the diffraction angles of CuO-doped NKN ceramics shifted to lower values because of the expan-

Fig. 9. Output waveforms of piezoelectric transformers driving a 13-W T5 fluorescent lamp.

Fig. 10. Photograph of 13-W T5 fluorescent lamp driven by piezoelectric transformers combined with circuit design.

sion of the lattice volume. Nb5+ ions were replaced by Cu2+ ions, inducing oxygen vacancies and preventing the domain walls from moving. CuO doping improved the homogeneity of the microstructure and the electrical properties of NKN ceramics, thus enhancing the performance of PTs. The PTs with an electrode inner diameter of 15 mm were chosen for application in electronic ballasts. A Dclass circuit design could drive the PTs to light a 13-W T5 fluorescent lamp. A low temperature rise of 6°C in the PTs was obtained when the 13-W T5 fluorescent lamp was driven. Moreover, the total efficiency of the PT and circuit reached 82.4%. References [1] G. Ivensky, I. Zafrany, and S. Ben-Yaakov, “Generic operational characteristics of piezoelectric transformers,” IEEE Trans. Power Electron., vol. 17, no. 6, pp. 1049–1057, 2002. [2] P. J. M. Smidt and J. L. Duarte, “Powering neon lamps through piezoelectric transformers,” in IEEE Power Electronics Specialists Conf. Rec., 1996, pp. 310–315. [3] M. Imori, T. Taniguchi, H. Matsumoto, and T. Sakai, “A photomultiplier high voltage power supply incorporating a piezoelectric ceramic transformer,” IEEE Trans. Nucl. Sci., vol. 43, no. 3, pp. 1427–1431, 1996. [4] Y. Fuda, K. Kumasaka, M. Katsuno, H. Sato, and Y. Ino, “Piezoelectric transformer for cold cathode fluorescent lamp inverter,” Jpn. J. Appl. Phys., vol. 36, no. 5B, pt. 1, pp. 3050–3052, 1997. [5] T. Zaitsu, Y. Fuda, Y. Okabe, T. Ninomiya, S. Hamamura, and M. Katsuno, “New piezoelectric transformer converter for ac adapter,” in IEEE Applied Power Electronics Conf. Proc., 1997, pp. 568–572. [6] H. Kakedhashi, T. Hidaka, T. Ninomiya, M. Shoyama, H. Ogasawara, and Y. Ohta, “Electronic ballast using piezoelectric transformers for fluorescent lamps,” in IEEE Power Electronics Specialists Conf. Rec., 1998, pp. 29–35. [7] J. Navas, T. Bove, J. A. Cobos, F. Nuño, and K. Brebol, “Miniaturised battery charger using piezoelectric transformers,” in IEEE Applied Power Electronics Conf. Proc., 2001, pp. 492–496. [8] D. Vasic, F. Costa, and E. Sarraute, “A new MOSFET & IGBT gate drive insulated by a piezoelectric transformer,” in IEEE Power Electronics Specialists Conf. Rec., 2001, pp. 1479–1484. [9] R. L. Lin, F. C. Lee, E. M. Baker, and D. Y. Chen, “Inductorless piezoelectric transformer electronic ballast for linear fluorescent lamp,” in IEEE Applied Power Electronics Conf. Proc., 2001, pp. 664–669.

yang et al.: fabrication of high-power piezoelectric transformers using lead-free ceramics [10] C. A. Rosen, K. Fish, and H. C. Rothenberg, “Voltage mode active clamp PWM controller for high speed operation,” U. S. Patent 2 830 274, Apr. 8, 1958. [11] P. Laoratanakul, A. V. Carazo, P. Bouchilloux, and K. Uchino, “Unipoled disk-type piezoelectric transformers,” Jpn. J. Appl. Phys., vol. 41, no. 3A, pp. 1446–1450, 2002. [12] D. A. Berlincourt, “Piezoelectric starter and ballast for gaseous discharge lamps” U. S. Patent 3 764 848, Oct. 9, 1973. [13] J. Yoo, K. Yoon, Y. Lee, S. Suh, J. Kim, and C. Yoo, “Electrical characteristics of the contour-vibration-mode piezoelectric transformer with ring/dot electrode area ratio,” Jpn. J. Appl. Phys., vol. 39, no. 5A, pp. 2680–2684, 2000. [14] J. Yoo, K. Yoon, S. Hwang, S. Suh, J. Kim, and C. Yoo, “Electrical characteristics of high power piezoelectric transformer for 28 W fluorescent lamp,” Sens. Actuators A, vol. 90, no. 1–2, pp. 132–137, 2001. [15] M. Matsubara, T. Yamaguchi, W. Sakamoto, K. Kikuta, T. Yogo, and S. Hirano, “Processing and piezoelectric properties of lead-free (K,Na)(Nb,Ta)O3 ceramics,” J. Am. Ceram. Soc., vol. 88, no. 5, pp. 1190–1196, 2005. [16] M. Matsubara, K. Kikuta, and S. Hirano, “Piezoelectric properties of (K0.5Na0.5)(Nb1−xTax)O3−K5.4CuTa10O29 ceramics” J. Appl. Phys., vol. 97, no. 11, art. no. 114105, 2005. [17] S. L. Yang, C. C. Tsai, Y. C. Liou, C. H. Hong, and S. Y. Chu, “Effects of modified-process on the microstructure, internal bias field, and activation energy in CuO-doped NKN ceramics,” J. Eur. Ceram. Soc., vol. 32, no. 12, pp. 1643–1650, 2012. [18] S. L. Yang, C. C. Tsai, Y. C. Liou, C. H. Hong, and S. Y. Chu, “Investigation of CuO-doped NKN ceramics with high mechanical quality factor synthesized by a B-site oxide precursor method,” J. Am. Ceram. Soc., vol. 95, no. 3, pp. 1011–1017, 2012. [19] S. L. Yang, C. C. Tsai, Y. C. Liou, C. H. Hong, B. J. Li, and S. Y. Chu, “Effects of improved process for CuO-Doped NKN lead-free ceramics on high-power piezoelectric transformers,” IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., vol. 58, no. 12, pp. 2555–2561, 2011. [20] D. Lin, M. S. Guo, K. H. Lam, K .W. Kwok, and H. L. W. Chan, “Lead-free piezoelectric ceramic (K0.5Na0.5)NbO3 with MnO2 and K5.4Cu1.3Ta10O29 doping for piezoelectric transformer application,” Smart Mater. Struct., vol. 17, no. 3, art. no. 035002, 2008. [21] E. A. Gurdal, S. O. Ural, H. Y. Park, S. Nahm, and K. Uchino, “High power (Na0.5K0.5)NbO3-based lead-free piezoelectric transformer,” Jpn. J. Appl. Phys., vol. 50, no. 2, art. no. 027101, 2011. [22] E. Li, H. Kakemoto, S. Wada, and T. Tsurumi, “Enhancement of Qm by co-doping of Li and Cu to potassium sodium niobate leadfree ceramics,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 55, no. 5, pp. 980–987, 2008. [23] K. Toshio, S. Toshimasa, T. Takaaki, and D. Masaki, “Effects of manganese addition on piezoelectric properties of Pb(Zr0.5Ti0.5)O3,” Jpn. J. Appl. Phys., vol. 31, no. 9S, pp. 3058–3060, 1992. [24] Y. D. Hou, M. K. Zhu, F. Gao, H. Wang, B. Wang, H. Yan, and C. S. Tian, “Effect of MnO2 addition on the structure and electrical properties of Pb(Zn1/3Nb2/3)0.2(Zr0.5Ti0.5)0.8O3 ceramics,” J. Am. Ceram. Soc., vol. 87, no. 5, pp. 847–850, 2004. [25] S. M. Lee, S. H. Lee, C. B. Yoon, H. E. Kim, and K. W. Lee, “Low-temperature sintering of MnO2-doped PZT-PZN piezoelectric ceramics,” J. Electroceram., vol. 18, no. 3–4, pp. 311–315, 2007. [26] S. T. Ho, “Modeling of a disk-type piezoelectric transformer,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 54, no. 10, pp. 2110– 2119, 2007.

Song-Ling Yang was born in Tainan, Taiwan, on October 19, 1983. He received the M.S. degree from the Department of Electronic Engineering, Kun Shan University, Tainan, Taiwan, ROC, in 2008. He is currently pursuing a Ph.D. degree in the Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, ROC. His present research is on the synthesis of lead-free piezoelectric ceramics using various processes, and the development of lead-free piezoelectric materials for application in ultrasonic resonators, transducers, and piezoelectric transformers.

413

Shih-Ming (Orion) Chen was born in Tainan, Taiwan, R.O.C. He received his B.S. and Ph.D. degrees in electrical engineering from the National Cheng-Kung University (NCKU), Tainan, Taiwan, in 2003 and 2011, respectively. Dr. Chen was employed as a research and design engineer at LUMIN Electronics from 1991 to 1995. He has served at Delta Electronics Inc., Taiwan, as a section manager of the engineering department from 1995 to 1999. From 1999 to 2002, he was employed as a manager in charge of the Tainan R&D Center of the Sino-American Electronic Co. Ltd., Taiwan. Since 2002, he has worked as a section manager of Product Development of LCD-TV Head Division, CHIMEI Optoelectronics Corp. Since 2009, he has rejoined Delta Electronics as senior manager in the division of LCD display power. Currently, he is a Postdoctoral Fellow in the Green Energy Electronics Research Center at NCKU. His research interests are dc-to-dc converters, photovoltaic inverters, switching power supplies, uninterrupted power systems, CCFL/EEFL inverters, and LED drivers and chromatics controls.

Cheng-Che Tsai was born in Kaohsiung, Taiwan. He received his M.S. degree in electrical engineering from the National Cheng Kung University in 1987 and his Ph.D. degree in electronic engineering from the Southern Taiwan University in 2011. Since 1990, he has worked as a technical consultant on PTC and PZT-based material systems development and device applications at Hwang Sun Enterprise Co. Ltd. He is also an associate professor in the Department of Electronics and Computer Science at the Tung Fang Design University. His present research includes development of piezoelectric functional materials for ultrasonic devices, piezoelectric transducer and transformer design, and ultrasonic therapeutic system design.

Cheng-Shong Hong is a professor of electronic engineering at the National Kaohsiung Normal University, Kaohsiung City, Taiwan, ROC. He received his B.S., M.S., and Ph.D. degrees in electrical engineering from the Cheng Kung University, Tainan City, Taiwan, ROC. Dr. Hong is interested in relaxation phenomena of nanopolarizations and piezoelectric properties in ferroelectric ceramics and how processing, polarization, and piezoelectric properties can be modified to optimize conditions. His work is primarily focused on physical models and applications for dielectrics and piezoelectrics in electronic ceramics.

Sheng-Yuan Chu was born in Taipei, Taiwan on February 11, 1965. He received his Ph.D. degree in electrical engineering from The Pennsylvania State University in 1994. He is a professor in the Department of Electrical Engineering, National Cheng Kung University, Taiwan. The main topics of his scientific activity are piezoelectric ceramic materials and their applications for resonators and SAW devices; electrical and microstructural properties of ZnO thin films and their applications for SAW filters and sensors; organic light-emitter diodes (OLEDs); and step and flash imprint lithography (SFIL) technology for OLED process.