High-power, multioutput piezoelectric transformers ... - IEEE Xplore

502

ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 51, no. 5, may 2004

High-Power, Multioutput Piezoelectric Transformers Operating at the Thickness-Shear Vibration Mode Jinlong Du, Senior Member, IEEE, Junhui Hu, Member, IEEE, and King Jet Tseng, Senior Member, IEEE Abstract—In this study, a piezoelectric transformer operating at the thickness shear vibration mode and with dual or triple outputs is proposed. It consists of a lead zirconate titanate (PZT) ceramic plate with a high mechanical qual20 4 mm3 . The PZT ity factor m and a size of 120 ceramic plate is poled along the width direction. The electrodes of input and output parts are on the top and bottom surfaces of the ceramic plate and separated by narrow gaps. A new construction of support and lead wire connection is used for the transformer. At a temperature rise less than 20 C and efficiency of 90%, the piezoelectric transformer with dual outputs has a maximum total output power of 169.8 W, with a power of 129.5 W in one output and 40.3 W in another. The one with triple outputs has a maximum total output power of 163.1 W, with a power of 36.9 W in the first output, 13.0 W in the second output and 113.2 W in the third output. The maximum efficiency of the piezoelectric transformer with dual outputs and triple outputs is 98% and 95.7%, respectively. The voltage gains of the transformers are less than one, and different outputs have different gains. Also, there is a driving frequency range in which the load resistance of one output has little effect on the voltage gain of another output.

Q

I. Introduction piezoelectric transformer converts electric energy into ultrasonic vibration by converse piezoelectric effect at the input part, and the ultrasonic vibration into electric energy with a different voltage by piezoelectric effect at the output part. Compared with a conventional electromagnetic transformer, a piezoelectric transformer has many advantages such as high-power density, high efficiency, low profile, small size, light weight, no windings, electromagnetic noise-free operation, and noninflammability. Since the Rosen-type transformer [1], many piezoelectric transformers have been proposed and developed [2]– [13]. However, up to now, the piezoelectric transformers mainly have been used to generate high voltage for the cold cathode fluorescent lamps in notebook computers. The output power levels in these transformers are typically in the range of 0.5–30 W. However, in many modern applications, for example the power supplies for portable equipment, there is a strong need for low-voltage transformers with power levels above 30 W. Furthermore, in

A

Manuscript received June 10, 2003; accepted January 9, 2004. The authors would like to acknowledge the financial support by Academic Research Fund RG4/02 from the Ministry of Education, Singapore. The authors are with the School of EEE, Nanyang Technological University, Singapore 639798 (e-mail: [email protected]).

order to miniaturize the power supplies further, the transformers with multiple outputs are needed to supply the electric power to different parts in a system. These lead to the demand for a new type of piezoelectric transformer that can manage larger amounts of electric power and have several different outputs. In this study, to widen the application range of piezoelectric transformers, rectangular, plate-shaped piezoelectric transformers with dual or triple outputs were proposed and investigated for low-voltage applications. They operate at the thickness-shear vibration mode and are able to provide maximum total output power of more than 160 W, with a temperature rise less than 20◦ C. The maximum efficiency of the piezoelectric transformer with dual outputs is 98%. The voltage gains of the two outputs are 0.76 and 0.51 at the frequency of 278 kHz, respectively. The maximum efficiency of the piezoelectric transformer with triple outputs is 95.7%. The voltage gains of the three outputs are 0.75, 0.84, and 0.39 at the frequency of 278 kHz, respectively. Compared with most other low-voltage piezoelectric transformers, these transformers can be competitive in practical applications because of the high power, multiple outputs, simple structure, and flexibility in design.

II. Construction and Principle Fig. 1 shows the construction and dimensions of the proposed piezoelectric transformers operating at the thickness-shear vibration mode with dual outputs [Fig. 1(a)] and triple outputs [Fig. 1(b)]. They are made of lead zirconate titanate (PZT) ceramic plates with a high mechanical quality factor Qm and dimensions of 120×20×4 mm3 . The PZT ceramic plates are poled along the width direction. The electrodes of the input and output parts are on the top and bottom surfaces of the ceramic plates and separated by the narrow insulating gaps. For the transformer with dual outputs, the electrode areas of the input, output 1, and output 2 are 240 mm2 , 720 mm2 , and 1440 mm2 , respectively. For the transformer with triple outputs, the electrode areas of the input, output 1, output 2, and output 3 are 240 mm2 , 480 mm2 , 240 mm2 , and 1440 mm2 , respectively. Fig. 2 illustrates the thickness shear vibration mode used in the transformers. The vibration direction of the piezoelectric transformers is in parallel with its poling direction, and the applied electric

c 2004 IEEE 0885–3010/$20.00

du et al.: characteristics of dual and triple output transformers

503 TABLE I Relevant Properties of the PZT Material.

Fig. 1. Rectangular, plate-shaped, piezoelectric transformer operating at the thickness-shear vibration mode (a) with dual outputs, (b) with triple outputs.

Properties

Value

Dielectric constant ε11 /ε0 Electromechanical coupling factor k15 Piezoelectric coefficient d15 , 10−12 C/N Mechanical quality factor Qm

1590 0.70 510 2500

Fig. 3. Supporting construction of the piezoelectric transformers.

Due to the thickness-shear vibration mode, the nodal planes of the proposed transformers are located at the middle of the ceramic plates (see Fig. 3). So, the supporting structure shown in Fig. 3 was proposed and used. The transformer is supported by a slice of hard metal (support 3) and clamped by two thin metal bars (support 1 and 2). By this supporting construction, stable support, less energy loss, and excellent heat dissipation can be attained. In this study, the transformer is perpendicular to the base. The reason for placing the transformer vertically rather than horizontally is that a vertically placed plate has a better heat dissipation than that of a horizontally placed plate [14]. Fig. 4 is a photograph of the piezoelectric transformer with dual outputs. The lead wires with Fig. 2. The thickness-shear vibration mode used in the proposed piezoelectric transformers.

field is perpendicular to its poling direction. The relevant properties of the PZT ceramic are shown in Table I. The rectangular plate structure can effectively decrease the temperature rise of the transformers, which is beneficial to increasing the maximum power of the transformers. The reason is that the heat dissipation of a transformer can be improved by increasing the ratio of perimeter to area of the cross section of the transformer [14]. The thicknessshear mode is used in the proposed transformers because it has the largest electromechanical coupling factor of the commonly used vibration modes for most piezoelectric materials.

Fig. 4. Photograph of the piezoelectric transformer with dual outputs.

504


Fig. 5. Structure of the piezoelectric transformer with its input and output electrodes separated along the width direction of the ceramic plate.

Fig. 6. Experimental setup for measuring the characteristics of the piezoelectric transformers.

proper elasticity are chosen to keep the connection stable. By this connection, the maximum operating temperature of the transformer can be increased. Conventionally, the connection between the lead wires and electrodes of a piezoelectric transformer is realized by soldering. When the transformer operates at a large vibration, the local temperature at the solder points of the transformer may be very high. If this temperature approaches the melting point of the solder, the solder points fall off the electrodes. By the connection shown in Fig. 4, the above problem is avoided. So the maximum operating temperature of the transformer can be raised. This increases the maximum output power of the transformer. The structure shown in Fig. 5 also was investigated, in which the input and output electrodes are separated along the width direction of the ceramic plate. Experimental results show that this structure has a very low electromechanical-coupling factor kef f (≈0.05). So it has poor performance. Further analysis shows this is because the width and length of the input part are too close.

III. Experimental Setup The characteristics of the piezoelectric transformers were measured by the experimental setup shown in Fig. 6. The transformer was driven by an alternating current (AC) voltage generated by a function generator (Tektronix

AFG320, Tektronix Corp., Tokyo, Japan) and amplified by a high-speed power amplifier (NF HSA4014, NF Corp., Yokohama, Japan). In high-power experiments, a highfrequency electromagnetic transformer was used to amplify the voltage from the power amplifier. In Fig. 6, RL1 , RL2 , and RL3 are pure resisters. It is known that the efficiency of a piezoelectric transformer attains a maximum value when the load resistance is equal to 1/ωCd2 , where Cd2 is the clamped capacitance of the output part. This load is called the matching load. In this study, RL1 , RL2 , and RL3 are the matching loads for the output 1, output 2, and output 3, respectively. For the transformer with dual outputs as shown in Fig. 1(a), RL1 = 305 Ω, RL2 = 330 Ω. For the transformer with triple outputs as shown in Fig. 1(b), RL1 = 400 Ω, RL2 = 1190 Ω, RL3 = 230 Ω. The voltage, current, and power of the input and outputs were measured by the oscilloscopes. The highest temperature in the surface of the transformer was measured by an infrared thermometer (Keyence IT2-50, Keyence Corp., Osaka, Japan) about 1 minute after applying the input voltage. It was observed that the input part of a piezoelectric transformer had a higher temperature rise than the output parts, and the temperature rise at the connection points between the lead wires and the electrodes was higher than that at other points. So the measuring point of the temperature rise was chosen at the connection point of the input part. Also, the temperature rise of the transformer was kept below 20◦ C in the experiments, which were controlled by properly tuning the amplitude of the input voltage.

IV. Results and Discussion In the following discussion, PT-A is the transformer with dual outputs as shown in Fig. 1(a), and PT-B is the transformer with triple outputs as shown in Fig. 1(b). Table II shows the measured equivalent circuit parameters of the input and output parts of PT-A and PT-B. Each part was measured under the condition that the other parts were all short circuited. An HP4194A impedance analyzer (Yokogawa-Hewlett-Packard Corp., Tokyo, Japan) was used in the measurement. From the data of the output parts of PT-A or PT-B, it is seen that the capacitance ratio γ and the mechanical quality factor Qm decrease with increasing the area of electrodes. Fig. 7 shows the measured impedance of the input part of the piezoelectric transformers with the matching loads. Compared with the data in Table II, it is seen that the resonance and antiresonance frequencies of PT-A and PT-B all increase when the transformers have the matching loads. The impedance responses have two minimum values because of the existence of different output parts. Fig. 8 shows the voltage gain versus driving frequency of PT-A and PT-B with the matching loads. In this experiment, the input voltages were 128 Vrms for PT-A and 97 Vrms for PT-B. The temperature rise of the transformers was below 5◦ C. For both PT-A and PT-B, the output voltage gains are below 1.0, and they can be used in volt-


505

TABLE II Parameters of the Piezoelectric Transformers With Dual and Triple Outputs. fr (kHz) PT-A (dual outputs) Input 264.0 Output 264.0 Output1 267.0 Output2 264.0 PT-B (triple outputs) Input 263.5 Output 263.5 Output1 266.3 Output2 266.3 Output3 263.5

fa (kHz)

R(Ω)

L(mH)

Ca (pF)

Cb (pF)

Qm

γ

268.3 280.6 275.2 287.3

10.93 1.12 2.79 1.54

7.537 0.614 1.789 0.797

48.21 591.62 198.51 455.86

1797.08 4567.83 3250.49 2495.37

1143.8 964.9 1109.1 934.7

37.3 7.7 16.4 5.5

267.3 280.8 274.2 274.0 286.9

36.57 2.24 9.29 37.21 3.19

10.232 0.678 5.126 22.381 0.943

35.65 537.93 69.68 15.96 386.33

1411.21 4075.97 1302.54 321.47 2132.92

469.7 528.0 950.4 1034.8 533.0

39.6 7.6 18.7 20.1 5.5

fr and fa are the resonance and antiresonance frequencies, respectively; R is the equivalent resistance; L is the equivalent inductance; Ca is the equivalent capacitance; Cb is the clamped capacitance; Qm is the mechanical quality factor; and γ is the capacitance ratio.

(a)

(a)

(b) Fig. 7. Measured impedance of the input part of the piezoelectric transformers with the matching loads.

(b) Fig. 8. Voltage gain versus driving frequency of PT-A and PT-B with the matching loads.

506


(a) (a)

(b) Fig. 9. Frequency dependence of the total output power and efficiency of PT-A and PT-B under the input voltage of 128 Vrms and 97 Vrms , respectively. (b)

age step-down systems. In practical applications, in order to attain specific output voltages, one may adjust the electrode areas of the input and output parts and control the driving frequency. Fig. 9 shows the frequency dependence of the total output power and the efficiency of PT-A and PT-B under the input voltages of 128 Vrms and 97 Vrms , respectively. In this experiment, their load resistances all were matched, and the maximum temperature rise was lower than 5◦ C. For PT-A, a maximum total output power of 98.1 W and maximum efficiency of 98% were obtained under an input voltage of 128 Vrms at the driving frequencies of 278 kHz and 284 kHz, respectively. For PT-B, a maximum total output power of 63.4 W and maximum efficiency of 95.7% were obtained under an input voltage of 97 Vrms at the driving frequencies of 300 kHz and 268 kHz, respectively. The curves of both the total output power and efficiency fluctuate greatly. The first reason for this phenomenon is that there are some spurious vibration modes in the driving frequency range (see Fig. 7). The second reason is that there are different outputs in these transformers, and they

Fig. 10. Output powers of different outputs measured for different driving frequencies.

arrive at their maximum output powers and efficiencies at different driving frequencies. The distributions of the output powers in Fig. 9 were measured, and the results are shown in Fig. 10. For PT-A, at the maximum efficiency frequency of 284 kHz, the powers of output 1 and 2 are 62% and 38% of the total output power, respectively. For PT-B, at the maximum efficiency frequency of 268 kHz, the powers of output 1, 2, and 3 are 34%, 13%, and 53% of the total output power, respectively. The difference of the output powers is due to the difference of the electrode area of these outputs. Fig. 11 shows the relationships among the maximum total output power, temperature rise, and input voltage of PT-A and PT-B with the matching loads. Here, the maximum total output power means the maximum value of the total output power with respect to the driving frequency for a given input voltage. For PT-A, a maximum output power of 169.8 W can be obtained at an input


507

(a) (a)

(b) (b) Fig. 11. Relationships among the maximum total output power, temperature rise, and input voltage of PT-A and PT-B with the matching loads.

voltage of 180.1 Vrms and a temperature rise lower than 20◦ C. For PT-B, a maximum output power of 163.1 W can be obtained at an input voltage of 176.8 Vrms and a temperature rise lower than 20◦ C. When the input voltage is greater than 180 Vrms , the temperature rise of both PT-A and PT-B increases rapidly, and the characteristics of the transformer become quite unstable. When the temperature of the piezoelectric transformers is too high, the internal loss of the PZT material becomes very large. This increases the temperature of the transformers further and makes the operation unstable. Fig. 12 shows the efficiency versus the input voltage when the total output power reaches the maximum value. The matching loads were used. The efficiency of the transformers decrease with increasing the input voltage. The reason for this phenomenon is that the vibration of the transformers is intensified with increasing the input voltage, and the internal loss of the transformers increases with the increase of the vibration.

Fig. 12. Efficiency versus input voltage when the total output power reaches the maximum with respect to the driving frequency.

Fig. 13 shows the distributions of the output power in Fig. 11. For PT-A, the output power is 129.5 W for output 1 and 40.3 W for output 2 when the total output power reaches the maximum value at the input voltage of 180.1 Vrms . For PT-B, the output power is 36.9 W for output 1, 13.0 W for output 2, and 113.2 W for output 3 when the total output power reaches the maximum value at the input voltage of 176.8 Vrms . The difference of the power of the outputs is due to the difference between the electrode areas of the outputs. Fig. 14 shows the effects of the output number on the frequencies of the maximum efficiency and maximum total output power. In this experiment, PT-A with the two outputs in parallel was used as the single output transformer, PT-A with two separately operating outputs as the dual output transformer, and PT-B with three separately operating outputs as the triple output transformer. The matching loads were used, and the maximum temperature rise was kept below 20◦ C. The frequency of the maximum total output power increases with the increase of the output number of the piezoelectric transformer. On the contrary, the frequency of the maximum efficiency de-

508


(a)

(b) Fig. 13. Distributions of the output powers when the total output power reaches the maximum with respect to the driving frequency.

Fig. 15. Relationships between the voltage gain of output 1 and the frequency of PT-A when its output 2 is short circuited, open circuited, and matched, respectively.

creases with the increase of the output number. As the increase of the output number, the area of the insulating gaps between the electrodes increases. This is responsible for the phenomena shown in Fig. 14. To investigate the effect of the load of one output on the voltage gains of the other outputs, the relationship between the voltage gain of output 1 and the operating frequency was measured for PT-A when its output 2 was short circuited, open circuited, and matched, respectively; the results are shown in Fig. 15. The voltage gain of output 1 is affected by the load of output 2 for most of the driving frequencies, and there is a frequency range in which the voltage gain of output 1 has little change as the load of output 2 changes. For some frequencies, the output impedance of output 2 is quite small; so the load resistance of output 2 has little effect on the voltage gain of output 1. In many power electronics applications, we want transformers with constant voltage gains. So, this result shows the competitiveness of the multioutput piezoelectric transformers in power electronic applications.

V. Summary

Fig. 14. Effects of the output number on the frequencies of the maximum efficiency and maximum total output power.

The high-power, multioutput piezoelectric transformers operating at the thickness-shear vibration mode have been proposed, and their characteristics have been investigated experimentally. With a novel supporting construction and lead wires connection, at a temperature rise less than 20◦ C and the efficiency of 90%, the piezoelectric transformer with dual outputs (PT-A) has a maximum total output power of 169.8 W, with a power of 129.5 W in output 1 and 40.3 W in output 2, and the transformer with triple outputs (PT-B) has a maximum total output power of 163.1 W, with a power of 36.9 W in output 1, 13.0 W in output 2, and 113.2 W in output 3. The maximum effi-


ciency of PT-A and PT-B is 98% and 95.7%, respectively. The effects of the output number on the frequencies of the maximum total output power and maximum efficiency were investigated. The frequency of the maximum total output power increases with the increase of the output number; and the frequency of the maximum efficiency decreases with the increase of the output number. Also, there is a frequency range in which the voltage gain of one output is less affected by the load of another output.

References [1] C. A. Rosen, “Ceramic transformers and filters,” in Proc. Electron. Component Symp., 1956, pp. 205–211. [2] N. Wakatsuki, M. Ueda, and M. Satoh, “Low-loss piezoelectric transformer using energy trapping of width vibration,” Jpn. J. Appl. Phys., vol. 32, pt. 1, no. 5B, pp. 2317–2320, 1993. [3] T. Zaitsu, O. Ohnishi, T. Inoue, M. Shoyama, T. Ninomiya, F. C. Lee, and G. C. Hua, “Piezoelectric transformer operating in thickness extensional vibration and its application to switching converter,” IEEE Power Electron. Specialists Conf. Record, pp. 585–589, 1994. [4] S. Kawashima, O. Ohnishi, H. Hakamata, S. Tagami, A. Fukuoka, T. Inoue, and S. Hirose, “Third order longitudinal mode piezoelectric ceramic transformer and its application to high voltage power inverter,” in Proc. IEEE Ultrason. Symp., 1994, pp. 525–530. [5] K. Nakamura, “Lame-mode piezoelectric resonator and transformers using LiNbO3 crystals,” in Proc. IEEE Ultrason. Symp., 1995, pp. 999–1002. [6] J. H. Hu, Y. Fuda, M. Katsuno, and T. Yoshida, “A study on the rectangular-bar-shaped multilayer piezoelectric transformer using length extensional vibration mode,” Jpn. J. Appl. Phys., vol. 38, pp. 3208–3212, 1999. [7] M. Shoyama, K. Horikoshi, T. Ninomiya, T. Zaitsu, and Y. Sasaki, “Steady-stage characteristics of the push-pull piezoelectric inverter,” IEICE Trans. Commun., vol. E82-B, no. 8, pp. 715–721, Aug. 1999. [8] J. S. Yang and W. Zhang, “A thickness-shear high voltage piezoelectric transformer,” Int. J. Appl. Electromag. Mech., vol. 10, pp. 105–121, 1999. [9] J. H. Hu, G. R. Li, Y. Zhang, H. L. W. Chan, and C. L. Choy, “An improved method of analyzing the performance of multilayer piezoelectric transformers,” in Proc. IEEE Ultrason. Symp., 1999, pp. 943–946. [10] J. H. Hu, H. L. Li, H. L. W. Chan, and C. L. Choy, “A ringshaped piezoelectric transformer operating in the third symmetric extensional vibration mode,” Sens. Actuators, vol. A 88, pp. 79–86, 2001. [11] P. Laoratanakul, A. V. Carazo, P. Bouchilloux, and K. Uchino, “Unipoled disk-type piezoelectric transformers,” Jpn. J. Appl. Phys., vol. 41, pp. 1446–1450, 2002. [12] T. Hemsel, W. Littmann, and J. Wallaschek, “Piezoelectric transformers—State of the art and development trends,” in Proc. IEEE Ultrason. Symp., 2002, pp. 645–648. [13] Y. Sasaki, M. Umeda, S. Takahashi, M. Yamamoto, A. Ochi, and T. Inoue, “High-power characteristics of multilayer piezoelectric ceramic transducers,” Jpn. J. Appl. Phys., vol. 40, pp. 5743– 5746, 2001. [14] J. H. Hu, “Analyses of the temperature field in a bar-shaped piezoelectric transformer operating in longitudinal vibration mode,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 50, no. 6, pp. 594–600, 2003.

509 Jinlong Du was born on September 24, 1975. He received the B.E. and M.E. degrees in automatic testing and controlling from Harbin Institute of Technology, Harbin, China in 1997 and 2002, respectively. Currently, he is a Ph.D. candidate in the school of Electrical and Electronic Engineering, Nanyang Technological University (NTU), Singapore. His main research interests focus on ultrasonic/piezoelectric devices and control. He is a Student Member of IEEE.

Junhui Hu received B.E. and M.E. degrees in electrical engineering from Zhejiang University, Hangzhou, China, and Engineering Doctor’s degree from Tokyo Institute of Technology, Tokyo, Japan, in 1986, 1989 and 1997, respectively. He was a research engineer at the R&D Center of NEC Tokin, Sendai, Japan, from November 1997 to February 1999. Now, he is an assistant professor at the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU), Singapore. Before joining NTU, he had been a research fellow and postdoctoral fellow at the Center for Smart Materials, Hong Kong Polytechnic University, Hong Kong SAR, China. His research field includes ultrasonic actuators, piezoelectric transformers, transducers and other electromechanical devices. Dr. Hu won the Paper Prize from the Institute of Electronics, Information and Communication Engineers (Japan) in 1998, and is the author and co-author of sixteen disclosed patents. He is a member of IEEE and the Acoustic Society of Japan.

King-Jet Tseng (S’85–M’88–SM’98) was born in Singapore. He received the B.Eng. (First Class) and M.Eng. degrees from the National University of Singapore, Singapore, and the Ph.D. degree from Cambridge University, Cambridge, UK. He is currently an Associate Professor at the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore. He teaches power electronics and drives at the undergraduate level and supervises a number of research students. He is also the Supervisor of the Power Electronics and Drives Laboratory. He has been involved in research in power electronics, drives, and motion control since 1988. He has been a Visiting Professor to the University of Yangon, Myanmar, and to the Ecole Superieure d’Ingenieur en Electrotechnique et Electronique, Amiens, France. Dr. Tseng is a Fellow of the Cambridge Commonwealth Society and the Cambridge Philosophical Society. He is a Member of the Institute of Engineers, Singapore, a Corporate Member of the Institution of Electrical Engineers (UK), and a Senior Member of the Institute of Electrical and Electronic Engineers (USA). In 1996, he was awarded the Swan Premium by the Institution of Electrical Engineers (UK), for his work on gate turn-off thyristors for use in traction drives. From 1996 to 1998, he was the Chairman of the IEEE Industry Applications Chapter of Singapore. He is a Chartered Engineer registered in the UK. In 2000, he was awarded the IEEE Third Millennium Medal. He has been listed in the Marquis Who’s Who in the World and Who’s Who in Science and Engineering for his contributions to engineering education and research.