Seminar report on PIEZOELECTRIC TRANSFORMERS Guided by Presented by Mrs. Jaseela M Resmara s Roll no 01105025 Piezoelectric transformers Seminar ‘04 ACKNOWLEDGEMENT I express my sincere gratitude to Dr. P.M.S Nambisan, Prof. and Head of Department of Electrical and Electronics Engineering, MES College of Engineering, Kuttipuram, for his cooperation and encouragement. I would also like to thank my seminar guide Mrs.Jaseela.M (Lecturer, Department of Electrical and Electronics Engineering), Asst. Prof. Gylson Thomas (Staff in-charge Department of Electrical and Electronics Engineering), for their invaluable advice and wholehearted cooperation without which this seminar would not have seen the light of day. Finally I would like to express my gracious gratitude to my family and friends for their valuable advice and encouragement. MESCE, Kuttippuram 1 EEE Dept. Piezoelectric transformers Seminar ‘04 PIEZOELECTRIC TRANSFORMERS ABSTRACT Small electronic devices which operate at high voltages require a compact transformer to step up the low voltages of available power supplies. Electromagnetic transformers, having thousands of wire turns around a ferrite core, have become an obstacle to the progress of miniaturization, as they are the most bulky devices on a circuit board. Here comes the importance of the new concept – piezoelectric transformer. Piezoelectric transformers are, in fact, not transformers. They have no wires or magnetic fields, so they are better categorized as dynamos. They form a better alternative to conventional electromagnetic transformers. This paper deals with its working principle. Their advantages and the disadvantages are also explained. MESCE, Kuttippuram 2 EEE Dept. Piezoelectric transformers Seminar ‘04 CONTENTS INTRODUCTION PIEZOELECTRICITY - THE CONCEPT PIEZOELECTRIC TRASFORMERS o WORKING PRINCIPLE o RECTIFICATION OF INADEQUACIES o CONSTRUCTION o THE ELECRICAL PARAMETER o PT SCHEMATIC o APPILICATIONS o ADVANTAGES o DISADVANTAGES A BRIEF ANALYSIS o DESCRIPTION o ANALYSIS PERFORMANCE MEASUREMENT CONCLUSION REFERENCE MESCE, Kuttippuram 3 EEE Dept. Piezoelectric transformers Seminar ‘04 INTRODUCTION With the onset of miniaturization, many applications in the electronics industry now require small, low profile components with a high efficiency of operation. Electromagnetic transformers, having thousands of wire turns around a ferrite core, have become an obstacle to the progress of miniaturization, as they are among the most bulky devices on a circuit board. Piezoelectric transformers have recently received some attentions as a possible alternative. Piezoelectric transformer is, in fact, not transformers. They have no wires or magnetic field, so they are better categorized as dynamos. These transformers work like a motor that is mechanically coupled to a generator. MESCE, Kuttippuram 4 EEE Dept. Piezoelectric transformers Seminar ‘04 PIEZO ELECRICITY – THE CONCEPT Piezoelectricity is a coupling between materials mechanical and electrical behaviors. In the simplest of terms, when a piezoelectric material is squeezed, an electric charge collects on its surface. Conversely, when a piezoelectric material is subjected to a voltage drop, it mechanically deforms. Many crystalline materials exhibit piezoelectric behaviour. A few materials exhibit the phenomenon strongly enough to be used in application that take advantage of their properties. These include quarts, Rochelle salt; lead titanate zirconate ceramics (eg; PZT-4, PZT-5A, etc), barium titanate, and polyvinylide (a polymer film). On a nanoscopic scale, piezoelectricity results from a non uniform charge distribution within a crystal’s unit cells. When such a crystal is mechanically deformed, the positive and negative charge centers displace by different amounts. So while the overall crystal remains electrically neutral, he difference in charge center displacement result in an electrical polarization within the crystal. Electric polarization due to mechanical input is perceived as piezoelectricity. In a setup that wasn’t expected to work, piezoelectricity was well observed in Rochelle salt crystals. When the crystal were deformed (with the gentle tap of wood or hammer), short (on the order of 4 milliseconds) relatively high voltage (maximum 20 v peak to peak with 10M input Z to scope). It was surprising to see such a well-defined bursts of electricity (very low current). MESCE, Kuttippuram 5 EEE Dept. Piezoelectric transformers Seminar ‘04 PIEZOELECTRIC TRANSFORMERS Working Principle With the direct effect, placing a force or vibration (stress) on the piezoelectric element generates a charge. The polarity of this charge depends on the orientation of the stress compared with the direction of polarization in the piezoelectric element. During the manufacturing process, poling, or applying a high a dc field in the range of 45 KV/cm to the piezoelectric transformer, set the polarization direction. The inverse piezoelectric effect is the opposite of the direct effect. Applying an electric field, or voltage, to the piezoelectric element results in a dimensional change, or strain. The direction of the change is like wise linked o the polarization direction. Applying a field at the same polarity of the element results in a decrease. An increase in one dimension in a structure results in a decrease in the other two through Poisson’s coupling, or the fact that lateral strain results in longitudinal strain at Poisson’s ratio. This phenomenon is an important factor in the operation of the transformer. In a piezoelectric transformer, the direct and converse piezoelectric effects are used to acoustically transform power from one voltage and current level to another through a vibrating structure. The converse piezoelectric effect, in which an applied electric field produces a resulting strain in a body, is used to convert an oscillating electric field applied to the left half of a structure, such as a bar, into a vibrational mode of the entire bar. If driven at resonance standing wave distributions of large amplitudes of stress and strain result. The resonantly amplified strain in the right half of each bar is converted to a voltage across the output terminals by the direct piezoelectric effect. Depending upon geometry and MESCE, Kuttippuram 6 EEE Dept. Piezoelectric transformers Seminar ‘04 materials parameters, we can obtain voltage amplification of various magnitudes, with associated step-down in current levels. Piezoelectric devices work on two different modes of operation (or a combination of the two), called d33 or d31, depending on the direction of extraction relative to the direction of polarization. The relative displacement in the d33 mode is approximately 3 times larger than that of the d31 mode. Displacement in the d33 is an expansion in the same direction as the electrical field and poling direction. Displacement in the d31 mode is a contraction perpendicular to the electrical and poling direction. Many materials, such as quartz, lithium niobate, and lead zirconate lead titanate (PZT) exhibits some form of the piezoelectric effect. The piezoelectric transformer uses PZT, hence, it is a PZT transformer. Figure1 Electrical input signals to the primary electrodes are piezoelectrically transferred to mechanical vibrations. MESCE, Kuttippuram 7 EEE Dept. Piezoelectric transformers Seminar ‘04 RECTIFICATION OF INADEQUACIES The piezoelectric technology has been object of a significant evolution to achieve current reliability in PTs based applications. Initially this technology suffered from serious instabilities and device failures related to, I. Immature materials fabrication technology. II. Mechanical reliability min the nodal point, and III. Driving circuits. These drawbacks have subsequently been fully addressed through improvements in materials, third mode vibrational excitation for reducing the stress in the central nodal point, and design of more stable power supplies incorporating self tracking feedback circuits and resonant converters. In spite of the induced modifications, the current state of the PTs still suffers from a few drawbacks. First, the current PTs includes an output and an input sections which each have different polarization (alternatively poled piezoelectric transformers), with several associated inconveniences. I. Different polarization requires a special preprocessing in order to provide separately polarized sections. II. Internal stress concentration is in the interface between transversal and longitudinal polarization. This in turn, used to cause internal cracks during electric polarization. This in turn, used to cause internal cracks during electric poling process or premature mechanical fatigue and breakdown during operation. MESCE, Kuttippuram 8 EEE Dept. Piezoelectric transformers Seminar ‘04 The second of the issue under significant research now a days, is the enhancement of the power capabilities of Rosen type double polarized PTs. Although piezoelectric transformers for CCFL invertors requiring a power in the range of 510 W has been successfully achieved, higher power applications are still limited to PTs. The conventional piezoelectric transformers have a lack of high output as well as low impedance characteristics, which are needed for use in lightning ballasts. Recently, new structures for piezoelectric transformers have been developed and proposed for high power and small load resistance applications. Their structures, however, are more complicated, causing more costly in mass production. One of the simplest designs to make a PT was proposed by Berlin court in 1973. This design consisted in a PT where the input and output sections had the same polarization (“unipoled” piezoelectric transformers). MESCE, Kuttippuram 9 EEE Dept. Piezoelectric transformers Seminar ‘04 CONSTRUCTION The piezoelectric transformer is constructed of PZT ceramic, but more precisely it is a multilayer ceramic. The manufacturing of the transformer is similar to the manufacturing of ceramic of chip capacitors. The process prints layers of flexible, unified PZT ceramic tape with metallic patterns, then allighs and stacks the layers to form the required structure. The next step involves pressing, dicing, and firing the stacks to create the stacks to create the final ceramic device. The input section of the transformer has a multilayer ceramic capacitor structure. The pattern of the metal electrodes creates an interdigitated plate configuration. The output section of the transformer has no electrode plates between the ceramic layers, so firing produces a single ceramic output structure. Conductive materials which forms the output electrode for the transformer, coats the end of the output section. The next construction step establishes the polarization directions for the two halves of the transformer. Poling of the input section across the interdigitated electrodes results in a polarization direction that aligns vertically to the thickness. Poling of the output section creates a horizontal or length oriented polarization direction. Operating the transformer drives the input in thickness mode, which means that an applied voltage between the parallel plates of the input causes the input section to become thicker and thinner on alternative half of the sine wave. The change in input thickness couples through to the output section, causing it to lengthen and shorten and thereby generating the output voltage. The voltage set up ratio is proportional to the ratio of the output length and thickness of the input layers. MESCE, Kuttippuram 10 EEE Dept. Piezoelectric transformers Seminar ‘04 Figure 2.1 Piezoelectric element. This device is composed of two electrode plates and a piezoelectric ceramic material, such as barium titanate-based Ceramics. Figure 2.2 Longitudinal mode piezoelectric element. The direction of the operating stress, T, is parallel o the polarization direction, P, with a corresponding resonant frequency. Figure 2.3 Transverse mode piezoelectric element. The direction of the operating stress, T, is perpendicular to the polarization direction, P, with a corresponding resonant frequency. MESCE, Kuttippuram 11 EEE Dept. Piezoelectric transformers Seminar ‘04 Figure 3.1 Rosen piezoelectric transformer. This piezoelectric transformer is a combination of a transverse mode piezoelectric actuator (primary side) and a longitudinal mode piezoelectric transducer (secondary side). Figure 3.2 Thickness vibration piezoelectric transformers. This piezoelectric transformer is a combination of a longitudinal mode piezoelectric actuator (primary side) and a longitudinal mode piezoelectric transducer (secondary side). Figure 3.3 Radial vibration mode piezoelectric transformer. This piezoelectric transformer is a combination of a transverse mode piezoelectric actuator (primary side) and a transverse mode piezoelectric transducer (secondary side). MESCE, Kuttippuram 12 EEE Dept. Piezoelectric transformers Seminar ‘04 THE ELECTRICAL PARAMETERS The equivalent circuit model for the piezoelectric transformer looks identical to that of its series resonant magnetic counter part. The difference, however, extend pasty the nominal values to the physical representations of the various components. The input and output capacitances are simply the result having a dielectric between two metal plates. The effective dielectric constant of PZT material is 400 to 5000, depending on composition. At this point, unfortunately, basic electronics ends. The rest of the components are more complicated. The inductances, Lm, are the mass of the transformer. The capacitance Cm is the compliance of the material, or the inverse of spring rates. Calculating the compliance requires using the applicable generalized beam equation and young’s modulus, which is a constant that expresses the ratio of unit stress to unit deformation. The resistor, Rm, represents the combination of dielectric loss and the mechanical Q of the transformer. The acoustic, as opposed to the electrical, resonant frequency is related to the product of the capacitance Lm. The transformer operates in length resonance, and the associated motions are identical to those of a vibrating string. The major difference between a PZT transformer’s frequencies are in the ultrasonic range and vary, by design, from 50 KHz to MHz. like the string, the transformer has displacement nodes and antinodes. Mechanically clamping a node prevents vibration, which reduces efficiency in the best case and prevents operation in the worst. Mounting the transformer is crucial, you simply can’t refolw solder the device to pc board. MESCE, Kuttippuram 13 EEE Dept. Piezoelectric transformers Seminar ‘04 PT SCHEMATIC The final model is the “ideal” transformer with ratio N. this transformer represents three separate transformations. The first is the transformation is the electrical energy into mechanical vibration. This transformation is a function of the piezoelectric constant, which is the electrical field divided by stress, the stress of area , and the electrical field length. The second transformation is the transfer of mechanical energy from input section to the output section and is a function of poisons ratio, or the ratio of lateral to longitudinal strain, for the material. The final transformation is the transfer of mechanical energy back into electrical energy, and the calculations are similar to those for input side. Figure 4 MESCE, Kuttippuram 14 EEE Dept. Piezoelectric transformers Seminar ‘04 APPLICATIONS Thinner laptop computers and flat-screen TVs may be possible with a simple change in the geometry of piezoelectric transformer that can increase the conversion ratio without adding additional volume or weight. The researchers are, however, looking into developing piezoelectric transformers for use on the battlefield to replace the enormous transformers needed to operate X-ray equipment. These machines can require as much as 10 kilovolts, but not much current. In these applications, 20 or more disk transformers might be stacked and still take up only a fraction of the weight and the volume of conventional transformer. In the other direction, the researchers are looking at step-down applications for audio amplification. Applications of piezoelectric transformers include high voltage power supplies for ring laser gyroscope, cathode ray tubes, image intensifiers, and munitions fuses. A novel application involves separation of the primary and secondary part of the transformer for use as power embedded sensors or devices. In this way, the primary can also be used to monitor the load on the secondary component. MESCE, Kuttippuram 15 EEE Dept. Piezoelectric transformers Seminar ‘04 Selected Markets in Which PT Technology Has Likely Application Today; Audio Equipment Cellular Phones Computer Data Transmission Computer power supplies DC-DC Converters Electric Light Ballasts Flat Panel Displays High Voltage gate Drivers Ignition systems for vehicles Invertors LCD Displays Motor Drives And Controls Power Transformers(up to 4 Amps) Radios Satellite transmitters and receivers Smart Appliance And Smart House Systems Televisions VCRs MESCE, Kuttippuram 16 EEE Dept. Piezoelectric transformers Seminar ‘04 ADVANTAGES Piezoelectric transformers are small in size and don’t require windings. So there is no short circuit possibility between the windings, and the transformer has the ability to generate a wide range of high voltage AC or DC outputs as well. The unique nature of these transformers enables innovative circuit design, such as operation above resonance for inductive behavior in order to achieve soft switching without compensating inductors. New models are being developed for calculating power density and voltage\current amplification, and for evaluating bulk piezoelectric materials for losses, high frequency performance, and vibration velocity limits for construction of a prototype transformers. MESCE, Kuttippuram 17 EEE Dept. Piezoelectric transformers Seminar ‘04 FEATURES POTENTIAL BENEFITS High power capacity PT works in more applications than any other electromagnetic transformers Low Profile\small Size Compact assembled package and product High efficiency Energy conservation &compliance with standards Low EMI emission Little or no interference with surrounding circuitry compact packaging with geometric flexibility. Pure Sine Wave Output Longer product life, such as lamps. Wide Design versatility Recalibration simplified or eliminated for different model types and products Fail Safe Design No Winding to burn, break, or short open circuit would not cause are induced failures No windings Low EMI High Breakdown Voltage Simply achieved in PTs, so circuitry, packing & manufacturing are facilitated Simple mounting Facilitates automated assembly of boards Wide Performance Range Versatility for high voltages ac or dc output MESCE, Kuttippuram 18 EEE Dept. Piezoelectric transformers Seminar ‘04 DISADVANTAGES However those piezoelectric transformers are not necessarily the answer to all power problems. For one thing, in any transformer, as the voltage is increased, the current decrease, so transformer is only applicable where current is unimportant. MESCE, Kuttippuram 19 EEE Dept. Piezoelectric transformers Seminar ‘04 A BRIEF ANALYSIS OF THE PIEZOELECTRIC TRANSFORMER-TO STUDY THEIR IMPORTANT CHARACTEISTICS Small electronic devices which operate at high voltages require a compact transformer to step up the low voltages of available power supplies. We have been exploring the feasibility of utilizing piezoelectric transformers (PTs) for use in low profile high voltage power supplies. The simplest design is the Rosen type PT has been theoretically modeled utilizing an equivalent circuit which accounts for both the electrical and mechanical behavior of transformer. The critical performance parameters of the transformer, such as voltage step-up ratio and efficiency, can be predicted from the equivalent circuit. To verify the equivalent circuit, we are constructing Rosen-type PTs and measuring their performance. THEORY Description A piezoelectric transformer is essentially an acoustic resonator\filter that is designed such that the output voltage is different from the input voltage. PTs like resonators can be classified according to their mode of vibration. The vibration mode that is most promising for high step-up ratio transformers is the length extensional mode. In this mode, along thin bar supports acoustic waves which propagate along its lengths. When the frequency of MESCE, Kuttippuram 20 EEE Dept. Piezoelectric transformers Seminar ‘04 vibration is such that an integer number of half wavelengths is equal to the length of the bar, the bar will be in mechanical resonance and large strains will appear in the bar. Accompanying the mechanical strain will be electrical fields due to the piezoelectric effect. The direction of the electric field will correspond to the direction in which the piezoelectric ceramic is poled. For a transformer, parts of the bar are poled in the length direction and other parts are poled in the thickness direction. Thus, the thickness poled portions of the bar will develop a voltage differential between the two large faces of the bar that is proportional to the thickness of the bar, and the length poled portions of the bar will develop voltage along the length of hat portion proportional to its length. To a first order, the voltage step-up ratio of the transformer will be proportional to the ratio of the thickness of the bar to the length of the bar. Figure 5 A schematic of the Rosen transformer. The poling direction by arrows. The input half of the transformer is poled along its thickness while the output half is poled along its length. MESCE, Kuttippuram 21 EEE Dept. Piezoelectric transformers Seminar ‘04 Figure 6 The complete equivalent circuit representing the Rosen transformer. The input electrodes are on the left side. The impedance is hyperbolic functions of the complex wave number. Possibly the simplest length extensional mode voltage step-up PT is the Rosen transformer (Figure 1) . It consists of two portions, the first (input portion) is poled in the thickness direction, and the second (output portion) is poled along the bar’s length. This transformer is operated at its first harmonic frequency such that one full standing wave exists on the bar. This design requires three electrodes, two covering the large faces of the input section, and one on the end face of the output section. Analysis The standard method of analyzing acoustic resonators is by utilizing a Mason equivalent circuit . The mechanical and electrical behaviors of the resonator are represented by parallel branches with the electromechanical conversion represented by an ideal transformer. An equivalent circuit was originally given by C. A. Rosen in his 1956 MESCE, Kuttippuram 22 EEE Dept. Piezoelectric transformers Seminar ‘04 Ph.D. thesis and represents the behavior of the transformer under the assumption that the mechanical impedance of the two halves of the transformer are matched. This circuit is reproduced in (Figure 2) . This circuit completely represents the transformer given the following assumptions: 1) the dimensions of the bar are such that no other resonance mode has a frequency near that of the operational mode, 2) the bar is sufficiently thin and narrow such that the transverse inertia of the bar is negligible, and 3) the dielectric losses in the material are negligible. Mechanical losses in the material are represented by the real component of the wave number (the argument in the hyperbolic impedances) which is describable in terms of the mechanical quality factor (or equivalently, the mechanical loss tangent). The mechanical quality factor (Qm) is determined by experiment, and in general this value decreases with frequency of vibration. Figure 7 The at-resonance equivalent circuit of the Rosen transformer. This circuit is used to predict performance of the transformer. MESCE, Kuttippuram 23 EEE Dept. Piezoelectric transformers Seminar ‘04 Figure 8 Theoretical transformer performance as a function of resistive load impedance without output matching circuitry. The efficiency peaks at a low load resistance while the power peaks at a substantially higher load. A more useful circuit is the near-resonance circuit. If the mechanical impedances of the two halves of the bar are approximated to be identical (that is, the velocity of sound in the two halves are the same), the general circuit can be simplified (see Figure 3). The circuit can be further simplified by representing the impedances near resonance with a series LCR branch. The resonance of the impedance is given by the values of the inductor and capacitor, and the mechanical losses are represented by the resistor. This circuit is utilized in predicting the performance of the Rosen transformer. MESCE, Kuttippuram 24 EEE Dept. Piezoelectric transformers Seminar ‘04 The performance of the Rosen transformer can be seen in Figure 4. Equations for voltage gain, efficiency, and power transfer derived from the at-resonance circuit have been plotted using typical values for transformer dimensions and material properties. As is evident from Figure 4, the performance is strongly load dependent. The gain increases with increasing load and reaches a maximum at infinite load (open circuit). Power peaks at a moderate load, and efficiency peaks at a relatively low load. For applications that demand high efficiency, the transformer is limited to operate within a small load range that corresponds to low values of power transfer and voltage gain. The loss of efficiency is due to current passing through the output capacitor (labeled C02 in Figure 3). This current also passes through the resistor and contributes to power dissipation within the transformer but does not contribute to power transferred to the load. For both efficiency and power transfer to be simultaneously large and inductor would need to be placed in parallel with the load such that the inductor-capacitor parallel circuit would be at resonance at the operation frequency. In such a case, efficiency would increase asymptotically with load power transfer peaking at a load value corresponding to the mechanical loss resistor. Also, a capacitor could be placed in series with the load so as to cancel the negative capacitor allowing greater power transfer. Of course, the addition of the extra circuit elements may in part negate the size advantage of the PT, and any design alteration which would eliminate the need for extra circuit elements would be a great improvement. MESCE, Kuttippuram 25 EEE Dept. Piezoelectric transformers Seminar ‘04 PERFORMANCE MEASUREMENT A computer controlled data acquisition system has been assembled (see Figure9).It consists of a PC, digital function generator, digital oscilloscope, and an amplifier. The PC controls the function generator and oscilloscope and downloads the captured waveforms from the oscilloscope. These waveforms are then analyzed to determine the transformer impedance, input power, output power, and voltage gain. With this setup, the transformer can be quickly and precisely characterized. Figure 9 A schematic of the data acquisition system. A PC controls an HP function generator and a Nicolet oscilloscope via a GPIB connection. The dotted boxes represent electromagnetic shielding. MESCE, Kuttippuram 26 EEE Dept. Piezoelectric transformers Seminar ‘04 CONCLUSION We have theoretically analyzed the Rosen transformer and have predicted its performance as a function of the load. It is evident that matching circuitry will be necessary in order for both efficiency and power transfer to be Satisfactory. We have developed a poling technique which allows us to successfully pole transformers without destructive electrical discharges. We are in the process of constructing and measuring the performance of Rosen transformers. Once the performance of the transformers has been characterized, we will be able to determine if the Rosen design is sufficient to replace magnetic transformers in field emission display power sources. MESCE, Kuttippuram 27 EEE Dept. Piezoelectric transformers Seminar ‘04 REFERENCES Dallago.E.Danioni, G.venchi, “Single chip, low voltage piezoelectric transformer controller”, IEEE Xplore Sept 2003 pp 273-276. Ray Lee Lin, “Piezoelectric transformer characterization and application”, Nov 2001, pp 1-9. Gregory Ivensky, “Generic operational characteristics of piezoelectric transformer”, IEEE Transaction on Power Electronics, vol 17, Nov 2003 pp1049-1052. M.A. Smith “piezoelectric ceramic transformer for micro power supplies” 1997, pp 1-4. Gregory ivenski, “Analysis and modeling of Piezoelectric transformer in high output voltage application” pp 1-2. www.morgan-electroceramics.com www.google.com MESCE, Kuttippuram 28 EEE Dept.
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