Patent Application
20140167571
The present invention relates to a piezoelectric transformer comprising an elongate piezoelectric ceramic body adapted to operate in fundamental thickness mode. The elongate piezoelectric ceramic body comprises a central input or primary section coupled to adjacently arranged first and second output sections. High power conversion efficiency and zero-voltage switching capability are achieved by providing a primary side effective electromechanical coupling factor, keff
Piezoelectric transformers are known alternatives to magnetics in power converters such as AC-AC step-up or step-down converters, DC-DC step-up or step-down converters or AC-DC step-up or step-down converters. Piezoelectric transformers have several advantages in comparison to their magnetic counterparts such as low level of conducted and radiated EMI because the power conversion is based on the piezoelectric effect. The size and electrical conversion efficiency of piezoelectric transformers are of outmost importance in numerous applications just as the case with magnetics based power converters. A piezoelectric transformer comprises a piezoceramic material shaped as an acoustic resonator with a fundamental resonance frequency typically situated well above the audible band. The piezoceramic material is shaped as piezoelectric body which can exhibit a dense storage of kinetic energy. A piezoelectric transformer is inherently capacitive in nature which means that an electrical interface to switch mode operating power transistors of an input driver advantageously can be made with certain precautions to obtain so-called zero-voltage switching.
The present invention is based on novel insights about physics of piezoelectric transformers as regards their zero-voltage switching factors (denoted “ZVS factor”). The present inventor has demonstrated that by configuring or designing the piezoelectric transformer such that it is easier to supply energy to the transformer than extract energy therefrom the piezoelectric transformer exhibits inductive input impedance across a certain or predetermined frequency band or range. In other words, the piezoelectric transformer can be forced to display a pseudo inductive behavior. In mathematical terms this constraint can be formulated by effective and straightforward mathematical expressions coupling the desired inductive behaviour to a relationship between the primary side effective electromechanical coupling factor, keff
The intrinsic pseudo inductive or inductive behavior of the present class of elongate piezoelectric transformers allows a power converter based thereon to operate without the ordinary external series inductor and still maintain zero voltage switching operation in the input driver/half-bridge portion of the power converter. The ability of the power converter to operate with good power conversion efficiency without the ordinary external series inductor entails several significant advantages such as smaller dimensions, a freely selectable height dimension, a decrease of inductive losses caused by reactive coil currents etc. In addition, the lack of the ordinary external series or parallel inductor leads to a lower level of conducted and radiated EMI because of the elimination of external magnetic components in the power converter.
WO 2010/097407 describes a piezoelectric transformer for semiconductor based light sources. The employed piezoelectric transformers are based on ring-shaped piezoelectric bodies with ZVS factors of 100% and 120%.
US 2010/0328969 describes an electronic power converter comprising a piezoelectric transformer driven by an input voltage signal with a burst frequency and a substantially constant excitation frequency. A ring shaped thickness mode piezoelectric transformer with a ZVS factor of about 100% or 120% is disclosed.
The paper “Parameterized Analysis of Zero Voltage Switching in Resonant Converters for Optimal Electrode Layout of Piezoelectric” ISBN: 978-1-4244-1667-7 discloses the design of ring shaped PT operating in thickness mode.
U.S. Pat. No. 6,215,227 discloses a rectangular thickness mode piezoelectric transformer with an input section arranged in-between a pair of adjoining output sections. Opposing end faces of the output sections have end-masses attached thereto to increase the thickness of the piezoelectric transformer and decrease its resonant frequency.
U.S. Pat. No. 5,440,195 discloses a rectangular ceramic piezoelectric transformer with an output section polarized in lengthwise direction arranged in-between a pair of adjoining input or driving sections. The driving sections have multi-layered electrode structures and are operating in planar mode.
A first aspect of the invention relates to a piezoelectric transformer comprising:
fres
fanti-res
fres
fanti-res
The cross-section of the elongate piezoelectric ceramic body may take any arbitrary shape or profile of which the following are exemplary shapes out of numerous different shapes such as a rectangular, circular, or elliptical or simple n-gon polygon with 5, 6, 7 or more sides. The term “elongate” means that a length of the elongate piezoelectric ceramic body is larger than any other dimension thereof such as a thickness, width or diameter of the elongate piezoelectric ceramic body. Furthermore, according to a preferred embodiment of the invention, the length of the elongate piezoelectric ceramic body is at least two times larger than any other dimension thereof to create a rod-shaped form. The latter constraint ensures that a half-wave length resonance frequency is excited when an input signal of appropriate frequency is applied between the first and second input electrodes such that the elongate piezoelectric ceramic body operates in the fundamental thickness mode to avoid or at least suppress influence from unwanted spurious modes. This mode is often designated as k33-k33-k33-mode or simply k333-mode.
The first and second secondary or output sections can either be polarized or configured to operate parallelly or to operate separately where the first and second output electrodes are coupled to separate electrical loads. One of the first and second input electrodes may comprise common electrode shared between the central input section and the first and/or second output section such that the common electrode serves as a ground or negative voltage reference for the piezoelectric transformer.
In accordance with the present invention, the central input or primary section of the elongate piezoelectric ceramic body is arranged in-between the first and second output sections because this topology or design provides a large mass for the input section to “push” against compared to each of the first and second output sections. This feature is therefore particularly helpful in making the primary side effective electromechanical coupling factor, keff
The elongate piezoelectric ceramic body preferably comprises a piezoelectric ceramics material such as a hard doped Lead Zirconate Titanate e.g. Pz26 available from the supplier Ferroperm Piezoceramics NS. Other suitable materials include hard doped piezoceramic materials like NCE40, NCE41 and NCE46 available from the supplier Noliac NS, which like the Pz26 material, all exhibit large k33 electromechanical coupling factors such coupling factors above 0.65. The Pz26 piezoelectric ceramics material has a typical value of k33 in the thickness direction, i.e. parallel to the direction of polarization, of 0.65 or 65%, compared to a k31 factor of 0.33 or 33% in a direction perpendicular to the direction of polarization. Another embodiment of the elongate piezoelectric ceramic body utilizes the hard doped piezoceramic material NCE46 which has a k33 coupling factor (in the thickness direction, i.e. parallel to the direction of polarization) of 0.68 while the corresponding k31 coupling factor is 0.33 (in a direction perpendicular to the direction of polarization). Furthermore, the respective values of the primary and secondary side effective electromechanical coupling factors keff
Hence, the primary side and secondary side effective electromechanical coupling factors can be made large due to the exploitation of the k33 electromechanical coupiing factors, and optionally large mechanical quality factors Qm, of the input and output sections of the PT. In preferred embodiments, the primary side effective electromechanical coupling factor keff
Generally, electromechanical coupling factors such as k33 and k31 of a piezoelectric ceramic material are non-dimensional coefficients which are useful for describing a particular piezoelectric ceramic material under a particular stress and electrical field configuration for conversion of stored energy to mechanical or electrical work. The electromechanical coupling factors consist of particular combinations of piezoelectric, dielectric, and elastic coefficients. Since the coupling factors are dimensionless, they serve as a useful comparison between different piezoelectric materials independent of the specific values of permittivity or compliance, both of which may vary widely. The effective electromechanical coupling factors are defined for measurements of frequencies near the resonance and the anti-resonance of a piezoelectric ceramic body. As such the effective electromechanical coupling factors are related to the quasi-static coupling factors of a given material in addition to the physical shape of the object in question.
In another embodiment of the piezoelectric transformer the primary side effective electromechanical coupling factor keff
According to a preferred embodiment of the invention, the elongate piezoelectric ceramic body is shaped and sized to provide a ZVS factor larger than 1.0 or 100%, preferably larger than 1.2 or 120%, such as larger than 1.5 or 150% or 2.0 or 200%; in which the ZVS factor is determined at a matched load condition as:
Even though a ZVS factor of 1.0 or 100% in theory may suffice to obtain the desired inductive behaviour of the piezoelectric transformer, practical considerations suggest that using the above-mentioned larger values is often advantageous. One reason is that a piezoelectric transformer with a ZVS factor of about 100% will solely exhibit the desired inductive behaviour in an extremely narrow frequency band or range such that it may be difficult to properly adjust the excitation frequency of the AC input signal.
In one advantageous embodiment, the ZVS factor is larger than 125% and a volume of the central input section occupies less than 50%, and more preferably less than 40%, of a volume of the elongate piezoelectric ceramic body. The short length of the central input section while maintaining a large ZVS factor leads to a half-wave length resonance frequency, which is utilised for operation in the fundamental thickness mode, situated at a high frequency because of the accompanying short length of the entire elongate piezoelectric ceramic body. A high frequency value of the half-wave length resonance frequency may lead to a higher power density in the piezoelectric transformer.
The simultaneous provision of a high ZVS factor and the low volumetric occupation of the central input section, relative to the entire elongate piezoelectric ceramic body, have in part been achieved by using the above-mentioned k33 electromechanical coupling factor of the piezoelectric ceramics of the central input section of the elongate piezoelectric ceramic body. By exploiting the k33 coupling factor of the piezoelectric ceramic material, a large effective electromechanical coupling factor is achieved to provide a piezoelectric transformer with compact dimensions without compromising the ZVS capabilities of the piezoelectric transformer.
As mentioned above, the length of the elongate piezoelectric ceramic body is preferably at least two times larger than any other dimension thereof because this constraint ensures the half-wave length resonance frequency is excited by the AC input voltage or signal during operation. This is important to maximize conversion efficiency. The elongate piezoelectric ceramic body may be shaped as a substantially rectangular “slab” having a length that is two or more times larger than its width. The width is furthermore preferably two or more times larger than a thickness of the body.
According to yet another preferred embodiment of the present piezoelectric transformer, the elongate piezoelectric ceramic body is formed as a single unitarily machined body of anisotropic piezoelectric compound without any junctions or joints at the first and second connection surfaces. After sintering of the elongate piezoelectric ceramic body, the resulting body structure is designated a co-fired structure. This unitarily body structure of the elongate piezoelectric ceramic body is often designated a ‘bulk component’. The unitary or bulk structure without junctions may lead to an piezoelectric ceramic body with high mechanical strength due to the lack of mechanical attachment means or compounds such as glue, welding or soldering agents between the central input section and the output sections at the first and second connection surfaces.
The unitary structure may be provided by machining or forming the elongate piezoelectric ceramic body by stacking a plurality of individual thin ceramic layers build uniformly lengthwise along the longitudinal body axis. In this manner, the elongate piezoelectric ceramic body is built layer by layer in the thickness direction thereof. Each of the thin ceramic layers may have a thickness between 10 and 200 μm such as between 20 and 50 μm after sintering. Each of the first and second input electrodes may be printed on each of the thin layers in transversal direction to the longitudinal body axis at appropriate locations in the input section. In this structure or build-up, each of the first and second input electrodes is formed as vertical stack of individual electric conductive fingers with intervening layers of piezoelectric compound. This structure of the elongate piezoelectric ceramic body is often designated “interdigital” construction, “33IDE3” build-up or IDE build-up and may serve to emulate a true 333 thickness structure or build-up of the elongate piezoelectric ceramic body. Each input electrode may comprise between 20 and 80 individual electrically conductive fingers depending on the thickness of the elongate piezoelectric ceramic body, the thickness of the individual thin ceramic layers and other factors. The IDE structure is normally simpler to manufacture in current production methods than a true 333 thickness structure which on the other hand has a potential to provide higher power conversion efficiency and/or power density because the primary side effective electromechanical coupling factor, keff
In another embodiment the elongate piezoelectric ceramic body is formed by between 2 and 5 separate piezoelectric ceramics structures. The elongate piezoelectric ceramic body may be fabricated from two, three or more separate ceramics structures that are firmly attached to each other in lengthwise direction by gluing, welding or soldering. The separate ceramics structures may be bonded together by a Low Temperature Co-firing Ceramic (LTCC) material.
According one such embodiment, the central input section of the elongate piezoelectric ceramic body is fabricated as a single unitary of bulk component, preferably including input electrodes such as the above-discussed IDE input electrodes, and the first and second output sections fabricated as separate bulk components or parts as well. The first and second input electrodes preferably comprise a first vertically extending input electrode and a second vertically extending input electrode, respectively. The first and second vertically extending input electrodes being separated by an intermediate section of piezoelectric material in direction of the longitudinal body axis. A distance between first and second vertically extending input electrodes sets an electric field strength applied to the input section for a given input voltage or primary side voltage. The distance, along the longitudinal body axis, between first and second vertically extending input electrodes may vary depending on requirements of a particular application and the respective dimensions of the input section and first and second output sections. In a number of useful embodiments, the distance lies between 100 and 1000 μm such as between 200 and 500 μm. The distance is measured from center to center of the first and second vertically extending input electrodes in view of their significant lengthwise extension (i.e. the electrode width) in some embodiments of the present invention.
Another embodiment, wherein the elongate piezoelectric ceramic body also comprises three separate, but mutually bonded ceramics structures, comprises a central input section fabricated as a true 333 thickness structure manufactured by stacking or adding thin ceramics layers in a lengthwise direction of the central input section. The first and second output sections are preferably fabricated as separate bulk or unitary components. Very large ZVS factors for example about 170% have also been achieved by this embodiment as evidenced by the below outlined experimental results.
In another embodiment of the invention, the first vertically extending input electrode comprises a plurality of first electrode members distributed along the longitudinal body axis and separated by intermediate sections of piezoelectric material. Likewise, the second vertically extending input electrode comprises a plurality of second electrode members distributed along the longitudinal body axis and separated by intermediate sections of piezoelectric material. The number of first electrode members is preferably between 2 and 8 and the number of first electrode members is preferably between 2 and 8. In one advantageous variant of this embodiment, the first and second electrode members are arranged in an interdigitated or braided pattern along the longitudinal body axis such that pairs of first and second electrode members are facing each other separated by the intermediate sections of piezoelectric material. This embodiment makes it possible to adjust the electric field strength applied to the central input section for a given input voltage by selecting the number of pairs of the first and second electrode members distributed across the input section.
As mentioned above in connection with the interdigital construction or 33IDE3 build-up of the elongate piezoelectric ceramic body, the first vertically extending input electrode, or each of the first electrode members, may comprise respective set(s) of electrically conductive horizontal fingers aligned vertically above each other and separated by intervening layers of the piezoelectric material. Likewise, the second vertically extending input electrode, or each of the second electrode members, may comprise respective set(s) of electrically conductive horizontal fingers aligned vertically above each other and separated by intervening layers of the piezoelectric material.
According to a preferred embodiment of the invention, the first vertically extending input electrode, or each of the first electrode members, is/are electrically connected to a first electrically conductive layer arranged at a first exterior surface of the elongate piezoelectric ceramic body. In addition, the second vertically extending input electrode, or each of the second electrode members, is/are electrically connected to a second electrically conductive layer arranged at a second exterior surface of the elongate piezoelectric ceramic body. The first and second electrically conductive layers may serve as first and second externally accessible input terminals for providing electrical connection to an input voltage source delivering the AC input voltage. layer, on an end surface of the first output section and the second output electr The respective distances, along the longitudinal body axis, between individual electrode members of the first and second vertically extending input electrodes may be set to about the double of the distance between the latter to facilitate the previously mentioned interdigitated pattern. The distances between the individual electrode members of the first and second vertically extending input electrodes may therefore lie between 200 μm and 2 mm, more preferably between 400 μm and 1 mm.
According to yet another embodiment of the invention, the first output electrode is arranged, e.g. by printing or deposition of a metallic ode arranged, e.g. by printing or deposition of a metallic layer, on an end surface of the second output section. In addition, a length of each of the first and second output electrodes is less than 1 mm, preferably less than 100 μm, even more preferably less than 20 μm. The length is measured along the longitudinal body axis. In this embodiment, the respective lengths of the first and second output electrodes will typically be much less than the respective lengths of the first and second output sections to ensure that large or high values of the primary side and secondary side effective electromechanical coupling factors are achieved. The small length, within the above-mentioned constraints, of each of the first and second output electrodes ensures that the desired difference between the primary side effective electromechanical coupling factor, keff
According to another embodiment of the invention, where the first output electrode is arranged on an end surface of the first output section and the second output electrode arranged on an end surface of the second output section, the length of each of the first and second output electrodes is significantly less than the length of the first and second output sections, such as less than 50%, or preferably less than 10%, or even more preferably less than 1%, of the length.
A second aspect of the invention relates to a resonant piezoelectric power converter comprising a piezoelectric transformer according to any of the above-described embodiments. The resonant piezoelectric power converter comprises a transistor driver having an output electrically coupled to the first and second input electrodes of the input section of the piezoelectric transformer. The transistor driver is furthermore adapted to applying an input AC voltage of predetermined amplitude and frequency characteristics to the first and at second input electrodes. A load impedance is electrically coupled to the first and second output sections of the piezoelectric transformer for receipt of a transformed AC voltage. The transistor driver may comprise a half-bridge or full bridge circuit comprising cascaded e.g. NMOS or IGBT transistors. The full bridge circuit comprises a pair of complementary outputs coupled to respective ones of the first and second input electrodes. In a preferred embodiment of the resonant piezoelectric power converter an excitation frequency of the input AC voltage is situated between 5 and 10% above a fundamental resonance frequency, i.e. the half-wave resonance, of the elongate piezoelectric ceramic body. This range of excitation frequencies allows the piezoelectric transformer to be operated in a frequency range wherein the transformer displays the desired inductive behaviour such that an external inductor can be avoided as discussed above in detail.
A preferred embodiment of the invention will be described in more detail in connection with the appended drawings, in which:
a), 1b) and 1c) illustrate schematically a perspective view, a horizontal cross-sectional view and a vertical cross-sectional view, respectively, of a piezoelectric transformer in accordance with a first embodiment of the invention,
The embodiments of the present piezoelectric transformer described in detail below are particularly well-suited for power converters providing AC voltage amplification or step-up. The AC voltage amplification may for example be exploited to generate output voltages above 2000 Volt or event 6000 Volt. Such high AC voltages are very useful for driving for example dielectric electroactive polymer actuators. However, the skilled person will understand that piezoelectric transformers in accordance with the present invention are highly useful for other types of applications both step-up and step down voltage converting applications.
a) is a perspective view of a piezoelectric transformer 100 comprising an elongate piezoelectric ceramic body 102 adapted to operate in fundamental thickness mode. Thickness mode operation is achieved by polarizing a central input section or primary section 112 in a direction substantially parallel to a longitudinal body axis, L-axis, of the elongate piezoelectric ceramic body 102 as indicated by a set of electrical field arrows inside the input section 112 (refer to
The first output section 116 comprises a first output electrode 120 arranged at an end surface thereof. Likewise, the second output section 114 comprises a second output electrode 122 deposited on an end surface of the second output section arranged oppositely lengthwise to the end surface carrying the first output electrode 120. Each of the first and second output electrodes 120, 122, respectively, preferably comprises a thin layer of metallic material or other electrically conductive material. Each of the first and second output electrodes 120, 122, respectively, may be fastened or arranged on the end surface of the output section in question by printing techniques or similar methods of depositing metallic material on ceramic surfaces. Alternatively, the first and second output electrodes 120, 122 may be fastened to the end surface in question by welding, gluing or soldering a thin slab of electrode material to the end surface. In both situations, a length (along the L-axis) of each of the first and second output electrodes 120, 122, respectively, may be less than 1 mm, preferably less than 100 μm, or even more preferably less than 20 μm.
As illustrated by the horizontal cross-sectional view depicted on
As illustrated on the vertical cross sectional view of
The present embodiment of the piezoelectric transformer 100 has been fabricated in several different versions with rectangular outlines for experimental test and performance verification.
In a first version, the length (L), width (W) and depth (D) of the elongate piezoelectric body 102 were 20 mm, 10 mm and 2 mm, respectively. Other variants were fabricated with substantially identical width and depth dimensions, but lengths increased to 25 mm and 30 mm, respectively. Table 1 below includes detailed electrical and mechanical data for each of the experimental prototype piezoelectric transformers according to the first version. Each electrode member of the first and second vertical input electrode members 103a-d, 105a-c, respectively, was printed in ceramic layers in width-wise transversal direction to the L-axis of the elongate piezoelectric body 102. After sintering each tape-cast piezoelectric ceramics layer had a thickness of about 33 μm. Each of these ceramic layers has an electrode finger printed thereon which means that the 2 mm thickness or depth (D) of the elongate piezoelectric body 102 comprises about 60 individual vertically aligned fingers. In the present embodiments of the invention, the elongate piezoelectric body 102 is fabricated by piezoelectric ceramics material designated Pz26 available from the manufacturer Ferroperm Piezoceramics NS. The Pz26 material is a hard doped Lead Zirconate Titanate (Pb[ZrxTi1-x]O3 0≦x≦1) (“PZT”) with low loss and well-suited for resonance applications.
The piezoelectric transformer prototypes are adapted for application of an AC input voltage of 48 V. This voltage may be delivered by a full-bridge transistor driver operating on a 24 Volt DC power supply voltage. The full-bridge transistor driver may for example be based on high-voltage CMOS power transistors where full-bridge output terminals are electrically coupled to the externally accessible input electrodes 108, 106. The target output voltage was about 800 V DC.
For the 20 mm transformer, the length of the input section 112 was 7.14 mm while a combined length of the first and second output sections 114, 116 was 12.86 mm. As indicated in table 1, a ZVS factor of 1.49 or 149% was expected for the 20 mm piezoelectric transformer while a ZVS factor of 1.43 or 143% was expected for the 30 mm transformer.
Each electrode member of the first and second vertical input electrode members 103a-d, 105a-c, respectively, has a nominal or design width of about 70 μm as indicated in Table 1 under “IDE Width”. However, measured data on the experimental 20 mm, 25 m and 30 mm transformer prototypes indicated a typical fabricated width between 45 and 50 μm. The electrode width is measured horizontally along the longitudinal body axis, L-axis. The primary layer width specified in Table 1 is a nominal length of piezoelectric material in-between a facing pair of the first and second vertical input electrodes 103a-d, 105a-c for example between vertical input electrodes 103a and 105a depicted in
The output/secondary side impedance curve 204 or function was obtained at the output electrodes of the above-described experimental 25 mm prototype piezoelectric transformer with shorted input electrodes. The measurements were made by an HP4194A Impedance/Gain-Phase analyzer. The input side impedance function or curve 202 allows direct determination of the primary side effective electromechanical coupling factor, keff
which leads to a numerical value of keff
which leads to a numerical value of keff
Finally, applying equation (3) above to these values leads to a ZVS factor of about 110% for the present experimental 25 mm piezoelectric transformer.
It is worthwhile to notice that the difference between the measured ZVS factors of the 25 mm and 30 mm experimental prototypes (i.e. 110% versus 136%) because this parameter would ideally be substantially identical when only the length of the piezoelectric body is varied as in the present case. However, the measured difference is largely caused by the fabricated and finite width of 45-50 μm, i.e. the IDE width as expressed in Table 1, of each electrode member. The piezoelectric material stuck in-between individual fingers of each electrode member (103a and 105a on
The skilled person will appreciate that ZVS factors above the demonstrated 1.36 or 136% are readily obtainable by increasing the length of the elongate piezoelectric body and/or decreasing the IDE width of each electrode member.
Experimental prototype PTs according to a second embodiment of the piezoelectric transformer 100 has been fabricated in two different versions with rectangular outlines for additional experimental test and performance verification. In a first version, the length (L), width (W) and depth (D) of the elongate piezoelectric body 102 were 27.3 mm, 10 mm and 2 mm, respectively. The length of the central input section was 9.9 mm build by 40 IDE layers. The elongate piezoelectric body 102 was fabricated in the NCE46 hard doped piezoelectric ceramics material. The second version was identical expected for a depth (D) of 1 mm.
Table 2 and table 3 below show the measured data for the primary side and second side effective electromechanical coupling factors keff
Each of the central input section 412 and secondary sections 412, 414 is manufactured in the previously discussed hard doped piezoceramic material NCE46. The length (L), width (W) and depth (D) dimensions of the elongate piezoelectric body are 10.7 mm, 6.4 mm and 6.4 mm, respectively. The length of central input section 412 is about 3.5 mm such that the volumetric occupation of the central input section amounts to about 33% of the elongate piezoelectric body 402. The central input section 412 comprises a first vertical input electrode comprising two individual first electrode members 403a-b. The two individual first electrode members 403a-b are terminated electrically in an externally accessible electrically conductive layer 408 arranged at a proximate exterior surface of the elongate piezoelectric ceramic body 402. A second vertical input electrode comprises three individual second electrode members 405a-c terminated in a shared externally accessible electrically conductive layer 406 arranged at an opposing exterior surface. By application of an AC input voltage between the electrically conductive layers 408, 406, coupled to the first and second sets of input electrode member, respectively, electrical fields are applied to the central input section 412 along its polarization direction as described in connection with
The first and second output sections 414, 416 are polarized in a direction substantially parallel to a longitudinal body axis, L-axis, of the body 402. The first output section 414 comprises a first output electrode 422 arranged at an end surface thereof. Likewise, the second output section 416 comprises a second output electrode 420 deposited on an end surface of the second output section arranged oppositely lengthwise to the end surface carrying the first output electrode 422. Each of the first and second output electrodes 422, 420, respectively, preferably comprises a thin layer of metallic material or other electrically conductive material. Each of the first and second output electrodes 422, 420, respectively, may be fastened or arranged on the end surface of the output section in question by printing techniques or similar methods of depositing metallic material on ceramic surfaces. Alternatively, the first and second output electrodes may be fastened to the end surface in question by welding, gluing or soldering a thin slab of electrode material to the end surface. In both situations, a length (along the L-axis) of each of the first and second output electrodes 422, 420, respectively, may be less than 1 mm, preferably less than 100 μm, or even more preferably less than 20 μm.
Table 4 below shows the measured data for the primary side and second side effective electromechanical coupling factors keff
The above measurement results demonstrate that very high ZVS factors are readily obtainable by utilizing a true 333 thickness structure or build-up of the central input section of the elongate piezoelectric ceramic body. The true 333 thickness structure eliminates the above-discussed inactive area/volume of piezoceramic material caught between the individual electrically conductive fingers of the IDE electrodes. The true 333 thickness structure is therefore capable of providing higher power conversion efficiency and/or power density of the piezoelectric transformer because the primary side effective electromechanical coupling factor keff
| Number | Date | Country | Kind |
|---|---|---|---|
| 11176929.5 | Aug 2011 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/065261 | 8/3/2012 | WO | 00 | 3/5/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61521635 | Aug 2011 | US |
