Piezoelectric transformer

Abstract: A piezoelectric transformer includes a substrate (12) of piezoelectric material, preferably a single crystal of LiNbO₃. The substrate is uniformly polarized in a selected direction P and vibrated at a resonant frequency in the length direction. An input voltage (Vi) is applied across the thickness of the substrate by input electrodes 14a, 14b and thus excites the vibration by way of the piezoelectric effect. This causes another voltage in the direction of the thickness of the substrate in an output portion, which voltage is taken out by output electrodes 16a, 16b as an output voltage (Vo). As a result, it is possible to decrease the output impedance, to obtain a large load current and thus fully to heighten the step-up ratio of the piezoelectric transformer.

Description: The present invention relates to a piezoelectric transformer, more particularly, to a structure of a piezoelectric transformer which employs a rotated Y-cut substrate of a piezoelectric single crystal, e.g., a single crystal of lithium niobate (LiNbO₃), enables a large load current to flow therein, and possesses a high stability in operation. There are various electrical appliances requiring a high voltage of five or six kilovolts or more, e.g., deflection of an electron beam in a cathode ray tube (CRT), electrification of a photoconductive drum in an electrostatic printing, a transformer for direct current (DC) converter, or the like. To supply the high voltage, electromagnetic transformers are generally employed at present. With a recent demand for small size and light weight, however, a piezoelectric transformer has been developed as a solid state device and some piezoelectric transformers have been concretely proposed. As an example of the proposed ones, a piezoelectric transformer is known which employs a ceramic substrate having an input portion polarized in a certain direction and an output portion polarized in another direction. In this piezoelectric transformer, however, a problem occurs in that, where an excited vibration is greatly displaced, a distortion is concentrated on the portion where the direction of the polarization is changed, and thereby the substrate is broken or destroyed. Also, since ceramic material generally presents saturation characteristics as the input voltage applied is increased, the drawback arises that the piezoelectric and elastic constants deteriorate due to the heat generated, or other causes. This results in a lowering in stability or reliability in operation of the piezoelectric transformer. The document US-A-3 764 848 discloses a piezoelectric transformer including a substrate of piezoelectric material, the substrate being substantially uniformly polarized in a selected direction, wherein an electric field is applied to a portion of the substrate in the thickness direction so as to give rise by way of the piezoelectric effect to another electric field in the direction of the thickness of the substrate in another portion thereof, to be taken out as an output voltage. The problems in the prior art will be explained in detail later by comparison with the preferred embodiments of the present invention. It is therefore desirable to provide a piezoelectric transformer of decreased output impedance, giving a large load current and thus fully heightened step-up ratio. The piezoelectric transformer should also be stable and reliable in operation. The above objects are achieved by a piezoelectric transformer as set forth in independent claim 1. Preferable aspects of the present invention are set forth in the dependent claims. Described is also a method of operating such a transformer at a suitable resonant frequency. BRIEF DESCRIPTION OF THE DRAWINGS Other objects and features of the present invention will be described hereinafter in detail by way of preferred embodiments with reference to the accompanying drawings, in which: Figs. 1a and 1b are a perspective view and a sectional view, respectively, illustrating a structure of a prior art piezoelectric transformer; Fig. 2 is a graph showing a dependency of the electromechanical coupling coefficient relative to the angle of rotation in a rotated Y-cut substrate of a single crystal of LiNbO₃; Figs. 3a and 3b are a perspective view and a sectional view, respectively, illustrating a structure of the piezoelectric transformer as a first embodiment of the present invention; Fig. 4 is a perspective view illustrating a state in which the piezoelectric transformer shown in Figs. 3a and 3b is mounted on a substrate; Fig. 5 is an explanatory view of a rotated Y-cut substrate; Fig. 6 is a sectional view illustrating a structure of the piezoelectric transformer as a second embodiment of the present invention; Fig. 7 is a graph showing a relationship between the input voltage and the output voltage of the piezoelectric transformer shown in Fig. 6; Fig. 8 is a graph showing dependencies of the step-up ratio and the output current relative to the load resistance; Fig. 9 is a graph showing a dependency of the input resistance in the resonant state relative to the load resistance; Fig. 10 is a perspective view illustrating a structure of the piezoelectric transformer as a third embodiment of the present invention; and Fig. 11 is a sectional view illustrating a structure of the piezoelectric transformer as a fourth embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the preferred embodiments of the present invention, the problems in the prior art will be explained with reference to Figs. 1a and 1b. Figures 1a and 1b illustrate a structure of a prior art piezoelectric transformer, showing a perspective view and a sectional view, respectively. The illustrated piezoelectric transformer is a typical one proposed by Rosen (C.A.Rosen: Proc., Electronic Components Symp., p.205, 1957). In the illustration, reference 2 denotes a ceramic substrate of piezoelectric material, e.g., barium titanate, in the form of a rectangular and thin plate. The portion in the left side of the central portion of the ceramic substrate 2 is polarized in the direction of the thickness of the substrate as shown by arrow marks Pi, while the portion in the right side thereof is polarized in the direction of the length thereof as shown by arrow marks Po. In the left polarized portion (Pi), input electrodes 4a and 4b are formed on both sides to thereby form an input portion. Also, in the right polarized portion (Po), one output electrode 6 is formed on an end portion to thereby form an output portion together with another output electrode shared with one input electrode 4b. When an input voltage Vi with a resonant frequency determined by the dimension in the direction of the length of the substrate 2 is applied across the input electrodes 4a and 4b, a vibration is excited in the direction of the length of the substrate 2 as shown by a solid line "d" and a broken line "d′". As a result, a longitudinal piezoelectric effect is caused to thereby generate an output voltage Vo across the output electrodes 6 and 4b. In this case, a step-up ratio (Vo/Vi) under no load at the output ends is expressed by the following equation: Vo/Vi ∝ K 31 · K 33 · Q 1 · L 2 /L 1 Note, reference K₃₁, denotes an electromechanical coupling coefficient of the transverse piezoelectric effect in the input portion; reference K₃₃ an electromechanical coupling coefficient of the longitudinal piezoelectric effect in the output portion; reference Q₁ a mechanical "Q" of the ceramic substrate 2; reference L₁ a thickness of the ceramic substrate 2; and reference L₂ a length of the right polarized portion Po. Namely, the step-up ratio (Vo/Vi) of the above piezoelectric transformer is proportional to a product of K₃₁, K₃₃ and Q₁. The piezoelectric transformer as a device is supported by a supporting member 8 at the central portion thereof corresponding to a node of the excited vibration (see Fig. 1b). In the above piezoelectric transformer using the piezoelectric ceramic substrate, however, a processing of polarization must be effected on the boundary of the input portion and the output portion, e.g., in the central portion of the ceramic substrate 2, such that the respective directions of polarization are orthogonal to each other. Accordingly, a problem occurs in that, where the amplitude of the excited vibration becomes large, a distortion is concentrated on the boundary, especially in the secondary side (output portion), to thereby cause a destruction of the substrate. Also, since ceramic material generally presents its saturation characteristics as an input voltage applied thereto is increased, a drawback arises in that, where the amplitude of the excited vibration becomes large, piezoelectric constants and elastic constants deteriorate due to the emitted heat or other causes. This leads to a lowering in a stability or reliability in operation. Furthermore, since the prior art piezoelectric transformer utilizes the longitudinal piezoelectric effect in the output portion thereof, a problem occurs in that the electrostatic capacitance is small and thus the output impedance becomes high, so that it is impossible to obtain a large load current. According to the piezoelectric transformer of the present invention, an input portion thereof is driven using the transverse piezoelectric effect and thus, from an output portion thereof, an output voltage is taken out using the transverse piezoelectric effect. In this case, a step-up ratio (Vo/Vi) under no load at the output ends is expressed by the following equation: Vo/Vi ∝ Ki · Ko · Q 2 · Lo/Li Note, reference Ki (See Fig. 3b) denotes an electromechanical coupling coefficient of the transverse piezoelectric effect in the input portion (Ki<1); reference Ko an electromechanical coupling coefficient of the transverse piezoelectric effect in the output portion (Ko <1); reference Q₂ a mechanical "Q" of the substrate of piezoelectric material; reference Li a distance between electrodes provided in the input portion; and reference Lo a distance between electrodes provided in the output portion. In the constitution of the present invention, the electromechanical coupling coefficient Ki is selected to be equal to the electromechanical coupling coefficient Ko and thus the step-up ratio is proportional to the square of the coupling coefficient of the transverse piezoelectric effect. Also, the factor Lo/Li is equal to one (1) in a typical structure of the piezoelectric transformer. Accordingly, from equation (2), the step-up ratio is presumed to be lowered compared with the prior art expressed by equation (1). By using a single crystal of LiNbO₃ as the substrate of piezoelectric material, however, it becomes possible to fully heighten the step-up ratio much more than that of a prior art piezoelectric transformer because the mechanical "Q" (Q₂) of the single crystal of LiNbO₃ is extremely high (approximately 10000). Additionally, since the piezoelectric transformer of the present invention utilizes the transverse piezoelectric effect in the output portion thereof, it is possible to increase the electrostatic capacitance and thus decrease the output impedance. Accordingly, even if a relatively large load is coupled to the piezoelectric transformer, it is possible to prevent a lowering in the step-up ratio and thus realize a power transmission as in an ordinary electromagnetic transformer. Also, it is possible to easily form the electrodes in the output portion, compared with the prior art utilizing the longitudinal piezoelectric effect. As a result, it is possible to carry out all of the processes on a wafer and realize a low cost of the product. Furthermore, since the single crystal is used as the piezoelectric material, it is possible to reduce a dielectric loss and a mechanical loss and thus heighten a conversion efficiency. Namely, it is possible to suppress a rise in temperature due to the emitted heat, stabilize the characteristics and the operation, and improve the reliability. Although, as to piezoelectric material such as a single crystal of LiNbO₃, it is normally impossible to change the respective directions of polarization in the input portion and the output portion, we found that the single crystal of LiNbO₃ is applicable to a substrate of the piezoelectric transformer by a suitable choice of the orientation of the cutting thereof. Figure 2 shows a dependency of the electromechanical coupling coefficient (K₂₃ of the transverse piezoelectric effect) relative to the angle of rotation in a rotated Y-cut substrate of the single crystal of LiNbO₃. It is found that the electromechanical coupling coefficient presents its peak value (approximately 137° ) within a range of 120° to 160° of the angle of rotation. Figures 3a and 3b illustrate a structure of the piezoelectric transformer as a first embodiment of the present invention, showing a perspective view and a sectional view, respectively. In the illustration, reference 12 denotes a substrate of piezoelectric material, e.g., a single crystal of lithium niobate (LiNbO₃), which is a rotated Y-cut substrate cut from the single crystal (LiNbO₃) uniformly polarized in a Z-axis direction thereof, as stated later. Note, arrow marks "P" indicate a direction or orientation of the polarization. References 14a and 14b denote input electrodes, which are formed on each side of a half portion in the left side of the central portion of the piezoelectric substrate 12, respectively. Each of the input electrodes 14a,14b is formed by first forming nickel chromium (NiCr) with a thickness of 30 nm on the substrate 12 by way of vacuum deposition and then forming gold (Au) with a thickness of 100 nm thereon by way of vacuum deposition. References 16a and 16b denote output electrodes, which are formed on each side of another half portion in the right side of the central portion of the piezoelectric substrate 12, respectively. Each of the output electrodes 16a,16b is also formed by first forming nickel chromium (NiCr) with a thickness of 30 nm on the substrate 12 by way of vacuum deposition and then forming gold (Au) with a thickness of 100 nm thereon by way of vacuum deposition. Although, in the illustration, the input electrodes 14a,14b and the output electrodes 16a,16b are formed separately from each other, one of the input electrodes (14b) and one of the output electrodes (16b) may be formed, for example, as a single common electrode. Reference 18 denotes a member for supporting the piezoelectric transformer. In order that the vibration can be effectively excited, the supporting member 18 supports the piezoelectric transformer at the central portion thereof corresponding to a node of the excited vibration (see a solid line "d" and a broken line "d′" in Fig. 3b). The supporting member 18 has a height of 50 µm and an area of approximately 0.5 mm² and is provided on a mounting substrate 30 of printed circuit board as shown in Fig. 4. In this case, the input electrodes 14a,14b and the output electrodes 16a,16b are connected by way of the respective bonding wires W₁,W₃ and W₂,W₄ of gold (Au) to the respective electrodes or terminals B₁,B₃ and B₂,B₃ formed on the substrate 30. Note, in order to prevent the excited vibration from being obstructed, it is preferable to take the lead portions for the electrodes 14a,14b,16a and 16b from a region in the vicinity of the node of the excited vibration. The conception of the rotated Y-cut substrate is explained with reference to Fig. 5. In the illustration, the coordinates including X-axis, Y-axis and Z-axis indicate reference orientations in the single crystal of LiNbO₃, and the coordinates including X-axis, Y′-axis and Z′-axis are obtained when the original coordinates (X,Y,Z) are rotated by angle  about X-axis. In the present example, the angle  is selected to be 140° and the single crystal of LiNbO₃ is cut such that the plane including X-axis and Z′-axis is parallel to that of the substrate 12. Note, the longitudinal direction of the plane is selected along the Z′-axis. In Figs. 3a and 3b, when an input voltage Vi with a resonant frequency determined by the dimension in the direction of the length of the substrate 12 is applied by an alternating current (AC) power source (not shown) across the input electrodes 14a and 14b, a fundamental mode vibration with half-wave-length resonance is excited in the direction of the length of the substrate 12 as shown by a solid line "d" and a broken line "d′". As a result, a transverse piezoelectric effect is caused to thereby generate a high output voltage Vo across the output electrodes 16a and 16b. Accordingly, the step-up ratio (Vo/Vi) can be determined by measuring the output voltage Vo, for example, by means of an AC voltmeter with high impedance. Figure 6 illustrates a structure of the piezoelectric transformer according to a second embodiment, showing a sectional view thereof. In the present embodiment, the substrate 12 has a length of 20 mm, a width of 7 mm and a thickness of 0.5 mm. Also, each of the input electrodes 14a,14b has a length of approximately 9.5 mm and a thickness of 130 nm, and each of the output electrodes 16a,16b has a length of approximately 8.5 mm and a thickness of 130 nm. Namely, the input electrodes 14a,14b and the output electrodes 16a,16b are formed separately from each other with a gap of approximately 2 mm. Note, each electrode is formed by the same process as in the embodiment of Figs. 3a and 3b, and thus, the explanation thereof is omitted. The embodiment of Fig. 6 shows a case in that an input voltage with a specific resonant frequency (one-wave-length resonance) is applied across the input electrodes 14a and 14b to thereby excite a vibration of secondary mode in the direction of the length of the substrate 12, as shown by a solid line "d" and a broken line "d′". In this case, since the excited vibration of secondary mode has two nodes in the substrate 12, two supporting members 18a and 18b are accordingly provided at corresponding portions of the piezoelectric transformer corresponding to the two nodes. These supporting members 18a,18b can be provided, in the same manner as in the embodiment of Figs. 3a and 3b, on the substrate 30 shown in Fig. 4. According to the embodiment of Fig. 6, it is possible to heighten a stability of the piezoelectric transformer in mounting thereof on the substrate 30 by the provision of the two supporting members 18a,18b. Also, by taking the lead portions for the electrodes 14a, 14b and 16a,16b from the respective regions in the vicinity of the two nodes of the excited vibration, it is possible to prevent a lowering in the mechanical "Q" (Q₂) and a disconnection of the lead portions (bonding wires). Figure 7 shows a graph representing a relationship between the input voltage and the output voltage in the piezoelectric transformer shown in Fig. 6. In this case, the resonant frequency of the vibration of secondary mode is approximately 303 kHz. The input voltage Vi is expressed by an effective value (root mean square value) and the output voltage Vo is expressed by a rectified direct current voltage. Also, the rectifying circuit employs a load resistor of 100 M Ω and a capacitor of 4700 pF for cutting off ripples. As seen from the illustration, a relatively high step-up ratio (approximately 820) can be obtained with a simple constitution, and the characteristics present a good linearity. Figure 8 shows dependencies of the step-up ratio (Vo/Vi) and the output current (Io) relative to the load resistance in the piezoelectric transformer shown in Fig. 6. The illustrated graph represents data obtained when the input voltage Vi is 1 [V]. As seen from the graph, it will be found that, for example, to take an output voltage of 500 [V] out of the load resistor of 5 MΩ, the input voltage Vi of approximately 3.3 [V] is only necessary. Although, in the embodiment of Fig. 6, the vibration of secondary mode is employed, it is not restrictive. For example, a higher harmonic vibration may be excited, if necessary. Figure 9 shows a dependency of the input resistance (Ri) in the resonant state relative to the load resistance (RL). In the illustration, the curve indicated by a solid line (a) shows the characteristics of the present invention (input portion→ transverse piezoelectric effect; output portion→ transverse piezoelectric effect), and the curve indicated by a broken line (b) shows the characteristics of the prior art of Figs. 1a and 1b (input portion→ transverse piezoelectric effect; output portion→ longitudinal piezoelectric effect). The respective characteristics are calculated by a computer simulation using a well-known Mason's equivalent circuit model. The piezoelectric substrate as a sample is a rotated Y-cut substrate cut from a single crystal of LiNbO₃, the angle of rotation being selected to be 128°. The substrate has a length of approximately 30 mm, a width of 6.6 mm and a thickness of approximately 0.5 mm, Also, each of the input electrodes and the output electrodes has a length of approximately 14.5 mm. In the graph of Fig. 9, the region indicated by an arrow mark "A" shows a step-up operation region of the piezoelectric transformer, and the region indicated by an arrow mark "B" shows a step-down operation region thereof. As seen from the graph, the piezoelectric transformer of the present invention can cope with both the step-up transforming operation and the step-down transforming operation. Note, from the characteristics of Fig. 9, it is possible to estimate an approximate step-up ratio or a magnitude of power transmissible from a given power source. Figure 10 illustrates a structure of the piezoelectric transformer according to a third embodiment, showing a perspective view thereof. In the above embodiments, when the input voltage is increased to a some extent, a possibility occurs in that a discharge is caused between the output electrodes in the output voltage of approximately five or six kilovolts. This is because the thickness of the substrate 12 is relatively small and thus dielectric breakdown voltage of air is reached. The embodiment of Fig. 10 is proposed to cope with the disadvantage. The present embodiment is characterized in that at least one electrode of the input electrodes and the output electrodes (in the present example, output electrode 16a′) is formed inside, leaving a width of approximately 1 mm, from a periphery of the substrate 12. By this structure, it is possible to obtain approximately twice the output voltage without causing any dielectric breakdown. Note, it is of course that other electrodes may also be formed inside from the periphery of the substrate 12, if necessary. Figure 11 illustrates a structure of the piezoelectric transformer according to a fourth embodiment, showing a sectional view thereof. The present embodiment is characterized in that a protection film 20 is formed on both sides of the substrate 12, except on regions corresponding to terminal lead portions 22a,22b,24a and 24b for the electrodes. By this structure, it is possible to protect the electrode portions and prevent a dielectric breakdown from occurring in the output electrodes. The protection film 20 is made of silicon dioxide (SiO₂) with a thickness of 0.5 to 1.0 µm and formed by means of sputtering. Alternatively, the protection film 20 is made of silicon nitride (SiNx) with the same thickness as SiO₂ and formed by means of chemical vapor deposition. Also, the terminal lead portions 22a,22b,24a and 24b are formed to have a suitable area, respectively, by means of ion etching. The bonding wires W₁ to W₄ are taken out of the terminal lead portions and connected to the corresponding external electrodes (see Fig. 4, electrodes B₁ to B₃). Additionally, the embodiment of Fig. 11 may be combined by the embodiment of Fig. 10 to thereby obtain a better effect caused by the respective features. Although the present invention has been disclosed and described by way of four embodiments, it is apparent to those skilled in the art that other embodiments and modifications are possible without departing from the invention as defined in the appended claims.

Claims: A piezoelectric transformer comprising a substrate (12) of piezoelectric material, means for applying a first electrical field to a portion of the substrate (12) in the direction of its thickness, thereby to give rise to a second electrical field by way of the piezoelectric effect in another portion of the substrate; and means acting across the thickness of the substrate for taking out an output voltage (Vo) due to the second electric field; characterised in that the substrate (12) is a single crystal of lithium niobate. A piezoelectric transformer as set forth in claim 1, in which the substrate is uniformly polarized in a selected direction (P). A piezoelectric transformer as set forth in claim 1 or 2, in which the input means comprises a pair of input electrodes (14a,14b) by which the electrical field is applied to the substrate, and the output means comprises a pair of output electrodes (16a,16b; 16a',16b) for taking out the output voltage. A piezoelectric transformer as set forth in claim 3, wherein the input electrodes (14a,14b) are formed, respectively, on each side of the substrate (12) in one half portion thereof, and the output electrodes (16a,16b; 16a',16b) are formed, respectively, on each side of the substrate in the other half portion thereof. A piezoelectric transformer as set forth in claim 4, wherein at least one of the input or output electrodes, preferably one (16a') of the output electrodes, is formed inwardly of the periphery of the substrate. A piezoelectric transformer as set forth in any preceding claim, further comprising a protection film (20) formed on at least one side of the substrate (12), apart from a region or regions kept free for terminal lead portions (22a,22b,24a,24b) for the electrodes (14,16) provided on the substrate. A piezoelectric transformer as set forth in claim 6, wherein the protection film (20) is made of silicon dioxide or silicon nitride. A piezoelectric transformer as set forth in any preceding claim, wherein the substrate (12) of piezoelectric material is a rotated Y-cut substrate uniformly polarized in the Z-direction. A piezoelectric transformer as set forth in claim 8, wherein the angle of rotation of the substrate is between 120° and 160°. A piezoelectric transformer according to any preceding claim, in which the substrate (12) is in the form of a thin rectangular wafer and is adapted to be vibrated in its longitudinal direction by the input voltage (Vi). A piezoelectric transformer as set forth in claim 10, wherein the vibration in the direction of the length of the substrate (12) is a fundamental wave vibration with half-wavelength resonance. A piezoelectric transformer as set forth in claim 10, wherein the vibration in the direction of the length of the substrate (12) is a vibration of secondary mode with one-wavelength resonance. A piezoelectric transformer unit comprising a piezoelectric transformer as claimed in any preceding claim; a second substrate (30) for mounting the piezoelectric transformer; and at least one supporting member (18) provided on the second substrate, for supporting the piezoelectric transformer at a portion or portions thereof corresponding to a node or nodes of the excited wave vibration.

Inventors: ADACHI YOSHINORI, NAKAMURA KIYOSHI, SATO MASUJI, WAKATSUKI NOBORU, ADACHI, YOSHINORI, NAKAMURA, KIYOSHI, SATO, MASUJI, WAKATSUKI, NOBORU, Sato, Masuji, c/o FUJITSU LIMITED, Wakatsuki, Noboru, c/o FUJITSU LIMITED
Applicants: FUJITSU LTD, NAKAMURA KIYOSHI, FUJITSU LIMITED, NAKAMURA, KIYOSHI
Application year: 1991
Main classification: H01L41
IPC codes: H01L41