JP2003282990A - Piezoelectric transformer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoelectric transformer whose power loss is small. <P>SOLUTION: The piezoelectric transformer 1 has a constitution wherein a primary side piezoelectric plate 2 and a secondary side piezoelectric plate 3 are laminated, a ground electrode 4 is arranged between the plates 2 and 3, an input electrode 5 is arranged on a surface of the plate 2 and an output electrode 6 is arranged on a surface of the plate 3. An input voltage is increased and decreased by stretching vibration in a longitudinal direction. By setting the amount of deviation in the longitudinal direction of the plate 2 and the plate 3 as at most 2% of the whole length of the piezoelectric transformer 1, power loss is restrained. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-voltage power supply for lighting a fluorescent lamp and a step-down power supply (AC / DC adapter) from a general household power supply to low-voltage equipment. Related to a piezoelectric transformer. 2. Description of the Related Art In recent years, a piezoelectric transformer, which has been used as a power source for lighting a backlight of a liquid crystal display device, was developed in 1957 by C.C. E. FIG. Rosen
On the basis of the invention invented. This Rose
The n-type piezoelectric transformer has a rectangular plate-shaped piezoelectric ceramic in which one half of the longitudinal direction is an input unit (primary side) and the other half of the longitudinal direction is an output unit (secondary side). Has a structure in which an input electrode is formed, and an output electrode is formed on an end face on the secondary side. [0003] Such a Rosen type piezoelectric transformer generally has a small secondary electrode area and a long distance between the electrodes, so that a high voltage can be generated.
On the other hand, the generated current is very small. Therefore, Japanese Patent Application Laid-Open No. 4-291773 proposes a piezoelectric transformer having a structure in which piezoelectric ceramics are laminated, and which raises and lowers a voltage and a current using the thickness longitudinal vibration. Such a piezoelectric transformer has a large electrode area on the secondary side and a short distance between the electrodes, so that the boosting ratio is small but the generated current can be increased. However, the piezoelectric transformer using the thickness longitudinal vibration disclosed in Japanese Patent Application Laid-Open No. Hei 4-291773 has a high driving frequency in the 10 ⁶ Hz region (megahertz region). In addition, when driving the piezoelectric transformer, since it is easily affected by the stray capacitance between the wires used for input and output and between these wires and the surrounding circuit,
There is a problem that power loss increases. [0005] The present invention has been made in view of such circumstances, and has as its object to provide a piezoelectric transformer with low power loss. That is, according to the present invention, a plurality of rectangular piezoelectric ceramics each having an electrode on each of the front and back surfaces and polarized in the thickness direction are stacked, and A piezoelectric transformer for increasing / decreasing an input voltage by expansion and contraction vibration, wherein a maximum displacement of the rectangular piezoelectric ceramic in a longitudinal direction is 2% or less of a total length of the piezoelectric transformer. Provided. [0007] In such a piezoelectric transformer, since a decrease in the mechanical quality factor Qm is suppressed, power loss can be suppressed to a small value. Further, since the laminated piezoelectric ceramic has a large electrode area and a short distance between the electrodes, it is possible to drive the piezoelectric ceramic at a low frequency by using longitudinal vibration while maintaining a large output-side capacitance. . This makes it less likely to be affected by the stray capacitance during driving. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing a schematic structure of a piezoelectric transformer 1 according to the present invention. The piezoelectric transformer 1 has a ground electrode 4 provided between the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3, an input electrode 5 provided on the surface of the primary side piezoelectric plate 2, It has a structure in which an output electrode 6 is provided on the surface. As shown in FIG. 1, the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 are each polarized in the thickness direction, but the polarization directions may be opposite to each other. The driving of the piezoelectric transformer 1 is limited to the driving in the longitudinal expansion and contraction mode, that is, the driving at the basic resonance frequency. If an even-numbered vibration mode such as twice or four times the fundamental resonance frequency is used, the secondary piezoelectric plate 3
In this case, a portion where a positive voltage is generated and a portion where a negative voltage is generated appear, and the mutually generated voltages are canceled out, and as a result, no voltage is generated. In addition, when an odd-numbered vibration mode such as three or five times the fundamental resonance frequency is used, although a slight output is obtained, most of the generated voltage is reduced for the same reason as when an even-numbered vibration mode is used. Since they cancel each other, they cannot effectively function as a transformer. [0010] In such a piezoelectric transformer 1, the addition of the fundamental resonance frequency near the drive power (input power) to the input electrode 5, the primary side piezoelectric plate 2, d ₃₁ mode - caused by (thickness transverse vibration) A longitudinal stretching vibration occurs. That is,
The primary-side piezoelectric plate 2 converts electric energy into vibration energy. This expansion / contraction displacement is transmitted to the secondary-side piezoelectric plate 3, and the secondary-side piezoelectric plate 3 converts vibration energy into electric energy. Thus, a predetermined output voltage / current is extracted from the output electrode 6. Although the size of the piezoelectric transformer 1 is not limited, when the basic resonance frequency is several tens of kHz or less, the wiring between the input and output of the piezoelectric transformer 1 and the wiring between these wirings and the surrounding circuit are not limited. Since the influence of the stray capacitance between them is less likely to occur, the power loss based on this can be kept low. Further, the piezoelectric transformer 1 has a feature that the output capacitance is large because the area of the output electrode 6 is large and the distance between the electrodes is short. The piezoelectric transformer 1 can be manufactured by a bonding method or an integral firing method. When using the bonding method, first, two piezoelectric plates having a uniform shape are prepared, and silver paste is applied to the front and back surfaces of these piezoelectric plates, respectively.
Bake at a predetermined temperature. Next, the piezoelectric plate having the electrodes formed on the front and back surfaces is polarized by applying a predetermined voltage between these electrodes, and then bonded with an organic polymer adhesive (resin adhesive). When the input / output power of the piezoelectric transformer 1 is large, it is expected that the piezoelectric transformer 1 will generate considerable heat. Further, it is necessary to efficiently transmit the vibration energy generated in the primary side piezoelectric plate 2 to the secondary side piezoelectric plate 3. For this reason, as the resin adhesive, an epoxy-based adhesive or a polyimide-based adhesive having high heat resistance and high elastic modulus is preferably used. The polarization of the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 can be performed after they are bonded by a resin adhesive. In general, the polarization treatment is performed at a temperature close to the Curie point while the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 are immersed in insulating oil or held in a thermostatic dryer. For this reason, when the polarization treatment is performed after bonding the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 with a resin adhesive, the resin adhesive has durability against polarization conditions, such as heat resistance and voltage resistance. , Oil resistance, washing resistance (durability to oil removal treatment) and the like. When the bonding method is used, an electrode plate (metal plate) having a predetermined thickness is provided between the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 in order to easily take out the ground electrode 4. May be interposed. In this case, as the thickness of the electrode plate is smaller, the vibration of the piezoelectric transformer 1 is not hindered. However, considering the work of connecting lead wires to the electrode plate, the thickness of the electrode plate is preferably 20 to 100 μm. . It is preferable that the surface of the electrode plate has a satin finish, because the adhesive strength by the resin adhesive increases. In addition, it is preferable to provide a through hole in the electrode plate because the adhesive strength is improved. When lead wires are provided for the input electrode 5 and the output electrode 6, respectively,
What is necessary is just to solder a lead wire. In such an adhesive method, an inorganic adhesive such as water glass, silver (Ag) for forming electrodes, or silver (Ag) / palladium (Pd) may be used instead of the resin adhesive. It is possible. Here, when a material that needs to be fired at a high temperature such as silver (Ag) is used as the adhesive, the polarization process is performed after the bonding between the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 is completed. In addition, a silver paste is applied to a solid piezoelectric plate, the piezoelectric plates are attached to each other while the applied silver paste is not dried, and then fired, whereby the primary side piezoelectric plate 2 is baked.
And the secondary-side piezoelectric plate 3 can be bonded by a silver electrode. That is, the formed ground electrode 4 can have a function as an adhesive for bonding the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 together. On the other hand, the piezoelectric transformer 1 is formed by using the simultaneous firing method.
First, a piezoelectric ceramic powder is formed into a sheet, and a conductive pattern serving as the ground electrode 4 is printed on the obtained green sheet using an Ag / Pd paste or a platinum (Pt) paste, and the resultant is laminated. Then, the ground electrode 4 and the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 are simultaneously fired by pressing and firing. Thereafter, a silver paste is applied to the surface of the obtained laminated body and baked, thereby forming the input electrode 5 and the output electrode 6, and performing a polarization process. When the ground electrode 4 is taken out by a lead wire, for example, a terminal having a predetermined area which is electrically connected to the ground electrode 4 on the side surface of the piezoelectric transformer 1 but is insulated from the input electrode 5 and the output electrode 6. And a lead wire may be soldered to this terminal. Also,
The method of forming the input electrode 5 and the output electrode 6 when the piezoelectric transformer 1 is manufactured by the simultaneous firing method is not limited to the method of applying and firing the silver paste described above. For example, nickel (Ni), platinum (Pt), copper (Cu), or the like can be formed by a sputtering method, an evaporation method, or the like. However, since an electrode film formed by a sputtering method or the like does not have high adhesion strength, care must be taken so that peeling or the like does not occur later. As the piezoelectric material used for the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3, a piezoelectric ceramic of lead zirconate titanate is generally preferably used, and among them, the mechanical quality factor Qm is about Materials having a dielectric loss tan δ of not less than 1000 and not more than about 0.005 are preferably used. The dimensions of the piezoelectric transformer 1 and the thickness ratio between the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 may be determined as necessary, and are not particularly limited. Although the thickness ratio between the primary side piezoelectric plate 2 and the secondary side piezoelectric plate 3 greatly affects the step-up ratio / step-down ratio, other factors (Qm or K
Value, load capacity, etc.), it is necessary to appropriately set suitable conditions. FIG. 2 is a sectional view showing a schematic structure of a piezoelectric transformer 10 according to another embodiment of the present invention, and FIG. 3 is a sectional view showing a schematic structure of a piezoelectric transformer 20 according to still another embodiment of the present invention. FIG. The piezoelectric transformer 10 has a structure in which a ground electrode 14a is provided between the primary side piezoelectric plate 12 and the secondary side piezoelectric plate 13. The primary side piezoelectric plate 12 has a structure in which an input electrode 15 is provided between two piezoelectric plates 12a and 12b, and a ground electrode 14b is provided on the surface of the primary side piezoelectric plate 12; An output electrode 16 is provided on the surface of the device 13. The piezoelectric transformer 20 has a structure in which a secondary piezoelectric plate 23 is sandwiched between two primary piezoelectric plates 21 and 22. The primary side piezoelectric plate 21 is composed of two piezoelectric plates 21.
a and 21b, and an input electrode 25a is provided between the first and second piezoelectric plates 22a and 22b.
It has a structure in which an input electrode 25b is provided between 2b. On the surface of the primary side piezoelectric plate 21, between the primary side piezoelectric plate 21 and the secondary side piezoelectric plate 23, between the primary side piezoelectric plate 22 and the secondary side piezoelectric plate 23, and on the surface of the primary side piezoelectric plate 22. Are provided with ground electrodes 24a, 24b, 24c and 24d, respectively. The secondary side piezoelectric plate 23 is composed of two piezoelectric plates 23a and 2
It has a structure in which an output electrode 26 is provided between 3b. In the piezoelectric transformer 20, the piezoelectric plates 21a and 21b constituting the primary side piezoelectric plate 21 are each polarized in the thickness direction, but the direction of the polarization is such that vibrations with respect to the input voltage do not cancel each other. In addition, it is necessary that each sheet be reversed. This is the same for the primary side piezoelectric plate 22, and the piezoelectric transformer 10
The same applies to the primary side piezoelectric plate 12. In the piezoelectric transformer 20, the polarization direction of the piezoelectric plates 23a and 23b needs to be reversed for each of the secondary piezoelectric plates 23 so that the output voltages do not cancel each other. is there. However, in the piezoelectric transformer 20, the piezoelectric plates 21a, 21b,
The polarization of the piezoelectric plates 22a and 22b and the polarization of the piezoelectric plates 23a and 23b constituting the secondary side piezoelectric plate 23 are independent of each other.
It need not be as shown. Also in the piezoelectric transformer 10, the piezoelectric plates 12a and 12
The direction of polarization of b may be the reverse of the direction shown in FIG. 2, and the direction of polarization of the secondary side piezoelectric plate 12 may be the reverse of the direction shown in FIG. When the primary and secondary piezoelectric plates are composed of a plurality of piezoelectric plates, as in the case of the piezoelectric transformers 10 and 20, the number of stacked piezoelectric plates constituting the primary piezoelectric plate and the secondary side There is no limitation on the number of laminated piezoelectric plates and the ratio of the number of laminated piezoelectric plates, and the thickness ratio between the primary side piezoelectric plate and the secondary side piezoelectric plate. When the number of stacked piezoelectric plates is large, the co-firing method described above is preferably used for the production. It is also possible to bond a laminate having a plurality of layers of piezoelectric plates manufactured by a simultaneous firing method using a resin adhesive. Not limited. Table 1 shows the results of manufacturing the piezoelectric transformer 10 and measuring its characteristics. [Table 1] Here, the piezoelectric transformer 10 was manufactured by the following method. That is, first, a lead zirconate titanate-based piezoelectric ceramic powder having a predetermined composition is 35 mm thick.
It is formed into a plate shape of square x 3 mm, and this is 1200 ° C in air.
Was fired. Next, 25 mm x 6 m
An mx 2 mm piezoelectric plate (for the secondary side piezoelectric plate 12) was cut out, and a silver paste was screen-printed on each of the front and back surfaces of the cut out piezoelectric plate, and baked at 850 ° C. Thereafter, an electric field strength of 3 kV / mm was applied to the piezoelectric plate on which the electrodes were formed in a state of being immersed in silicon oil at 120 ° C.
Min, and polarized in the thickness direction. According to the same method, 25 mm × 6 mm × 1
mm piezoelectric plate (for the primary side piezoelectric plate 12) was prepared and polarized under the same conditions. These two types of piezoelectric plates were bonded together with a room-temperature-curable epoxy adhesive so as to obtain the state shown in FIG. 2, and the epoxy adhesive was cured by being left for about 48 hours. When bonding the piezoelectric plates, the positions of the piezoelectric plates were deliberately shifted in the longitudinal direction so that a deviation of 0 mm to 1 mm occurred at the end face. Lead wires were soldered to the electrodes exposed on the side surfaces of the manufactured piezoelectric transformer 10 and to the electrodes on the surface. The output lead wire of the obtained piezoelectric transformer 10 and the ground lead wire (earth wire) are connected by 100 kΩ.
A power meter is connected between the input lead wire and the ground lead wire, and a sine wave (voltage: 5 Vrms) near the fundamental resonance frequency (approximately 64 kHz) in the longitudinal stretching mode is connected. The efficiency (ratio between input power and output power) was determined by inputting and keeping the output constant at 5 W. The measurement of the efficiency is performed by setting the frequency of the drive signal to 60 kHz to 70 kHz.
Table 1 shows the values obtained when the highest efficiency was obtained in the obtained efficiency-frequency curve by changing the frequency up to z in steps of 0.1 kHz. As shown in Table 1, the piezoelectric transformer 10
It can be seen that, in order to enable high-efficiency driving of 90% or more, the displacement of the piezoelectric plate must be within 2%. This is considered to be caused by a decrease in the mechanical quality factor Qm of the piezoelectric transformer 10 due to the displacement in the longitudinal direction. Therefore, when the piezoelectric transformer 10 is manufactured by the bonding method, a jig used at the time of the bonding process is devised so as to enhance the lamination accuracy, or processing such as trimming both end surfaces of the piezoelectric transformer after bonding is performed. Is preferred. When the piezoelectric transformer is manufactured by the simultaneous firing method, the end face of the sintered body is cut,
By performing cutting or the like, the displacement of the end face can be substantially reduced to 0%. As described above, according to the present invention, a decrease in the mechanical quality factor Qm can be suppressed by keeping the amount of displacement of the piezoelectric plate within a predetermined range. As a result, power loss can be suppressed. Further, in the piezoelectric transformer of the present invention, since the electrode area of the laminated piezoelectric plates is large and the distance between the electrodes is short, the capacitance on the output side can be kept large. Furthermore, since driving at a fundamental resonance frequency of several tens of kHz or less is possible, it is hardly affected by stray capacitance during driving, and power loss due to this can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing an embodiment of a piezoelectric transformer according to the present invention. FIG. 2 is a sectional view showing another embodiment of the piezoelectric transformer of the present invention. FIG. 3 is a sectional view showing still another embodiment of the piezoelectric transformer of the present invention. [Description of Signs] 1; piezoelectric transformer, 2; primary side piezoelectric plate, 3; secondary side piezoelectric plate, 4; ground electrode, 5; input electrode, 6; output electrode, 1
0: piezoelectric transformer, 12: primary side piezoelectric plate, 12a.12
b: piezoelectric plate, 13: secondary side piezoelectric plate, 14a / 14b; ground electrode, 15; input electrode, 16; output electrode, 20; piezoelectric transformer, 21/22; primary side piezoelectric plate, 21a / 21
b, 22a, 22b; piezoelectric plate, 23; secondary-side piezoelectric plate, 2
4a / 24b / 24c / 24d; ground electrode, 25a /
25b; input electrode, 26; output electrode

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Junko Seki             2-4-2 Daisaku, Sakura City, Chiba Prefecture Pacific Ocean             Central Research Institute of Cement Co., Ltd. (72) Inventor Ryoichi Fukunaga             2-4-2 Daisaku, Sakura City, Chiba Prefecture Pacific Ocean             Central Research Institute of Cement Co., Ltd. (72) Inventor Ryuji Tsukamoto             2-4-2 Daisaku, Sakura City, Chiba Prefecture Pacific Ocean             Central Research Institute of Cement Co., Ltd.

Claims (1)

Claims: 1. A plurality of rectangular piezoelectric ceramics each having an electrode on each of a front surface and a rear surface and polarized in a thickness direction are stacked, and a step-up / step-down of an input voltage is caused by expansion and contraction vibration in a longitudinal direction. Wherein the maximum displacement of the rectangular piezoelectric ceramic in the longitudinal direction is 2% or less of the total length of the piezoelectric transformer.