JP2006093169A - Piezoelectric transformer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoelectric transformer in which, since a drive frequency can be raised even if the transformer has the same shape and capacity as a conventional art, it is easy to raise power density, and which is easy to raise a power capacity. <P>SOLUTION: In the case of constituting a piezoelectric transformer 30 of a square plate type which operates by a profile widening basic oscillation mode or a disk-like piezoelectric transformer 40 which operates by a radial widening basic oscillation mode, an insulting part 15 made of sapphire or an aluminum nitride is formed between a drive part 20 and a power generation part 10, thereby solving the subject. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a piezoelectric transformer, and more particularly, to a piezoelectric transformer with high output power that can be used for power transmission such as an AC-DC converter.

  Piezoelectric transformers, which do not require windings, have a feature that they are simple in structure, easy to reduce thickness and weight, and do not generate electromagnetic noise. Development is progressing for various uses, and it has already been put into practical use as a step-up transformer in backlight inverters for color liquid crystal display devices.

  The above-mentioned piezoelectric transformer used for the inverter for the backlight is a Rosen type, and the output power of a general Rosen type piezoelectric transformer is about 5 W. Apart from such a low-output-power piezoelectric transformer, development of a high-power power that can be used for power transmission, such as an AC-DC converter, for example, a piezoelectric transformer with an output power of several tens of watts or more is also underway.

  For example, Patent Document 1 discloses a piezoelectric transformer (piezoelectric transformer) that has a circular plate shape or a square plate shape and has a structure in which a power generation unit, an unpolarized insulating layer, and a drive unit are stacked in this order in the thickness direction. Is described. 3A is a plan view schematically showing a square plate-like piezoelectric transformer described in Patent Document 1, and FIG. 3B is a cross-sectional view taken along the line III-III shown in FIG. It is the schematic of a line cross section.

  The piezoelectric transformer 70 shown in these drawings has a structure in which a power generation unit 50, an unpolarized insulating layer 55, and a drive unit 60 are stacked in this order in the thickness direction. The power generation unit 50 is provided with a pair of external electrodes 65a and 65b that reach the bottom surface from the side surfaces thereof, and the drive unit 60 has a pair of external electrodes 67a that reach the top surface from the side surfaces. 67b are provided facing each other.

  The power generation unit 50 has a plurality of internal electrodes 51 stacked via piezoelectric ceramic layers polarized in the thickness direction, and adjacent piezoelectric ceramic layers are polarized in opposite directions via the internal electrodes 51. . Similarly, the drive unit 60 has a plurality of internal electrodes 61 stacked via piezoelectric ceramic layers polarized in the thickness direction, and the adjacent piezoelectric ceramic layers via the internal electrodes 61 are polarized in opposite directions. Has been. The power generation unit 50 has low impedance characteristics, and the drive unit 60 has high impedance characteristics. Specifically, the unpolarized insulating layer 55 is made of unpolarized piezoelectric ceramic. In FIG. 3 (b), for convenience, the individual internal electrodes 51, 61 are shown by one implementation line, the entire piezoelectric ceramic layer in the power generation unit 50 is denoted by reference numeral 53, and in the drive unit 60. The entire piezoelectric ceramic layer is indicated by reference numeral 63.

  The external electrode 65 a is connected to every other internal electrode 51 in the power generation unit 50, and the external electrode 65 b is connected to the remaining internal electrode 51 in the power generation unit 50. In addition, the external electrode 67 a is connected to every other internal electrode 61 in the drive unit 60, and the external electrode 67 b is connected to the remaining internal electrode 61 in the drive unit 60. The external electrodes 65a and 65b are connected to a set of electrode terminals not shown, and the external electrodes 67a and 67b are connected to another set of electrode terminals not shown.

  The piezoelectric transformer 70 having the above-described structure operates in the contour spreading basic vibration mode, and the electromechanical coupling coefficient when using lead zirconate titanate-based piezoelectric ceramics easily exceeds 50%. For this reason, according to the piezoelectric transformer 70, the energy conversion efficiency is high and a high-power one can be obtained as compared with the electromagnetic transformer. Note that the circular plate-shaped piezoelectric transformer described in Patent Document 1 also has the same structure as the piezoelectric transformer 70 described above. This piezoelectric transformer operates in the fundamental vibration mode with an increased diameter, and its electromechanical coupling coefficient is slightly larger than the electromechanical coupling coefficient of the piezoelectric transformer 70. Therefore, even with the circular plate-like piezoelectric transformer described in Patent Document 1, a high energy conversion efficiency can be obtained compared to an electromagnetic transformer.

  Patent Document 2 includes two plate-shaped piezoelectric bodies that have been previously polarized and an unpolarized insulating layer disposed between the two plate-shaped piezoelectric bodies, and operates in a spreading vibration mode. A piezoelectric transformer is described. FIG. 4 is a front view schematically showing the piezoelectric transformer described in Patent Document 2. As shown in FIG. As described above, the piezoelectric transformer 90 shown in the figure includes two plate-like piezoelectric bodies 80 and 82 and an unpolarized insulating layer 84 disposed between the two plate-like piezoelectric bodies 80 and 82. The input electrodes 86a and 86b are provided on the upper and lower surfaces of the plate-like piezoelectric body (piezoelectric drive unit) 80, and the output electrodes 88a and 86a are provided on the upper and lower surfaces of the plate-like piezoelectric body (piezoelectric power generation unit) 82, respectively. 88b are provided. As a material of the insulating layer 84, an adhesive, ceramics, or the same material as that of the plate-like piezoelectric body and an unpolarized material are used. According to the description of Patent Document 2, the piezoelectric transformer 90 is suitable for high power use. In addition, the white arrow in FIG. 4 has shown the polarization direction in each plate-shaped piezoelectric material 80 and 82. As shown in FIG.

Patent Document 3 includes a rectangular plate-like drive unit that uses the piezoelectric longitudinal effect and two rectangular plate-like power generation units that use the piezoelectric lateral effect, and has a low dielectric constant on each of the upper and lower surfaces of the drive unit. A piezoelectric transformer having a structure in which power generation units are arranged one by one through a substrate is described. The drive unit is polarized in the length direction, and each power generation unit is polarized in the thickness direction. Further, it is described that the drive unit and the power generation unit may be interchanged with each other. As the low dielectric constant substrate, alumina or the like is used, and particularly those having a relative dielectric constant of 10 or less. According to the description of Patent Document 3, these piezoelectric transformers can obtain a high step-up ratio or a high step-down ratio.
Japanese Patent Laid-Open No. 10-215011 (see claims, columns of embodiments of the invention, columns of effects of the invention, and FIGS. 1 to 6) Japanese Patent Laid-Open No. 11-330580 (see claim 3, claim 4, stage 0043-0049 and FIG. 5) Japanese Patent Laid-Open No. 2001-44527 (refer to claims, 0023-0030, column of effect of invention, and FIG. 1)

  In order to obtain a piezoelectric transformer having a high output power that can be used for power transmission such as an AC-DC converter, for example, a piezoelectric transformer having an output power of 50 W or more, it is necessary to increase the power capacity by enlarging the piezoelectric transformer. However, the power generation unit and the drive unit of the piezoelectric transformer are formed by laminating a necessary number of green sheets each having a composition for forming an internal electrode applied on one side, sintering, and then performing a polarization treatment. For this reason, when the size of the piezoelectric transformer is increased, it becomes difficult to sufficiently sinter the green sheet laminate, and it becomes difficult to manufacture a piezoelectric transformer having desired characteristics with high reliability.

Therefore, it is desirable to suppress the enlargement of the piezoelectric transformer as much as possible. This is also desirable from the viewpoint of obtaining a piezoelectric transformer that is highly competitive with existing electromagnetic transformers. As a result, the power density in the piezoelectric transformer is set to a high value of about 30 W / cm ³ or more. It will be necessary. Considering that the power density of the electromagnetic transformer is about 8W / cm ^3, the power density of 30 W / cm ³ is extremely high.

  If the drive frequency of the piezoelectric transformer can be increased, its power density can be increased relatively easily. However, for example, in a piezoelectric transformer used for an AC-DC converter, there is an electromagnetic noise restriction of 150 kHz or more. The driving frequency is suppressed to less than 150 kHz. A piezoelectric transformer used in an AC-DC converter is usually driven at a frequency 2 to 5% higher than its basic resonance frequency. Therefore, in order to increase the power density of the piezoelectric transformer for such an application, the basic is required. It is desirable to increase the resonance frequency.

  However, the basic resonance frequency of the piezoelectric transformer decreases as the size of the piezoelectric transformer increases. In particular, as in the piezoelectric transformer (piezoelectric transformer) described in Patent Document 1 and the piezoelectric transformer described in Patent Document 2, the insulating layer interposed between the power generation unit and the drive unit is unpolarized. In piezoelectric transformers formed of piezoelectric ceramics, adhesives, or ceramics, the fundamental resonance frequency decreases with increasing size.

  Even if the basic resonance frequency of the piezoelectric transformer is high, if the heat dissipation is low, the temperature rise due to driving increases, and it becomes difficult to achieve high power capacity and high power density. In particular, in a piezoelectric transformer in which the insulating layer interposed between the power generation unit and the drive unit is formed of unpolarized piezoelectric ceramics, the temperature rise associated with the drive is relatively large, so that high power capacity and high power density are achieved. hard.

  Furthermore, in the piezoelectric transformer described in Patent Document 3, because of the structure, the impedance of the power generation unit is extremely large (for example, 100 times or more) as compared with the impedance of the drive unit, so an attempt is made to increase the power capacity. Then, the insulating property of the insulating layer made of the low dielectric constant substrate becomes insufficient. For this reason, in the piezoelectric transformer described in Patent Document 3, it is difficult to achieve high power capacity and high power density.

  The present invention has been made in view of such circumstances, and its purpose is to easily increase the power density because the drive frequency can be increased even with the same shape and size as the conventional one. Another object of the present invention is to provide a piezoelectric transformer that can easily achieve high power capacity.

  The piezoelectric transformer of the present invention that achieves the above object is a square plate-shaped piezoelectric transformer that operates in a contour spreading basic vibration mode or a disk-shaped piezoelectric transformer that operates in a diameter spreading fundamental vibration mode, A drive unit having an internal electrode arranged in parallel; a power generation unit having an internal electrode arranged in parallel with the main surface; and an insulating unit arranged between the drive unit and the power generation unit, The insulating portion is made of sapphire (hereinafter, this piezoelectric transformer may be referred to as “piezoelectric transformer I”).

  This piezoelectric transformer I has the above-described insulating portion made of sapphire (aluminum oxide single crystal), and the sound speed (sound speed) of sapphire is lead zirconate titanate-based piezoelectric ceramics used for the piezoelectric transformer ( Hereinafter, it is faster than the speed of sound (sound speed) of piezoelectric ceramics such as “PZT-based piezoelectric ceramics”. Since the resonance frequency of a certain material layer is proportional to the sound speed in the material layer, in the piezoelectric transformer I in which the insulating portion is formed of sapphire having a large sound speed, the basic resonance frequency can be easily increased. Compared with a conventional piezoelectric transformer having the same shape and size, a high power density can be easily obtained.

  In particular, the insulating part made of sapphire is located in the center of the piezoelectric transformer in the thickness direction, and this part is the part where the vibration stress is the largest in the piezoelectric transformer. Easy to make use of. The mechanical strength of sapphire is higher than that of piezoelectric ceramics such as PZT-based piezoelectric ceramics, and the mechanical strength of the insulating portion made of sapphire is 1 than the mechanical strength of the insulating portion made of piezoelectric ceramic if the thickness is the same. An order of magnitude higher. For this reason, even when the piezoelectric transformer I is vibrated with a large amplitude, it is easy to suppress the breakage from the insulating portion. Here, the “mechanical strength” in the present specification means a tensile strength that is the least difficult to achieve the mechanical strength.

  In addition, since the thermal conductivity of sapphire is higher than that of piezoelectric ceramics such as PZT-based piezoelectric ceramics, it is possible to improve the heat dissipation of the piezoelectric transformer as a whole by forming the insulating portion of the piezoelectric transformer with sapphire. it can. As a result, the temperature rise associated with driving can be suppressed, and it is easy to achieve high power capacity.

Furthermore, the dielectric constant of sapphire is extremely small compared to the dielectric constant of piezoelectric ceramics such as PZT-based piezoelectric ceramics, and the volume resistivity of sapphire is as high as 1 × 10 ¹⁴ Ω · cm or more. Can be easily formed. From this point, the piezoelectric transformer I can easily achieve high power capacity.

  For these reasons, according to the piezoelectric transformer I of the present invention, the drive frequency can be increased even with the same shape and size as the conventional one, so that it is easy to achieve high power density and high power capacity. It becomes easy to obtain what is easy to plan.

  In the piezoelectric transformer I of the present invention, the sound velocity in the insulating portion is 8000 m / s or more, the thermal conductivity is 30 W / m · K or more, and the mechanical strength of the insulating portion is the power generation unit or the It is preferable that the mechanical strength of the piezoelectric ceramic constituting the drive unit is higher. In addition, said sound speed means the sound speed of the longitudinal wave in 27 degreeC and 30-150 kHz area, and said heat conductivity means the heat conductivity under 80 degreeC.

  Another piezoelectric transformer of the present invention that achieves the above-described object is a square plate-shaped piezoelectric transformer that operates in a contour spreading fundamental vibration mode or a disk-shaped piezoelectric transformer that operates in a diameter spreading fundamental vibration mode. A drive unit having an internal electrode arranged in parallel with the surface; a power generation unit having an internal electrode arranged in parallel with the main surface; and an insulating unit arranged between the drive unit and the power generation unit. The insulating portion is made of aluminum nitride (hereinafter, this piezoelectric transformer may be referred to as “piezoelectric transformer A”).

  This piezoelectric transformer A has the above-described insulating portion made of aluminum nitride, and the sound speed (sound speed) in aluminum nitride is the sound speed (sound speed) in piezoelectric ceramics such as PZT-based piezoelectric ceramics used in piezoelectric transformers. ) Faster than. Also, compared to piezoelectric ceramics such as PZT piezoelectric ceramics used in piezoelectric transformers, the thermal conductivity of aluminum nitride is several tens of times, the mechanical strength is several times to one digit, and the volume resistivity is one digit or higher. . The dielectric constant of aluminum nitride is extremely small compared to the dielectric constant of piezoelectric ceramics such as PZT piezoelectric ceramics used in piezoelectric transformers.

  Therefore, according to the piezoelectric transformer A, for the same reason as the piezoelectric transformer I of the present invention described above, the drive frequency can be increased even if it has the same shape and size as the conventional one, so that high power density is achieved. It is easy to obtain a product that is easy to achieve high power capacity.

  In the piezoelectric transformer A of the present invention, the sound velocity at the insulating portion is 8000 m / s or more, the thermal conductivity is 150 W / m · K or more, and the mechanical strength of the insulating portion is the power generation portion or the It is preferable that the mechanical strength of the piezoelectric ceramic constituting the drive unit is higher. In addition, said sound speed means the sound speed of the longitudinal wave in 27 degreeC and 30-150 kHz area, and said heat conductivity means the heat conductivity under 80 degreeC.

  As described above, according to the present invention, the drive frequency can be increased even with the same shape and size as in the prior art, so that it is easy to achieve high power density and high power capacity. Since it becomes easy to obtain a piezoelectric transformer, it is easy to provide a small piezoelectric transformer with high output power that can be used for power transmission such as an AC-DC converter.

  As described above, the piezoelectric transformer of the present invention is a square plate-shaped piezoelectric transformer that operates in a broadened fundamental vibration mode or a disk-shaped piezoelectric transformer that operates in a broadened fundamental vibration mode. An insulating part made of sapphire or aluminum nitride is provided between the two. Hereinafter, embodiments of the piezoelectric transformer of the present invention will be described in detail with reference to the drawings as appropriate.

<First form>
FIG. 1A is a plan view schematically showing an example of a square plate-like piezoelectric transformer among the piezoelectric transformers of the present invention, and FIG. 1B is a plan view of the I− shown in FIG. It is the schematic of a I line cross section. The piezoelectric transformer 30 shown in these drawings has a structure in which a power generation unit (secondary side) 10, an insulating unit 15, and a drive unit (primary side) 20 are stacked in this order in the thickness direction. An external electrode 25a is provided on one of a pair of side surfaces facing each other in the power generation unit 10, and an external electrode 25b is provided on the other side surface. In addition, an external electrode 27a is provided on one of a pair of side surfaces facing each other in the drive unit 20, and an external electrode 27b is provided on the other side surface.

  The power generation unit 10 includes a plurality of (9 layers in the illustrated example) adjacent internal electrodes 11 via piezoelectric ceramic layers (hereinafter referred to as “piezoelectric active layers”) polarized in the thickness direction. Each internal electrode 11 is made of, for example, an inorganic conductive material such as a silver (Ag) -palladium (Pd) alloy, and these internal electrodes 11 are parallel to the main surface of the power generation unit 10 and eventually the main surface of the piezoelectric transformer 30. They are arranged in parallel. The piezoelectric active layer is made of, for example, an inorganic piezoelectric material such as PZT-based piezoelectric ceramic, and adjacent piezoelectric active layers are polarized in opposite directions via the internal electrode 11. A lower exterior layer made of unpolarized piezoelectric ceramic is provided below the lowermost internal electrode 11, and an upper exterior layer made of unpolarized piezoelectric ceramic is provided on the uppermost internal electrode 11. It has been.

  The drive unit 20 has the same structure as the power generation unit 10 described above. In the illustrated example, the number of internal electrodes 21 in the drive unit 20 is three layers, which is smaller than the number of internal electrodes 11 in the power generation unit 10. However, the number of internal electrodes 11 in the power generation unit 10, the interval between adjacent internal electrodes 11 (thickness of the piezoelectric active layer), the number of internal electrodes 21 in the drive unit 20, and the adjacent internals The distance between the electrodes 21 (the thickness of the piezoelectric active layer) can be appropriately selected according to the application of the piezoelectric transformer 30 and the impedance characteristics required of the piezoelectric transformer 30.

  Every other internal electrode 11 in the power generation unit 10 is connected to the external electrode 25a, and the remaining internal electrodes 11 are connected to the external electrode 25b. In addition, every other internal electrode 21 in the drive unit 20 is connected to the external electrode 27a, and the remaining internal electrodes 21 are connected to the external electrode 27b. Each external electrode 25a, 25b, 27a, 27b is formed of a conductive material such as silver (Ag).

  In FIG. 1B, for the sake of convenience, the individual internal electrodes 11 and 21 are shown by one execution line, and the entire piezoelectric ceramic layer in the power generation unit 10 is denoted by reference numeral 13 and in the drive unit 20. The entire piezoelectric ceramic layer is indicated by reference numeral 23.

  The insulating part 15 provided between the power generation part 10 and the driving part 20 is a characteristic constituent member in the piezoelectric transformer of the present invention. The insulating part 15 is made of sapphire (aluminum oxide single crystal) or aluminum nitride. Become. The power generation unit 10 and the drive unit 20 are firmly bonded to the insulating unit 15 via a bonding material (not shown) by a method such as brazing.

  When the insulating portion 15 is formed of sapphire, the sound speed is usually 8000 m / s or more, and the thermal conductivity is 30 W / m · K or more. From the viewpoint of increasing both the power density and the power capacity of the piezoelectric transformer 30, the thickness of the insulating portion 15 is preferably in the range of about 1 to 2 mm. If the thickness is less than 1 mm, the improvement of the driving frequency is small, and it is difficult to maintain the insulation between the power generation unit 10 and the driving unit 20 in the piezoelectric transformer 30. If the thickness exceeds 2 mm, the piezoelectric transformer 30 is effective. Electromechanical coupling coefficient decreases.

  On the other hand, when the insulating portion 15 is formed of aluminum nitride (sintered body), the sound velocity is preferably 8000 m / s or more, and the thermal conductivity is preferably 150 W / m · K or more. Further, from the viewpoint of increasing both the power density and the power capacity of the piezoelectric transformer 30, the thickness is preferably within a range of about 1 to 3 mm. If this thickness is less than 1 mm, the improvement of the driving frequency is small, and it is difficult to maintain the insulation between the power generation unit 10 and the driving unit 20 in the piezoelectric transformer 30. The mechanical coupling coefficient decreases.

In using the piezoelectric transformer 30 having the above-described structure, a contour spreading basic vibration mode (primary contour spreading vibration mode) is used. An alternating current having a frequency that can vibrate each piezoelectric active layer in the drive unit 20 in the contour spreading basic vibration mode is applied to the external electrodes 27a and 27b, and the piezoelectric active layers are forced to fall under the contour spreading basic vibration mode. The piezoelectric active layer in the power generation unit 10 is strongly excited under the contour spreading fundamental vibration mode by the piezoelectric inverse effect depending on the electromechanical coupling coefficient k _{p ′} of the contour spreading fundamental vibration mode.

The value of the electromechanical coupling coefficient k _{p ',} since approximately twice the electromechanical coupling factor k ₃₁ in the longitudinal direction longitudinal oscillation mode due to the piezoelectric transverse effect large, the energy conversion efficiency of the piezoelectric transformer 30 is high. As a result, the power transmission efficiency by the piezoelectric transformer 30 is also increased. If the power transmission efficiency of the piezoelectric transformer 30 is high, the piezoelectric transformer 30 can be further downsized.

  In addition, the speed of sound of sapphire or aluminum nitride, which is the material of the insulating portion 15, is as fast as 8000 / s or more, and this value is 2.5 times or more that of piezoelectric ceramics such as PZT-based piezoelectric ceramics. Compared with the case where unpolarized piezoelectric ceramic is used as the material, the basic resonance frequency of the piezoelectric transformer 30 becomes higher. Therefore, the power density of the piezoelectric transformer 30 using sapphire or aluminum nitride as the material of the insulating portion 15 is significantly higher than the power density of the piezoelectric transformer using unpolarized piezoelectric ceramics as the material of the insulating portion.

For example, when PZT piezoelectric ceramic is used as the piezoelectric ceramic and the insulating portion 15 is formed of sapphire having a thickness of 1.5 mm, if the size of the piezoelectric transformer 30 is 20 mm × 20 mm × 7 mm, the basic resonance frequency is 120 kHz. Thus, a power density of 30 W / cm ³ or more can be easily obtained. On the other hand, when the insulating part is formed by using unpolarized PZT piezoelectric ceramics instead of sapphire, the basic resonance frequency of the piezoelectric transformer is greatly reduced to 96 kHz, and the power density is about 18 W / cm ³ .

  The insulating portion 15 is located in the central portion of the piezoelectric transformer 30 in the thickness direction, and this portion is the portion having the largest vibration stress in the piezoelectric transformer 30, so that the characteristic of sapphire or aluminum nitride that the sound speed is fast is effective. Easy to make use of. In addition, the mechanical strength of each of sapphire and aluminum nitride is higher than that of piezoelectric ceramics such as PZT-based piezoelectric ceramics. If the thickness is the same, the mechanical strength of the insulating portion 15 made of sapphire or aluminum nitride is higher than that of piezoelectric ceramics. It becomes several times to one digit higher than the mechanical strength of the insulating part. For this reason, even when the piezoelectric transformer 30 is vibrated with a large amplitude, breakage from the insulating portion 15 is easily suppressed.

  Furthermore, since the thermal conductivity of sapphire and aluminum nitride is much higher (tens of times) than that of piezoelectric ceramics such as PZT piezoelectric ceramics, the piezoelectric transformer in which the insulating portion 15 is formed of sapphire or aluminum nitride. The heat radiation performance of the entire 30 is significantly higher than the heat radiation performance of a piezoelectric transformer in which an insulating portion is formed of unpolarized piezoelectric ceramics. As a result, in the piezoelectric transformer 30, the temperature rise associated with driving can be suppressed, and it is easy to achieve high power capacity.

  For example, when lead zirconate titanate piezoelectric ceramic is used as the piezoelectric ceramic and the insulating portion 15 is formed of sapphire having a thickness of 1.5 mm, if the size of the piezoelectric transformer 30 is 20 mm × 20 mm × 7 mm, 50 W output The temperature rise over time is only about 15 ° C. or less.

The dielectric constant of sapphire and aluminum nitride is extremely small compared to the dielectric constant of piezoelectric ceramics such as PZT piezoelectric ceramics, and the volume resistivity of sapphire and aluminum nitride is as high as 1 × 10 ¹⁴ Ω · cm or more, respectively. In the piezoelectric transformer 30, the insulating characteristics of the insulating portion 15 can be easily increased. Also from this point, the piezoelectric transformer 30 can easily achieve high power capacity.

  For example, if the thickness of the insulating portion 15 made of sapphire or aluminum nitride is 1.0 mm, it can sufficiently withstand 50 Hz, 5 kV AC voltage, and 10 kV DC voltage. Further, the leakage current flowing out from the drive unit 20 to the power generation unit 10 can be reduced to about 1/100 compared to the case where the insulating unit is formed of piezoelectric ceramics such as PZT-based piezoelectric ceramics. It is possible to reduce the order of magnitude.

<Second form>
FIG. 2 (a) is a plan view schematically showing an example of a disk-shaped piezoelectric transformer of the piezoelectric transformers of the present invention, and FIG. 2 (b) is a cross-sectional view taken along the line II- shown in FIG. 2 (a). It is the schematic of the II line cross section. The piezoelectric transformer 40 shown in these drawings has the same structure as the piezoelectric transformer 30 shown in FIGS. 1A and 1B except that the outer shape is a disk shape. 2 (a) and 2 (b), the components shown in FIG. 1 (a) or FIG. 1 (b) are functionally common to the components shown in FIG. 1 (a) or FIG. 1 (b). The same reference numerals as those used in 1 (b) are attached, and the description thereof is omitted.

When this piezoelectric transformer 40 is used, a diameter broadening fundamental vibration mode (primary diameter broadening vibration mode) is used. An alternating current having a frequency that can vibrate each piezoelectric active layer in the drive unit 20 in the wide-diameter fundamental vibration mode is applied to the external electrodes 27a and 27b, and the piezoelectric active layer is biased under the wide-diameter fundamental vibration mode. When excited by a piezoelectric reverse effect depends on the electromechanical coupling coefficient k _p of径広rising fundamental vibration mode, the piezoelectric active layer in the power generation portion 10 is excited to stress under径広rising fundamental vibration mode.

Since the piezoelectric transformer 40 has the insulating portion 15 made of sapphire or aluminum nitride, for the same reason as the piezoelectric transformer 30 shown in FIG. 1A and FIG. Since the drive frequency can be increased even if the size is high, it is easy to achieve high power density and high power capacity. In general, the electromechanical coupling coefficient k _p in the diameter broadening fundamental vibration mode is slightly larger than the electromechanical coupling coefficient k _{p ′} in the contour spreading fundamental vibration mode. For this reason, if the drive frequency is the same, the power density of the piezoelectric transformer 40 is about 5 to 10% higher than the power density of the piezoelectric transformer 30 described above.

<Example>
The piezoelectric transformer 30 is the same as the piezoelectric transformer 30 shown in FIGS. 1A and 1B, using PZT-based piezoelectric ceramics as the piezoelectric ceramics and using artificial synthetic sapphire having a flat plate shape with a thickness of 1.5 mm as the insulating portion. A 20 mm × 20 mm × 7.0 mm square plate-shaped piezoelectric transformer having a structure was manufactured. In this piezoelectric transformer, the thickness of the power generation unit is 3.0 mm, and the thickness of the drive unit is 2.5 mm.

  The power generation unit has nine layers of internal electrodes made of a silver (Ag) -palladium (Pd) alloy, and thus has eight layers of piezoelectric active layers. The thickness of the individual piezoelectric active layer is 0.35 mm. On the other hand, the drive unit has three layers of internal electrodes made of a silver (Ag) -palladium (Pd) alloy, and thus has two layers of piezoelectric active layers. The thickness of the individual piezoelectric active layer is 1.2 mm. The difference between the number of internal electrode layers in the power generation unit and the number of internal electrode layers in the drive unit is that an external load connected to the AC-DC converter when the piezoelectric transformer is used as a step-up transformer of the AC-DC converter. For impedance matching.

  The power generation unit and the drive unit are subjected to parallel plane polishing (lapping) and brazed to the insulating unit (artificial synthetic sapphire) at 650 ° C. The external electrodes provided in each of the power generation unit and the drive unit are each made of a silver (Ag) -palladium (Pd) alloy.

  The piezoelectric transformer having the above-described structure was manufactured as follows. First, a required number of green sheets coated with a composition for forming an internal electrode on one side were laminated, punched to a predetermined size, and then sintered to produce a piezoelectric ceramic laminate for a power generation unit. Similarly, a piezoelectric ceramic laminate for the drive unit was produced. Next, each piezoelectric ceramic laminate was subjected to parallel plane polishing and then brazed to the insulating portion. Thereafter, a polarization treatment was performed by applying a direct current high voltage to each internal electrode of the piezoelectric ceramic laminate serving as the power generation unit and each internal electrode of the piezoelectric ceramic laminate serving as the drive unit, thereby obtaining a piezoelectric transformer.

A 20 Ω external load was connected to this piezoelectric transformer, and in this state, the piezoelectric transformer was operated under the broadened basic vibration mode, and a high power test was performed. The basic resonance frequency of the piezoelectric transformer at this time is 123 kHz, and the driving frequency is 128 kHz. When it was defined that the piezoelectric transformer reached the power limit when the temperature was raised by 30 ° C., the maximum output power was 95 W, and the maximum power density was 33.9 W / cm ³ . These values are considerably higher than those of conventional piezoelectric transformers and considerably higher than those of general electromagnetic transformers.

  In addition, when an alternating current having a frequency of 60 Hz and an effective voltage of 5000 V was applied to the power generation unit and the drive unit of the piezoelectric transformer, almost no current flowed even after 5 minutes. Furthermore, when the conduction noise was measured, it was 1/100 or less of the conduction noise in the piezoelectric transformer of the comparative example described later. If the piezoelectric transformer of this embodiment with a small conduction noise is used for an AC-DC converter, the volume of the noise filter can be suppressed to 1/10 or less as compared with an AC-DC converter equipped with a conventional piezoelectric transformer. The AC-DC converter can be further downsized.

<Example 2>
A piezoelectric transformer having the same structure as the piezoelectric transformer of Example 1 was manufactured except that the outer shape was a disk shape with a diameter of 2.26 mm and a thickness of 7 mm. Then, an external load of 20Ω was connected to this piezoelectric transformer, and in this state, the piezoelectric transformer was operated under the basic vibration mode with a large diameter, and a high power test was performed.

The basic resonance frequency of the piezoelectric transformer at this time is 136 kHz, and it is defined that the piezoelectric transformer has reached the power limit when the temperature is raised by 30 ° C. The maximum output power is 116 W, and the maximum power density is 41.6 W / cm. ³ .

<Comparative example>
A piezoelectric transformer was manufactured under the same conditions as in Example 1 except that the insulating part was formed of unpolarized PZT-based piezoelectric ceramics, and the basic resonance frequency was measured with an external load of 20Ω connected to the piezoelectric transformer. However, it was less than 100 kHz. The maximum output power was 33W.

FIG. 1A is a plan view schematically showing an example of a square plate-like piezoelectric transformer among the piezoelectric transformers of the present invention, and FIG. 1B is a plan view of the I− shown in FIG. It is the schematic of a I line cross section. FIG. 2 (a) is a plan view schematically showing an example of a disk-shaped piezoelectric transformer of the piezoelectric transformers of the present invention, and FIG. 2 (b) is a cross-sectional view taken along the line II- shown in FIG. 2 (a). It is the schematic of the II line cross section. FIG. 3A is a plan view schematically showing an example of a conventional square plate-like piezoelectric transformer, and FIG. 3B is a schematic cross-sectional view taken along line III-III shown in FIG. FIG. It is a front view which shows schematically the other example of the conventional piezoelectric transformer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Power generation part 11 Internal electrode 15 Insulation part which consists of sapphire or aluminum nitride 20 Drive part 21 Internal electrode 25a, 25b, 27a, 27b External electrode 30 Square-shaped piezoelectric transformer 40 Disc-shaped piezoelectric transformer

Claims (4)

A square plate-shaped piezoelectric transformer that operates in a contour spreading fundamental vibration mode, or a disk-shaped piezoelectric transformer that operates in a diameter spreading fundamental vibration mode,
A drive unit having an internal electrode arranged in parallel with the main surface, a power generation unit having an internal electrode arranged in parallel with the main surface, and an insulating unit arranged between the drive unit and the power generation unit. And a piezoelectric transformer, wherein the insulating portion is made of sapphire.   Piezoelectric ceramics having a sound velocity in the insulating portion of 8000 m / s or higher, a thermal conductivity of 30 W / m · K or higher, and a mechanical strength of the insulating portion constituting the power generation unit or the driving unit The piezoelectric transformer according to claim 1, wherein the piezoelectric transformer has a mechanical strength higher than that of the piezoelectric transformer. A square plate-shaped piezoelectric transformer that operates in a contour spreading fundamental vibration mode, or a disk-shaped piezoelectric transformer that operates in a diameter spreading fundamental vibration mode,
A drive unit having an internal electrode arranged in parallel with the main surface, a power generation unit having an internal electrode arranged in parallel with the main surface, and an insulating unit arranged between the drive unit and the power generation unit. And a piezoelectric transformer, wherein the insulating portion is made of aluminum nitride. Piezoelectric ceramics having a sound velocity in the insulating portion of 8000 m / s or higher, a thermal conductivity of 150 W / m · K or higher, and a mechanical strength of the insulating portion constituting the power generation unit or the driving unit The piezoelectric transformer according to claim 3, wherein the piezoelectric transformer has a mechanical strength higher than that of the piezoelectric transformer.

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