JP2002299717A - Laminated piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated piezoelectric transformer that is superior in efficiency. SOLUTION: In this laminated piezoelectric transformer, a primary-side internal electrode is formed on the surfaces of piezoelectric sheets, with an external electrode connected to the internal electrode, and a secondary-side external electrode are formed on the end faces of the laminate of the piezoelectric sheets. A secondary-side internal electrode is formed on the surfaces of the sheets, near the end sections of the sheets and a ratio (la/L) of a length la of the secondary-side internal electrode in the lengthwise direction to a length L of a piezoelectric crystal element is adjusted to 0.01-0.2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer suitable for a cold cathode tube lighting circuit and an inverter circuit component for lighting a backlight of a small liquid crystal display. The present invention relates to the improvement of the characteristics, particularly the transformer efficiency, of a piezoelectric transformer formed by laminating and integrating the attached piezoelectric transformers.

[0002]

2. Description of the Related Art In general, in a liquid crystal display, a liquid crystal display does not emit light by itself, and thus a backlight system in which a discharge tube such as a cold cathode tube is disposed on the back and side surfaces of a liquid crystal display is mainly used. In order to drive this discharge tube, an AC high voltage of several hundred volts or more is usually required, depending on the length and diameter of the discharge tube used. As a method of generating this AC high voltage, a piezoelectric transformer is used. An inverter using the device has been proposed. The piezoelectric transformer used here has a very simple structure because no winding is required. Therefore, the piezoelectric transformer device can be reduced in size, thickness, and cost, and is particularly used for portable OA equipment. Have been.

FIGS. 4 to 6 show an example of the laminated piezoelectric transformer. 4 is a perspective view, FIG. 5 is a longitudinal sectional view, and FIG. 6 is a plan sectional view. Reference numeral 9 in the figure denotes a piezoelectric transformer using, for example, a lead zirconate titanate (PZT) -based piezoelectric material.
This piezoelectric transformer 9 prints an internal electrode 92 on a part of a green sheet 91 made of PZT ceramics, laminates and press-bonds this sheet, sinters it, and then performs cutting and polishing.
Input electrodes 94 and 95 and an output electrode 96 are formed on the outer surface. The central part of the piezoelectric transformer is polarized in the thickness direction on the primary side, and the secondary sides disposed on both sides thereof are polarized in the length direction. In such a piezoelectric transformer, when an AC voltage having substantially the same frequency as the mechanical resonance frequency in the longitudinal direction of the piezoelectric transformer 9 is applied between the input electrodes 94 and 95 from an AC voltage source, the piezoelectric transformer 9 is moved in the longitudinal direction. A strong mechanical vibration is generated, whereby electric charges are generated by the piezoelectric effect on the secondary side, and an output voltage Vo is generated between the output electrode 96 and one of the input electrodes, for example, the input electrode 94. The application of the AC voltage to the input electrodes 94 and 95 of the piezoelectric transformer is performed via strip-shaped lead terminals provided integrally with the insulating case.
The output voltage Vo generated at the output electrode is taken out by a lead terminal connected to the output terminal. The lead terminal is formed in a band shape with a rigid material such as phosphor bronze,
One end of the piezoelectric transformer is fixed to an insulating case accommodating the piezoelectric transformer by, for example, press-fitting. The other end is electrically connected to the electrodes 94, 95, and 96 of the piezoelectric transformer by soldering. Holding mechanically.

[0004]

With the recent downsizing of OA equipment, further downsizing and thinning of piezoelectric transformer devices are strongly desired. Since the external dimensions of the piezoelectric transformer are inversely proportional to the excitation frequency, the excitation frequency may be increased as much as possible to reduce the size of the piezoelectric transformer device.However, if the frequency is too high, for example, in the backlight of a liquid crystal display,
A high-frequency current flows through the metal part of the device main body via the stray capacitance from the discharge tube or wiring, causing a problem that a sufficient tube current does not flow through the discharge tube and the lamp does not light. For this reason, it is necessary to lower the excitation frequency as much as possible. In recent piezoelectric transformer devices, the resonance frequency of the piezoelectric transformer is reduced by exciting the piezoelectric transformer in the λ / 2 resonance mode, and the size of the piezoelectric transformer device is reduced. It is planning to make it.

As a piezoelectric transformer exhibiting excellent characteristics in the λ / 2 resonance mode, a primary side is arranged at the center of the element as shown in FIG. 4 to FIG. 6 disclosed in JP-A-9-74236. There is a piezoelectric transformer having a structure in which secondary sides are disposed at both ends. Although the piezoelectric transformer having this structure exhibits excellent transformer characteristics with high transformer efficiency and high step-up ratio, it is necessary to form external electrodes on the four side surfaces of the piezoelectric transformer, and there is a problem that the work required for the formation is complicated. Was. As a method of improving this, in JP-A-11-68183, a secondary internal electrode is formed in advance at the end of the piezoelectric sheet, and at least one of the external electrodes connected to the secondary internal electrode and the primary electrode are formed. It is disclosed that by forming at least one of the external electrodes connected to the internal electrodes on the same end face of the laminate, the number of working steps can be significantly reduced. FIG. 1 is a perspective view of the laminated piezoelectric transformer having this configuration, and FIG.
FIG. 3 shows a sectional plan view.

The laminated piezoelectric transformer of this configuration has features that the operation required for forming the external electrodes can be greatly reduced, and that the case can be easily mounted. However, in the structure of the piezoelectric transformer having the above-described configuration, the transformer efficiency is somewhat low, and there is room for some improvement. In particular, the size of the liquid crystal screen is increasing year by year, and the power consumption of the backlight used in proportion to the size is increasing. There is a strong demand for higher power consumption, especially for mobile OA equipment. Therefore, higher efficiency of a transformer used for a backlight inverter is required. Further, it has been necessary to improve the transformer efficiency in order to suppress the heat generation of the piezoelectric transformer during use. Therefore, the present invention has been made to solve the above-mentioned problems, and is disclosed in
The present invention aims to improve the transformer efficiency by utilizing the structure of a piezoelectric transformer having excellent productivity disclosed in US Pat.

[0007]

According to the present invention, a primary-side internal electrode is formed on the surface of a piezoelectric sheet, and an external electrode connected to the primary-side internal electrode and a secondary-side external electrode are formed as follows. In the laminated piezoelectric transformer formed on the end face of the laminated body of the sheet, the secondary side internal electrode is formed on the sheet surface near the end of the piezoelectric sheet, and the element length is increased.
L, the ratio of the longitudinal length la of the secondary side internal electrode,
A laminated piezoelectric transformer having la / L of more than 0.01 and less than 0.2 is configured. In addition, the length of the primary side internal electrode in the longitudinal direction lb
By setting the ratio lb / L to 0.25 to less than 0.65, a piezoelectric transformer having excellent transformer efficiency can be provided.

As a result of intensive studies, the present inventors have studied variously the relationship between the structure of the piezoelectric transformer having the above-described structure and the characteristics of the transformer, and have found that the efficiency of the piezoelectric transformer depends on the element length L and the longitudinal direction of the secondary-side internal electrode. It has been found that a piezoelectric transformer having excellent transformer efficiency can be obtained by depending on the ratio to the length la and setting the ratio to a predetermined ratio. Furthermore, the ratio of the internal electrode length lb in the longitudinal direction on the primary side to the element length L, lb / L, is set to 0.
By setting the ratio to 3 to 0.6, the effect of the present invention can be enhanced. Hereinafter, the effects of the present invention will be specifically described with reference to examples.

[0009]

(Embodiment 1) First, a method of preparing a sample will be described. First, lead oxide, titanium oxide, zirconium oxide, and strontium carbonate, which are powders of each starting material, are weighed in predetermined amounts, mixed with a wet ball mill, dried, and then calcined powder is crushed by a raikai machine. After being charged into a ball mill and pulverized by a wet method, the raw material was dried to prepare a calcined raw material powder. Further, the powder was calcined at 850 ° C. for 2 hours to prepare a calcined powder. The calcined powder was pulverized by a raikai machine, charged into a ball mill, pulverized by a wet method, and dried to prepare a calcined raw material powder. Since the piezoelectric transformer of the present invention has a structure in which the piezoelectric ceramics and the electrodes are alternately stacked as described above, it can be easily manufactured by the multilayer ceramic technology. The calcined piezoceramic powder and the binder are wet-mixed, a green sheet is prepared by a doctor blade method, and an Ag-Pd or Pt paste serving as an electrode is printed on the surface of the green sheet by a printing method. After printing, the sheets are laminated and pressure-bonded under heating at 60 ° C. to be integrated. Porcelain is formed by sintering at about 1000 to 1200 ° C. After sintering, an external electrode for taking out is formed by printing and baking an Ag paste on the side surface of the piezoelectric transformer. Solder the lead wires to the external electrodes. After connecting the lead wires, the piezoelectric transformer is polarized by applying a DC voltage of 1 kV / mm in oil at 150 ° C. for 10 minutes. Table 1 summarizes the evaluation results of the piezoelectric transformer. The sample subjected to the test has a width of 4.7 mm, a thickness of 1.7 mm, the number of layers is 25, the internal electrode length lb on the primary side is constant at 12 mm, and the element length L
Is set to 30 mm, and the ratio la / L to the longitudinal length la of the secondary-side internal electrode is set to 0.01 to 0.2. The evaluation of the transformer characteristics was performed under the condition of an output of 3 W with a 100 kΩ resistor connected as a secondary load resistance.

[0010]

[Table 1]

Table 1 also shows the evaluation results outside the present invention for comparison. Table 1 shows that a high efficiency piezoelectric transformer of 94% or more can be obtained in the present invention. The la / L ratio is
In the case of No. 1 which is smaller than 0.01 of the present invention, the efficiency is 91.5% and the transformer efficiency is low. On the other hand, the la / L ratio is 0.2, which is larger than that of the present invention.
In No. 7, the boost ratio is as low as 30, and the sufficient boost characteristics required for the transformer cannot be obtained.

(Example 2) A multilayer piezoelectric transformer was manufactured in the same manner as in Example 1. Table 2 shows the evaluation results.

[0013]

[Table 2]

Table 2 also shows the evaluation results outside the present invention for comparison. Table 2 shows that a high efficiency piezoelectric transformer of 92% or more can be obtained in the present invention. lb / L ratio of 0.2
5 and No8 smaller than the present invention, the efficiency is as low as less than 90%,
And the boost ratio is also low. On the other hand, even with No13 having an lb / L ratio of 0.65 which is larger than that of the present invention, an efficiency of less than 90% can be obtained.

[0015]

As described above, the present invention provides a piezoelectric transformer which is excellent in workability for forming external electrodes and has excellent transformer characteristics, especially excellent transformer efficiency.

[Brief description of the drawings]

FIG. 1 is a perspective view of a piezoelectric transformer according to the present invention.

FIG. 2 is a longitudinal sectional view of a piezoelectric transformer according to the present invention.

FIG. 3 is a plan sectional view of a piezoelectric transformer according to the present invention.

FIG. 4 is a perspective view of a conventional piezoelectric transformer.

FIG. 5 is a longitudinal sectional view of a conventional piezoelectric transformer.

FIG. 6 is a plan sectional view of a conventional piezoelectric transformer.

[Description of Signs] 1 Piezoelectric transformer 2 Insulating case 3a, 3b Input electrode 4a, 4b Output electrode 5a, 5b, 6a, 6b Lead terminal

Claims (2)

[Claims] An internal electrode on the primary side is formed on a surface of a piezoelectric sheet, and an external electrode connected to the internal electrode on the primary side and an external electrode on the secondary side are provided on an end surface of a laminate of the sheet. In the formed laminated piezoelectric transformer,
A secondary-side internal electrode is formed on the sheet surface near the end of the piezoelectric sheet, and the ratio la / L of the element length L to the longitudinal length la of the secondary-side internal electrode is 0.01. Multilayer piezoelectric transformer characterized by being less than ultra-0.2. 2. The laminated piezoelectric transformer according to claim 1, wherein the ratio of the length lb of the primary internal electrode in the longitudinal direction to the element length L, lb / L, is more than 0.25 and less than 0.65.