JPH0936452A - Piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set a resonance frequency at 100kHz or below without sacrifice of boost ratio or durability by forming a thin insulation area on an inner electrode only in the vicinity of part for fixing an external input electrode and supporting and securing a piezoelectric transformer body such that a node will appear substantially in the center in the longitudinal direction. SOLUTION: Only a very thin insulation area 58A or 58B for ensuring insulation with respect to any one unconnected external input electrode 48A, 48B does not form the inner electrode on the side of oscillatory part. Since only that area is not subjected to sufficient polarization, undisplaced part is reduced significantly and thereby a piezoelectric transformer is deformed and oscillated uniformly as a whole. Consequently, cracking and delamination of inner electrode can be suppressed significantly and the durability can be enhanced. More specifically, the resonance frequency can be set at 100kHz or below without sacrifice of boost ratio or durability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer used for supplying electric power to a discharge tube for a backlight of a liquid crystal display, for example.

[0002]

2. Description of the Related Art Generally, in a liquid crystal display,
Since the liquid crystal itself does not emit light, a backlight system in which a discharge tube such as a cold cathode tube is arranged on the back surface or the side surface of the liquid crystal display and the liquid crystal is displayed using light from the discharge tube is predominant. In order to drive this discharge tube, a high AC voltage of several hundreds of volts or more is required, depending on the length and diameter of the discharge tube itself. An inverter using a piezoelectric transformer is known as a method for generating this high AC voltage.
No. 492 publication. Since the piezoelectric transformer has a very simple structure, it can be made compact, thin, and low cost.

The structure of this piezoelectric transformer will be described with reference to FIG. The illustrated example is a perspective view showing a Rosen type piezoelectric transformer. For example, for example, lead zirconate titanate (PZ)
Plate-shaped piezoelectric ceramic element 2 made of T) ceramic
, A pair of input electrodes 4 and 6 are formed on the upper and lower surfaces of the left half in this figure by, for example, silver baking, and an output electrode 8 is formed on the right end surface in the same manner. The left half of the ceramic element 2 is polarized in the thickness direction, and the right half of the power generating portion is polarized in the longitudinal direction.

In the piezoelectric transformer thus formed, when an AC voltage having a frequency substantially the same as the resonance frequency in the longitudinal direction of the ceramic element 2 is applied from the AC voltage source 10 between the input electrodes 4 and 6, the ceramic element 2 will be A strong mechanical vibration is generated in the longitudinal direction, and as a result, electric charges are generated by the piezoelectric effect in the power generation section in the right half, and one of the output electrode 8 and the input electrode
For example, the output electrode Vo is generated between the output electrode Vo and the input electrode 6. By applying this output voltage Vo across the discharge tube 12, it can be made to emit light. As shown in FIG. 14, there are two vibration modes, a full-wavelength mode that resonates at one wavelength in the longitudinal direction (FIG. 14A) and a half-wavelength mode that resonates at a half-wavelength (FIG. 14B).

The ceramic element 2 is supported at two points having a length of 1/4 from both ends in the one-wavelength mode, and is supported at the central portion in the half-wavelength mode. The outer dimensions of the piezoelectric transformer should be as small as possible in order to miniaturize the device, but since the outer dimensions are inversely proportional to the frequency, the smaller the size, the higher the frequency inevitably becomes. However, if the frequency is too high, in the backlight of the liquid crystal display body, a high frequency current will flow into the housing via stray capacitance such as wiring, and a sufficient tube current will not flow into the discharge tube, resulting in a significant decrease in efficiency. Will end up. For example, in the current inverter circuit, it is desirable that the frequency is 100 KHz or less, but the outer dimension length of the piezoelectric transformer for that purpose is as long as 34 mm or more. As described above, in a discharge tube driven by a high frequency voltage, the influence of stray capacitance is great, and therefore it is desired to make the frequency as low as possible, for example, 100 KHz or less.

From this point of view, if the ceramic element 2 has the same length dimension, the resonance frequency of the above-mentioned half-wavelength mode piezoelectric transformer element is half that of the full-wavelength mode, which is advantageous. The full-wavelength mode is often used because the piezoelectric transformer is easily fixed and held (two-point support) and the step-up ratio is high. In recent years, new ceramics (199
5) NO. 3, Electronic Ceramics 26 [1
[26] As shown on page 55, a piezoelectric transformer having a ceramic element having a multilayer structure has been proposed. FIG.
5 to 17 show the structure of this laminated piezoelectric transformer. As shown in FIGS. 15 (A) and 15 (B), a very thin ceramic sheet 14 with a thickness of about 100 μm is shown in the left half of the drawing. The internal electrode 16 is formed on the upper surface with silver or the like. In this case, in the sheet shown in FIG. 15A, the elongated insulating region 20A has a narrow width at one end edge portion in the width direction, that is, an upper end edge portion in the drawing, in order to ensure insulation with respect to one external input electrode 18A. Formed, on the other hand, FIG. 15 (B)
The sheet shown in FIG. 2 has a narrow end portion in the drawing with a narrow width to form an elongated insulating region 20B for ensuring insulation with respect to the other external input electrode 18B. I am configuring.

As described above, a plurality of piezoelectric transformer element plates 22A and 22B of two kinds having slightly different electrode positions are formed, and they are alternately arranged as shown in FIG. A piezoelectric transformer body 24 as shown in is formed. Further, a pair of external input electrodes 18A and 18B made of, for example, silver are formed by coating on both end faces in the width direction of the drive portion of the left half of the piezoelectric transformer body 24 in the figure. As a result, the external input electrodes 18A and 18B are different from each other in the internal electrodes 1 of different layers.
6 will be electrically connected. Further, in the power generation section, the output electrode 26 is formed on the end face in the length direction of the transformer body, and a piezoelectric transformer is constituted by this.

According to this structure, the distance between the internal electrodes is
Since it is very small unlike the case shown in FIG. 13, low voltage driving is possible and the boost ratio is greatly improved.

[0009]

By the way, in this laminated piezoelectric transformer, as shown in FIG. 15, for ensuring insulation between the internal electrode 16 and the external input electrode 18A or 18B which is not connected thereto. Insulation area 20A or 2
Although 0B is provided, these regions 20A and 20B are non-polarized regions that are not polarized because the electrodes on the upper and lower surfaces of the ceramic material do not overlap. Therefore, it becomes a non-displacement portion that does not vibrate even when an input voltage is applied between the external input electrodes 18A and 18B, and hinders the vibration of the original displacement portion where the electrodes overlap. Furthermore, the vibration of the original drive unit is not transmitted to the portion 27 of these regions 20A and 20B corresponding to the longitudinal direction of the transformer, that is, the region surrounded by the broken line in the figure, and at the same time the vibration of the entire piezoelectric transformer is reduced. The system becomes complicated. Therefore, there is a problem that power loss occurs due to the non-uniformity of the vibration, and the efficiency decreases. Furthermore, when the piezoelectric transformer is driven, a large internal stress is generated at the boundary between such a non-polarized region that does not vibrate and a polarized region that vibrates, which causes a further problem that cracks occur.

Further, since this laminated piezoelectric transformer is premised to be used in all wavelength mode, its length L, width W and thickness T are 42 mm and 12 m, respectively.
The resonance frequency for the length L = 42 mm is about 75 KHz, which is smaller than 100 KHz, which is the upper limit value of the frequency described above, but the external dimension is 42 mm, which is long. It has become.

Further, as another related art, for example, Japanese Patent Laid-Open No.
As disclosed in JP-A-209579, input electrodes are provided on approximately half of both surfaces of a piezoelectric substrate, output electrodes are provided on the other half, and island electrodes are prepared in the gap between the input side and the output side. There is also a piezoelectric transformer designed to input and output current from this, but this is not a laminated structure piezoelectric transformer,
1 shows a single-plate piezoelectric transformer capable of forming a current input / output structure relatively easily. Therefore, it cannot be directly applied to the structure of a multi-layer piezoelectric transformer which requires a devise in the connection structure between the external input electrode and the internal electrode.

Moreover, the piezoelectric transformer of this publication has a structure in which the input side and the output side are polarized in the thickness direction, which is different from the transformer in which the power generation part is polarized in the longitudinal direction of the transformer. The present invention has been devised in view of the above problems and effectively solving them. An object of the present invention is to reduce the resonance frequency to 100 KHz or less without deteriorating the step-up ratio and durability by devising the shape of the internal electrode and adopting the laminated half-wave mode vibration mode. It is to provide a piezoelectric transformer.

[0013]

Means for Solving the Problems The present inventors have adopted a laminated structure of ceramics and provided the internal electrodes on substantially the entire surface except for the peripheral portion in the vicinity of the external input electrode mounting portion which is not connected. The half-wave vibration mode
The present invention has been achieved by finding that a piezoelectric transformer having a resonance frequency of KHz or less and capable of withstanding practical use can be obtained.

That is, according to the present invention, a plurality of piezoelectric ceramic base plates on which internal electrodes are formed are formed so as to cover a substantially half surface of a thin plate made of an electromechanical conversion material, and the piezoelectric ceramic base plates are laminated. Forming a piezoelectric transformer body, providing a pair of external input electrodes commonly connected to different internal layers of the internal electrodes to form a drive unit, and providing an output electrode at a power generation unit opposite to the drive unit. In the piezoelectric transformer configured as described above, an insulating area having a slight width is formed in the internal electrode only in the vicinity of a mounting portion of the external input electrode that is not connected to the internal electrode, and the insulating area in the length direction of the piezoelectric transformer body is formed. The central portion is supported and fixed so as to serve as a vibration node.

With this structure, the piezoelectric transformer is supported at the central portion in the longitudinal direction and can vibrate in the half-wavelength mode. In this case, since the internal electrode structure is a multi-layer structure, a sufficient boosting ratio can be secured, and the insulating area is formed with a small width only in the vicinity of the external electrode mounting portion. A non-polarized region that is not polarized does not occur in the (driving part), and efficiency and durability can be maintained high. Therefore, it is possible to manufacture a piezoelectric transformer of a half-wavelength mode having a resonance frequency of 100 Kz or less that can be practically used, and the size of the transformer can be reduced.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a piezoelectric transformer according to the present invention will be described in detail below with reference to the accompanying drawings. 1 is a block diagram showing a discharge tube driving device using a piezoelectric transformer according to the present invention, FIG. 2 is a perspective view showing the piezoelectric transformer according to the present invention, and FIG. 3 shows a state in which the piezoelectric transformer shown in FIG. 2 is supported. It is a top view shown.

First, a discharge tube driving device using the piezoelectric transformer of the present invention will be described. As shown in FIG. 1, the driving device 28 is a chopper circuit for stepping up or down the DC voltage from the DC power supply 30 to an arbitrary voltage. The output of the chopper circuit 32 is input to the excitation circuit 34. The excitation circuit 34 applies a voltage that is periodically inverted to the piezoelectric transformer 36 of the present invention. The piezoelectric transformer 36 is composed of a piezoelectric ceramic element as described later, and generates a high voltage on the output side by applying the above voltage. By applying the output of the piezoelectric transformer 36 to the discharge tube 12 such as between cold cathodes, it is ignited and the discharge is continued.

The discharge tube 12 has a tube current detection circuit 38 as a tube current detection section for detecting a current flowing through the discharge tube 12.
Is connected to the piezoelectric transformer 36.
Is input to the error amplifier circuit 40 in order to control the output of the discharge tube 12 and adjust the amount of light emitted from the discharge tube 12.
Amplify the voltage difference between the two. The error voltage from this circuit 40 is input to a pulse width control circuit (PWM) 44 whose output pulse width is changed, for example, using pulse width modulation, and the chopper circuit is output by a control signal output from this control circuit 44. Is controlled to adjust the output voltage V2 of the chopper circuit.

Next, the piezoelectric transformer 3 characterized by the present invention
The structure of No. 6 will be described. As shown in FIGS. 2 and 3, the piezoelectric transformer 36 has a piezoelectric transformer body 46 formed by laminating a plurality of very thin piezoelectric ceramic base plates. The piezoelectric transformer body 46 has a rectangular parallelepiped shape like a thin plate. Has become. Although not explicitly shown in this example, internal electrodes are formed in the left half of the drawing to form a drive unit, and the right half is formed to function as a power generation unit. In this piezoelectric transformer body 46, a pair of external input electrodes 48A, 48B made of silver or the like is formed substantially in the center of both side surfaces in the width direction in the thickness direction.
Is formed, and the output electrode 50 on the entire end face of the transformer body 46 in the longitudinal direction is made of silver or the like.
Are formed.

The pair of external input electrodes 48A,
The output voltage from the excitation circuit 34 is input to 48B, and the high frequency voltage generated by the piezoelectric effect is supplied to the discharge tube 12 from the output electrode 50. In the present invention, in order to vibrate in the half-wavelength mode, the piezoelectric transformer 36 has a central portion in the transformer longitudinal direction, that is, in the illustrated example, a portion provided with the external input electrodes 48A, 48B is provided with a pair of fixing members 52, It is sandwiched by 52 and is supported and fixed by using, for example, an adhesive or a spring mechanism. Therefore, both ends in the longitudinal direction of the transformer become free ends and are in a deformable state, and the central portion is fixed and does not vibrate or deform, thus forming a node. The fixing members 52, 5
If a conductive member is used for 2, the output voltage V2 of the vibration circuit can be input to the piezoelectric transformer via this conductive member. As a result, the lead wires 46, 46 to the external input electrodes 48A, 48B as shown in FIG. 2 and FIG. 3 are not required, and not only the number of steps such as soldering can be reduced, but also
Accidents such as lead wire breaks can be prevented and reliability is improved. Further, the fixing members 52, 52 are external input electrodes 48A, 4
8B is sandwiched between the side portions provided with the internal electrode 56.
The same effect can be obtained by sandwiching (not shown) the central portion of the entire length of the piezoelectric transformer on the formation surface of A and 56B.

Next, a method of forming the piezoelectric transformer 36 will be described. FIG. 4 is a plan view showing the piezoelectric ceramic base plate, FIGS. 5 and 6 are partially enlarged views showing essential parts of the piezoelectric ceramic base plate shown in FIG. 4, and FIG. 7 is a perspective view showing the piezoelectric transformer body. First, as shown in FIG. 4, an electromechanical conversion material,
For example, a rectangular thin plate 54, a so-called green sheet, is formed of a lead zirconate titanate (PZT) ceramic material. Then, the inner electrodes 56A and 56B made of, for example, a silver-palladium alloy are formed on half of the upper surface of the thin plate 54, or substantially on the left half in the figure. To form this thin plate 54, for example, Pb (Zr, Ti) O
_A plate is formed by using a raw material obtained by adding an organic binder, a plasticizer, an organic solvent, etc. to ₃ powders and mixing the powder by, for example, a doctor blade method so that the thickness after firing becomes approximately 100 μm. At this time, the length L1 and the width W1 are each 2
It is set to about 0 mm and 5 mm. Next, this thin plate 54
A paste made of a silver-palladium alloy or a platinum paste is fixed to the surface of the left half of the above by screen printing to form the internal electrodes 56A and 56B. In this case, the internal electrodes 56A and 56B are both formed on the left half of the thin plate surface in the figure, but the external electrode 48 that is not connected is formed.
Insulation areas 58A and 58B having a small width are formed only near the mounting portions of A and 48B to insulate the two. As shown in FIGS. 4 (A) and 4 (B), both insulating areas 58A and 58B are provided at substantially the central portion in the length direction of the transformer and at opposite end portions in the width direction of the transformer. To be

Therefore, as will be described later, both internal electrodes 56
A and 56B are external input electrodes 4 located on opposite sides of each other.
It will be electrically connected to 8B and 48A. The insulating areas 58A and 58B are formed in a minute rectangular shape, but it is preferable that the area is as small as possible in order to minimize the non-polarized region and minimize the non-displacement portion.
Therefore, as shown in FIG. 5, an external input electrode, for example, 48B
The distance D1 between the inner electrode 56A and the internal electrode 56A or the width D1 of the insulating area 58A interposed therebetween can be reduced to a distance at which both can sufficiently secure an insulating state even during driving, for example, about 100 μm. it can. This relationship is the same in the insulating area 58B on the opposite side. This 100 μm corresponds to the thickness of one thin plate 54, and if the thickness of one thin plate 54 is smaller than 100 μm, the width D1 can be further reduced in proportion thereto.

Further, the width D1 is limited to a maximum of 2 mm at the maximum in order to secure mechanical strength, efficiency, etc. to a certain degree, and this value and a gap substantially equal to the plate thickness of the ceramic thin plate 54 described above. For example, it is preferable to set it in the range of 100 μm. In this way, the insulation area 58A,
Although the width of 58B is very small, in FIG. 5, the width is exaggerated to make it easier to understand. In addition, although the shape of the insulating area is a minute rectangular shape in FIG. 5, the shape does not matter as long as the insulating properties of both can be secured and the area is small, and therefore, as shown in FIG. For example, a fan shape having a small diameter D1 around the corner of 48B may be used.

As described above, the surface of the left half of the thin plate 54 is completely covered with the internal electrode 56A except for the minute insulating area.
Alternatively, it is covered with 56B, and the piezoelectric ceramic base plates 60A and 60B are formed. Next, a plurality of piezoelectric ceramic base plates 60A and 60B formed in this manner are prepared, and a plurality of these are alternately laminated, for example, 5 to 20 (see FIG. 16), and after pressure bonding, 1050 to 12
By firing at a high temperature of 00 ° C. for 1 to 5 hours, a piezoelectric transformer body 46 as shown in FIG. 7 is obtained. in this case,
Of course, the internal electrodes are arranged so as to overlap each other in the vertical direction with a ceramic plate interposed therebetween. In the figure, the left half is configured as a drive unit, and the right half is configured as a power generation unit. Further, the end portion of the internal electrode is exposed on the end surface of the piezoelectric transformer body 46 on the drive portion side.

The piezoelectric transformer body 46 has a thickness T1 of five piezoelectric ceramic base plates 60A and 60B each having a thickness of about 100 μm shown in FIGS. 4 (A) and 4 (B).
When 0 sheets are used, the distance is about 1 mm. For the purpose of protecting the internal electrodes, a dummy piezoelectric ceramic base plate having no internal electrodes may be formed on the upper surface, the lower surface, or the upper and lower surfaces of the piezoelectric transformer body. The fixing members 52 and 52 in this case may be installed on the side surface of the element as shown in FIG. 2, but the external input electrodes 48A and 48B are respectively extended around the dummy piezoelectric ceramics base plate (see FIG. (Not shown), this dummy piezoelectric ceramics base plate may be sandwiched. After the piezoelectric transformer body 62 is formed in this manner, a pair of external input electrodes 48A, 48B are formed in the thickness direction on both side surfaces of the center portion of the transformer body 62 in the longitudinal direction, that is, the portions corresponding to the insulating areas. This electrode 48
The width L2 of A and 48B is about 1 to 3 mm. Also,
Also on the power generation section side, the output electrode 50 is formed over the entire longitudinal end surface of the transformer body 62, and the piezoelectric transformer as shown in FIG. 2 is completed. The electrodes can be formed with high precision by screen printing as described above. The outer electrodes 48A and 48B are electrically connected to different one-layered inner electrodes in the half portion in the width direction.

As materials for the external input electrodes 48A, 48B and the output electrode 50, silver-palladium, Ni plating, Ni.Au plating, or the like can be used. In this way, when the piezoelectric transformer is completed, the external input electrode 48
A, 48B and the output electrode 50 to the lead wire 64,
65 is connected, and a DC voltage is applied between the internal electrodes on the drive unit side to perform polarization processing in the stacking direction of the ceramic base plates, that is, in the vertical direction as shown by the arrow in FIG. On the power generation section side, a DC voltage is applied between the internal electrode and the output electrode to perform polarization treatment in the longitudinal direction of the transformer as shown by the arrow in the figure. The conditions for the polarization treatment are 2-3 KV per 1 mm distance at a temperature of 130 ° C.
The DC voltage is applied for 5 minutes for about 5 minutes. Therefore, a DC voltage of about 300 V is applied between the internal electrodes having a distance between the electrodes of about 100 μm to perform polarization processing, while the length of the power generation unit is about 10 mm, so the internal electrode-output electrode A DC voltage of 30 KV is applied between them. In this way, when the polarization process is completed,
As shown in FIG. 3, the central portion in the longitudinal direction of the transformer, that is, the portion where the external input electrodes 48A and 48B are provided,
The pair of fixing members 52, 52 are supported and fixed by using an adhesive or the like so as to be sandwiched. Therefore, both ends in the longitudinal direction of the transformer become free ends and become deformable, and the central portion becomes a node that is fixed by the fixing members 52, 52 and is not vibrated or deformed, and it is possible to vibrate in the half-wavelength mode. Become. In the present embodiment, the polarization treatment
The lead wires 64 and 65 were respectively brought into contact with the external input electrodes 48A and 48B and the output electrode 50 to perform polarization processing.
Connect the conductive member to the external input electrode 48 without connecting the lead wire.
It is also possible to press a spring member (not shown) on the A and 48B and the output electrode 50 to polarize the drive unit and the power generation unit, respectively.
If a conductive member is used as the conductive member and the polarization process is performed via this fixing member, the number of steps can be further reduced.

Next, the operation of the present embodiment configured as described above will be described. First, the operation of the entire drive circuit will be described with reference to FIG. 1. The DC voltage from the DC power supply 30 is applied to the pulse width modulation circuit 4 by the chopper circuit 32.
A DC signal V2 is output by being stepped up or stepped down by a control signal from 4 and this voltage is input to the excitation circuit 34.
The excitation circuit 34 applies a voltage V3 that is periodically inverted at a frequency close to the natural frequency to the piezoelectric transformer 36 according to the present invention, and causes the piezoelectric transformer 36 to expand and contract.
Then, in the piezoelectric transformer 36, a high AC voltage is generated due to the piezoelectric effect, and this voltage is supplied to the discharge tube 12 to ignite it and continue the discharge.

The tube current of the discharge tube at this time is detected by the tube current detection circuit 38, and the detected voltage is the error amplification circuit 40.
Is compared with the reference voltage 42 and an error voltage is output. Based on this error voltage, the pulse width control circuit 44 performs pulse width modulation to form a control signal and adjust the light emission amount.

When the output voltage V3 from the excitation circuit 34 is applied to the external input electrodes 48A and 48B of the piezoelectric transformer 36 through the lead wires 64 and 64, for example, the operation of the piezoelectric transformer 36 will be described in more detail. Since the center portion of the longitudinal direction of 36 is fixed by the fixing member 52, the center portion does not vibrate, but both end sides thereof vibrate or are displaced, which is a so-called half-wavelength vibration mode. That is, when an AC voltage close to this natural frequency is applied between the internal electrodes 56A and 56B on the drive section side of the transformer, the output electrodes 50 on the power generation section side resonate due to the electromechanical conversion action, and a high output voltage having a predetermined frequency. Is obtained.

Here, in the present invention, as shown in FIG. 4, one of the areas where the internal electrodes are not formed on the drive unit side is the external input electrode 48A which is not connected,
Alternatively, it is only a portion of the insulating area 58A or 58B having a very small area for ensuring the insulating property with respect to 48B. As described above, since the area where the internal electrodes are not formed is only a small area around the mounting portion for the external input voltage 48A or 48B, this area is the only area where the polarization process is not sufficiently performed. Since the displacement portion is extremely small, the piezoelectric transformer deforms and vibrates uniformly as a whole. Therefore, it is possible to significantly suppress the occurrence of cracks and the peeling of the internal electrodes and improve the durability and the like.

With respect to this point, in the conventional transformer, as shown in FIG.
0B (non-formed area) is long and narrow, and its area is quite wide.
Strain is generated at the boundary between the displaced portion and the non-displaced portion, the internal stress is concentrated, which has been a cause of cracks and the like, but in the present invention, the internal electrode non-formed area is extremely large as described above. Since the amount is small, the deformation (vibration) of the piezoelectric transformer becomes uniform, so that not only the power efficiency is improved, but also the mechanical strength and durability are not deteriorated. Especially,
Since the external input electrodes 48A and 48B are provided in the central portion in the longitudinal direction of the transformer, that is, in the non-displacement portion to which the fixing member 52 is attached, peeling of these electrodes can be suppressed, and durability and the like can be further improved. .

Further, in the conventional transformer, the non-displacement portion acts so as to inhibit the vibration of the displacement portion, which causes the inferior efficiency. However, in the case of the present invention, as described above. Moreover, the non-displacement portion has a very small area, and the vibration of the displacement portion is hardly suppressed by the non-displacement portion, and the efficiency can be maintained high. Further, in the present invention, since the transformer is supported and fixed at the central portion in the longitudinal direction to vibrate in the half-wavelength mode, a relatively low resonance frequency is obtained even when the length dimension of the transformer is suppressed. be able to. For example, in the current drive circuit of the discharge tube 12, in order to suppress high frequency leakage and maintain high power efficiency, the drive frequency is set to 100K.
It is necessary to suppress below Hz, but the length L of the transformer
1, width W1 and thickness T1 are 20 mm, 5 mm and 1 respectively.
As a result of vibrating in a half-wavelength mode as in the present invention by setting to ~ 2 mm, a resonance frequency of about 80 KHz can be obtained, the transformer can be downsized, and high frequency leakage can be suppressed to improve power efficiency (98%). I was able to raise it. Especially, regarding the length dimension of the transformer, the dimension of the conventional transformer is 42
However, in the case of the transformer of the present invention, it is 2 mm.
It can be reduced to 0 mm, and the resonance frequency is 100 KH.
I was able to hold it below z.

Further, although the step-up ratio tends to be inferior by using the half-wave mode, the step-up ratio (40 to 100 times) is practically sufficient even if the input voltage is comparatively small by the laminated structure of the internal electrodes. Can be obtained. The dimensions shown in the above embodiments are merely examples, and the number of laminated piezoelectric ceramic base plates is not limited to 10, and may be increased or decreased as necessary. Since one sheet is about 100 μm, the thickness of the transformer is not significantly affected even if the number of sheets is increased.

In the above embodiment, the external input electrode 48
The connection state of A and 48B and the internal electrodes 56A and 56B is connected to the internal electrodes in a half area in the width direction of the external input electrodes, but the electrical connection distance between them is increased to increase the reliability of the connection between them. In order to improve the property, it is preferable to extend the connecting line between them, and for that purpose, the constitution may be made as shown in FIG. As shown in the illustrated example, at the end of the internal electrode on the side connected to the external input electrode, that is, at the position opposite to the transformer width direction with respect to the position where the insulating area 58A or 58B is formed, the transformer longitudinal direction (power generation) is generated. Internal auxiliary electrode 66 in the shape of a rectangle extending a little length
Are formed of the same material as the internal electrodes. By connecting the internal auxiliary electrode 66 also to the external input electrode 48A or 48B, the distance between the two connecting portions can be increased, and the reliability can be increased accordingly. In this case, in order to maintain the polarization uniformity of the power generation unit, the length L3 of the internal auxiliary electrode 66 is set to a length that can cover the entire length of the external input electrode in the width direction, and is made as short as possible. Good.

In the above embodiment, the external input electrode 48
The mounting positions of A and 48B were set at the node with the least amount of vibration, that is, at the center in the longitudinal direction of the transformer.
The present invention is not limited to this, and for example, it may be provided at the end portion of the driving portion as shown in FIGS. 9A and 9B, or FIGS. As shown in B), it may be provided on one end face in the longitudinal direction of the drive unit. Also in this case, the insulating areas 58A and 58B are formed only in the vicinity of the mounting portions of the external input electrodes 48A and 48B, and the other portions are formed by internal electrodes to suppress the generation of internal strain or internal stress as much as possible. To do. Furthermore,
The supporting and fixing position of the transformer is the central portion in the longitudinal direction of the transformer in the same manner as in the case of FIG. 3 described above in order to cause the half-wavelength mode vibration as shown in FIG. 10 and FIG. Are fixed and supported by the fixing member 52. still,
Also in the embodiment shown in FIGS. 8 to 12, if a conductive member is used as the fixing member and the element is sandwiched and also used as an electrode connection, the lead wire 64 becomes unnecessary, as in the previous embodiment. In this case, since the external input electrodes 48A and 48B are installed in a portion having a large vibration amplitude, the Ni plating film and Cr-Au are used.
It is desirable that the vapor-deposited film be as thin as possible.

In each of the above embodiments, the case where the piezoelectric transformer of the present invention is used in the drive power supply circuit of the backlight discharge tube of the liquid crystal display device has been described, but the present invention is not limited to this.
It can be applied to a small-sized power supply circuit requiring a high voltage, for example, a power supply circuit for a high-pressure ozone generator, a power supply circuit for adhering toner in a copying machine, and the like.

[0037]

As described above, according to the piezoelectric transformer of the present invention, the following excellent operational effects can be exhibited. The entire structure has a laminated structure, and the insulation area between the internal electrode and the external input electrode that is not connected is extremely small and the half-wavelength vibration mode is used. It is possible to obtain an output voltage of a relatively low frequency in a state in which is shortened. Further, in the drive section, the insulating area is provided only in the vicinity of the attachment section of the external input electrode, and the internal electrode is formed entirely on the other sections. Therefore, not only can the cracking and the like be suppressed and the durability can be improved, but also the vibration of the displacement portion is not disturbed so much, so that the efficiency can be increased accordingly.

Particularly, the durability and efficiency can be further improved by matching the mounting position of the external input electrode with the supporting and fixing position of the transformer, which serves as a vibration node. Furthermore, since miniaturization can be achieved, it can contribute to cost reduction, and since an output of 100 KHz or less can be obtained in the miniaturized state, it is applied to, for example, a drive circuit of a backlight discharge tube of a liquid crystal display device. By doing so, high frequency leakage can be suppressed and efficiency can be significantly improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a discharge tube driving device using a piezoelectric transformer according to the present invention.

FIG. 2 is a perspective view showing a piezoelectric transformer according to the present invention.

3 is a plan view showing a state in which the piezoelectric transformer shown in FIG. 2 is supported.

FIG. 4 is a plan view showing a piezoelectric ceramic base plate.

5 is a partially enlarged view showing a main part of the piezoelectric ceramic base plate shown in FIG.

6 is a partially enlarged view showing a modified example of the main part of the piezoelectric ceramic base plate shown in FIG.

FIG. 7 is a perspective view showing a piezoelectric transformer body.

FIG. 8 is a plan view showing a modified example of the piezoelectric ceramic base plate.

FIG. 9 is a plan view showing another modification of the piezoelectric ceramic base plate.

10 is a plan view showing a piezoelectric transformer using the piezoelectric ceramic base plate shown in FIG.

FIG. 11 is a plan view showing still another modification of the piezoelectric ceramic base plate.

12 is a plan view showing a piezoelectric transformer using the piezoelectric ceramic base plate shown in FIG.

FIG. 13 is a principle diagram showing a driving principle of a piezoelectric transformer.

FIG. 14 is a diagram showing a vibration mode of a piezoelectric transformer.

FIG. 15 is a plan view showing internal electrodes of a conventional piezoelectric transformer.

FIG. 16 is a diagram showing a laminated state of piezoelectric ceramic sheets.

FIG. 17 is a perspective view showing a conventional piezoelectric transformer.

[Explanation of symbols]

 28 Discharge tube drive circuit 30 DC power supply 32 Chopper circuit 34 Excitation circuit 36 Piezoelectric transformer 44 Pulse width control circuit 46 Piezoelectric transformer body 48A, 48B External input electrode 50 Output electrode 52 Fixing member 54 Thin plate 56A, 56B Internal electrode 58A, 58B Insulation area 60A, 60B Piezoelectric ceramic plate 64, 65 Lead wire 66 Internal auxiliary electrode

Claims (4)

[Claims] 1. A piezoelectric transformer body in which a plurality of piezoelectric ceramic base plates on which internal electrodes are formed are formed so as to cover a substantially half surface of a thin plate made of an electromechanical conversion material, and the piezoelectric ceramic base plates are laminated. A piezoelectric transformer having a pair of external input electrodes commonly connected to the internal electrodes of different layers placed to form a drive section, and an output electrode provided on the power generation section on the opposite side of the drive section. In the above, in the internal electrode, an insulating area having a slight width is formed only in the vicinity of the attachment portion of the external input electrode which is not connected to the internal electrode, and the substantially central portion in the length direction of the piezoelectric transformer body is formed. A piezoelectric transformer characterized by being supported and fixed so that it becomes a node of vibration. 2. The piezoelectric transformer according to claim 1, wherein the internal electrode is formed on the front surface of the thin plate except for the insulating area on the drive unit side. 3. The width of the insulating area formed between the non-contact external input electrode and the internal electrode is not less than the thickness of the piezoelectric ceramic base plate.
Alternatively, the piezoelectric transformer described in 2. 4. The piezoelectric transformer according to claim 1, wherein the external input electrode is provided at a supporting and fixing position which is a substantially central portion in the length direction of the piezoelectric transformer body.