JP2004031382A - Piezo-electric transformer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To save cost of inverter and improve characteristic thereof by simultaneously lighting many cold cathode lamps with only one piezoelectric transformer without use of a ballast capacitor. <P>SOLUTION: The piezoelectric transformer is provided with input electrodes formed on the front and rear surfaces of a piezoelectric ceramic plate, and output electrodes formed at the end surface not in contact with the input electrodes. In this piezoelectric transformer, the piezoelectric ceramic plate is formed in the shape of cross, the input and output electrodes are formed on the front and rear surfaces of the crossing portion, and the output electrodes are formed at the respective front ends of four projected portions. Independent outputs can be obtained from the four output ends, and the four cold cathode lamps can be lighted with respective outputs. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a structure of a piezoelectric transformer, and more particularly to a multi-output piezoelectric transformer using a contour vibration (plane spreading vibration) mode.
[0002]
[Prior art]
The functional element utilizing the natural resonance of the piezoelectric ceramic has features that it has a small loss, can be miniaturized, has a large electromechanical coupling coefficient, and has excellent temperature characteristics and aging characteristics. Piezoelectric transformers, which are a type of such functional elements, utilize the vibration of elastic bodies, so their stored energy is large compared to electromagnetic types, so they can be miniaturized; they are nonflammable and excellent in safety; The inverter using a piezoelectric transformer is widely used as a power source for a cold cathode lamp.
[0003]
FIG. 5 is a perspective view showing an example of a conventional piezoelectric transformer. Input electrodes 52 and 53 are formed by forming conductor films on the front and back surfaces of one side half of a piezoelectric ceramic plate 51 having a square main surface. This portion serves as a driving portion of the piezoelectric transformer, and the opposite half serves as a power generation portion. A conductor film is formed on an end face of the power generation portion to serve as an output electrode 54.
[0004]
Such a piezoelectric transformer has been widely used in the field of a single lamp such as a small liquid crystal monitor. In the case of a large-sized liquid crystal monitor such as a liquid crystal television or a personal computer display, a large number of cold cathode lamps are used, so that a conventional lighting method in which one inverter is arranged for one lamp is not desirable in terms of cost.
[0005]
On the other hand, when a large number of lamps are connected in parallel to one piezoelectric transformer, if one of the lamps is turned on first, the impedance of the tube is greatly reduced. Then, since the output voltage of the piezoelectric transformer is greatly reduced, other lamps cannot be turned on. As a solution to this problem, a ballast capacitor 67 is connected in series with each lamp 66 as shown in FIG. As a result, even if one lamp is turned on first, a high output voltage can be secured at the output section of the piezoelectric transformer, and a large number of lamps can be turned on simultaneously by one inverter.
[0006]
However, such a lighting method has the following problems.
{Circle around (1)} Since a high voltage that is constantly lit is required, it is not preferable for safety.
(2) A piezoelectric transformer having a higher step-up ratio is required.
{Circle around (3)} The high voltage ballast capacitor is expensive and causes a problem of cost.
(4) The stability of the tube current is poor, and the variation is large.
(5) Ballast capacitors cause power loss.
Therefore, if the piezoelectric transformer has a large number of independent output terminals, a large number of lamps can be simultaneously turned on with a single piezoelectric transformer without a ballast capacitor, which is advantageous in terms of cost reduction and improved characteristics.
[0007]
[Problems to be solved by the invention]
The present invention makes it possible to simultaneously turn on a large number of cold cathode lamps with one piezoelectric transformer without using a ballast capacitor, thereby reducing the cost of an inverter using a piezoelectric transformer and its characteristics. Is to improve.
[0008]
[Means for Solving the Problems]
The present invention solves the above-mentioned problem by forming a piezoelectric ceramic plate into a shape capable of forming one input electrode and a plurality of output electrodes.
[0009]
That is, in a piezoelectric transformer having an input electrode on the front and back surfaces of a piezoelectric ceramic plate and an output electrode on an end surface not in contact with the input electrode, the piezoelectric ceramic plate has a cross shape and input and output electrodes on the front and back surfaces of the intersection. The present invention is characterized in that an output electrode is provided at each tip of the two projecting portions.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
In the piezoelectric transformer according to the present invention, the piezoelectric ceramic plate has a cross shape, and an input electrode is formed at the center. Output electrodes are formed at the tips of the four protrusions of the cross, and outputs are individually obtained from these four output electrodes, and the outputs turn on a plurality of cold cathode lamps. In order to increase the step-up ratio, the input electrode portion may have a laminated structure.
[0011]
【Example】
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing a piezoelectric transformer according to the present invention, in which a cross-shaped piezoelectric ceramic plate 11 is used as a piezoelectric ceramic plate. Conductive films are formed on the front and back surfaces of the center crossing portion of the piezoelectric ceramic plate 11 to form input electrodes 12 and 13, and this portion serves as a driving unit of the piezoelectric transformer.
[0012]
The four arm portions extending outward from the driving portion serve as the power generation portion, and a conductor film is formed on each of the ends to form the output electrode 14. The drive unit was polarized in the thickness direction, and the power generation unit was polarized in the length direction from the input electrode to the output electrode.
[0013]
2 (A) and 2 (B) show displacement diagrams of the primary and secondary modes of surface spreading vibration (contour vibration) of the piezoelectric transformer according to the present invention, respectively. In the primary vibration mode, the vibration node (displacement = 0) is at the center similarly to the radial vibration of the disk, and the main surface vibrates so as to shrink or expand simultaneously in all ranges. In the case of the secondary vibration mode, since the end of the output section and the inside of the element vibrate in opposite phases, there are four vibration nodes at the output section in addition to the center of the element (N in FIG. 2B). point). This is similar to the second-order spread vibration mode of a square plate. Therefore, there is an advantage that the secondary mode is easier to support than the primary mode.
[0014]
The sample of the piezoelectric transformer according to the present invention was obtained by press-molding a PZT-based piezoelectric ceramic having a high Qm of about 2,000 and having a shape as shown in FIG. The thickness was 1.5 mm, the interval between the two opposing output electrodes was 45 mm, and the width of the driving section was 20 mm. Here, in order to adjust the electromechanical coupling coefficient and move the spurious out of the operating band, an inclination angle of 10 ° was taken toward the output section.
[0015]
FIGS. 3 and 4 show the characteristics when operating in the primary and secondary modes, respectively, and show the impedance characteristics of the input section when the output terminal is open, the output terminal is short-circuited, and the load is 130 kΩ per output. Since a large coupling coefficient can be obtained in both modes, any mode can be used as a piezoelectric transformer. The input capacitance C1 of this sample = 3500 pF, the output capacitance per output terminal C2 = 20 pF, the operating frequency in the primary mode is about 50 kHz, the matching impedance per output is about 160 kΩ, and the operating frequency in the secondary mode Is about 110 kHz, and the matching impedance per output is about 70 kΩ. It is possible to select and drive two operating frequencies depending on the arrangement of the cold cathode lamps, the stray capacitance, and the like.
[0016]
In the lighting test using the piezoelectric transformer according to the present invention, a long cold cathode lamp having a length of φ = 2.6 mm and a length of 320 mm was used, and each of the four lamps was connected to four output electrodes without a ballast capacitor. In the case of using the secondary mode, four lamps could be simultaneously turned on by fixing the input voltage of 150 Vp-p or more and sweeping the frequency from 135 kHz to 110 kHz.
[0017]
In spite of the simultaneous lighting described above, there is a slight difference in the starting voltage or frequency at the time of lighting of each lamp due to the variation in tube impedance actually. In the above example, a time difference of 1-2 ms is expected at a sweep speed of 20 ms. Once one lamp is turned on first, the load causes damping to the piezoelectric transformer, so that the open-circuit voltage of the output unit connected to the lamp that has not been turned on becomes low. Therefore, the open-circuit voltage of each output unit gradually decreases in the order of lighting, and when the lamp that is lit last turns on, the open-circuit voltage of the output unit connected to it is the lowest. Nevertheless, in the piezoelectric transformer according to the present invention, an open circuit voltage for lighting all the lamps was secured. When the above sample is constantly lit, when the output is about 3.5 W per output, the heat generation of the element is as low as about 10 ° C.
[0018]
The piezoelectric transformer according to the present invention can be applied not only to a single plate but also to a laminated type. When the input portion has a laminated structure, not only the boost ratio can be increased, but also the torsional vibration that easily occurs can be completely suppressed.
[0019]
【The invention's effect】
According to the present invention, four cold cathode lamps can be turned on without a ballast capacitor by a piezoelectric transformer having four output terminals, and the following effects can be obtained.
{Circle around (1)} An expensive ballast capacitor is not required, and cost can be reduced.
{Circle around (2)} It is not necessary to maintain a high voltage at the time of constant lighting, safety is improved, and the demand for the boost ratio of the transformer is reduced.
(3) The power loss due to the ballast capacitor is eliminated, and the power efficiency is improved.
(4) Variation in tube current is reduced.
(5) The operating frequency can be selected according to the arrangement of the cold cathode lamp, the stray capacitance, and the like.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an embodiment of the present invention. FIG. 2 is an explanatory view of a displacement state. FIG. 3 is an explanatory view of impedance characteristics of a piezoelectric transformer according to the present invention. FIG. FIG. 5 is a perspective view of a conventional piezoelectric transformer. FIG. 6 is an explanatory view showing a conventional lighting method.
11, 51: piezoelectric ceramic plates 12, 13, 52, 53: input electrode 14, 54: output electrode

Claims (3)

In a piezoelectric transformer having an input electrode on the front and back surfaces of a piezoelectric ceramic plate and an output electrode on an end surface not in contact with the input electrode,
The piezoelectric ceramic plate is cross-shaped, with input and output electrodes on the front and back of the intersection,
A piezoelectric transformer comprising an output electrode at each tip of each of the four projecting portions. 2. The piezoelectric transformer according to claim 1, which operates in a spreading vibration mode. 2. The piezoelectric transformer according to claim 1, wherein the input electrode portion has a laminated structure.

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