JP2005175244A - Piezoelectric transformer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To optimize an electrode structure and to improve electrical characteristics to reduce problems caused by a shape of an electrode. <P>SOLUTION: A piezoelectric transformer includes an input electrode on a front and a rear surfaces with a lengthwise end side of a piezoelectric ceramic sheet of which main surface is rectangular as a driving portion, and an output electrode on an end face with an opposite side as an output portion. The main input electrode is formed with a length almost half of the piezoelectric ceramic sheet, and a widthwise central portion is formed so that it may curve and protrude towards an output electrode side. Moreover, an auxiliary input electrode is formed which is confronted with the main input electrode on a main surface at a certain space, and which is driven by a reverse phase to the main input electrode. A main driving electrode and an auxiliary driving electrode are driven by the reverse phase. The reverse phase driving is obtained either by connecting to power supply so that the phase may be reversed but with the same polarization direction or by driving through polarization in an opposite direction but at the same phase. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a structure of a piezoelectric transformer suitable for being used as a lighting power source for a cold cathode fluorescent lamp, and aims to improve the characteristics thereof.

  Piezoelectric transformers use vibrations of elastic bodies, so their stored energy is large compared to electromagnetic types, nonflammability and superior safety, and magnetic shields are unnecessary. . Inverters using piezoelectric transformers are widely applied as power supplies for cold cathode lamps for liquid crystal display panels.

  A so-called Rosen type that uses a vibration mode in the longitudinal direction has been the mainstream of conventionally used piezoelectric transformers. This type has a problem that the step-up ratio in the load state is low and the resonance impedance is large, so that high output cannot be expected. On the other hand, if a spread-surface vibration mode that couples vibrations in the length and width directions can be used, a piezoelectric transformer with high power, high efficiency, and high step-up ratio can be realized because the electromechanical coupling coefficient and operation Q are larger than the Rosen type. it can.

A piezoelectric transformer using secondary surface spreading vibration as shown in FIG. 5 has already been realized. This uses a piezoelectric ceramic plate (1) having a ratio of length L to width W of about twice, and the driving electrodes (2, 3) are placed on the front and back surfaces of the half section in the opposing length direction. With the structure where the output electrode (4) is formed on the end face. As shown in FIG. 5, the vibration of this rectangular plate is that the area of the other power generation section (output section) shrinks when the area of the peristaltic side section widens, and then the power generation section spreads when the drive section shrinks. Alternating spreading vibration is caused. Such alternating spreading vibration is called a diagonally symmetric plane spreading vibration mode.

  As can be seen from the features of the obliquely symmetric surface spreading vibration mode, there are two vibration nodes (points where the displacement is 0) at the center of the drive unit and the center of the output unit, and vibration occurs between the drive unit and the output unit. There is a boundary line that reverses the phase. However, since the polarization direction and the electrode structure of the drive unit and the output unit are different, it has become clear from the FEM analysis that the inversion boundary line of the vibration phase is not a straight line but is bent to some extent. The greater the width W with respect to the element length L, the greater the degree of bending. In the case of the Rosen type using the vibration mode in the longitudinal direction, the L / W ratio is 4 times or more, whereas in the oblique symmetry mode in which the L / W ratio is about 2 times, the bending of the boundary line becomes remarkable. . In the conventional obliquely symmetric mode piezoelectric transformer in which the boundary between the drive unit and the output unit is a straight line, a part of the drive unit electrode slightly enters the antiphase range, which causes power loss and temperature rise.

Japanese Patent No. 3349046 Japanese Patent No. 2850216

  The present invention optimizes the electrode structure and improves the electrical characteristics in order to reduce the problems caused by the electrode shape as described above.

  This invention solves said subject by providing a peristaltic electrode according to a vibration phase inversion boundary. That is, in a piezoelectric transformer having an input electrode on the front and back surfaces on one end side in the length direction of a piezoelectric ceramic plate having a rectangular main surface and an output electrode on the opposite end surface, the input electrode is approximately the length of the piezoelectric ceramic plate. It is characterized in that it is formed with a half length and is formed so that the central portion in the width direction is curved and protrudes toward the output electrode.

  According to the present invention, in the piezoelectric transformer using the obliquely symmetric surface spreading vibration mode, the peristaltic electrode is provided in accordance with the position of the vibration phase inversion boundary, so that the power efficiency is improved and the heat generation can be suppressed to be small. Further, by providing the auxiliary electrode, the output portion can be easily polarized, and the length of the output portion can be arbitrarily adjusted.

A piezoelectric transformer with a piezoelectric ceramic plate with a rectangular main surface with one end in the length direction as the drive unit and input electrodes on the front and back surfaces, and an output electrode on the other end as the output unit. The main input electrode is a piezoelectric ceramic. The plate is formed so as to be approximately half the length of the plate, and the central portion in the width direction is formed to be curved and protrude toward the output electrode. In addition, an auxiliary input electrode that is driven in a phase opposite to that of the main input electrode is formed so as to face the main input electrode at a certain interval on the main surface. The main peristaltic electrode and the auxiliary drive electrode are driven in opposite phases. In order to drive in the opposite phase, either the polarization direction is the same and the power source is connected so as to be in the opposite phase, or it is polarized in the opposite direction and driven in the same phase.

  As described above, it cannot be said that the drive part region of the conventional piezoelectric transformer in the obliquely symmetrical plane spreading vibration mode vibrates completely in the same phase. That is, when the drive unit swells, the drive unit is partly contracted. Therefore, there is a possibility that the power of this part is lost. Power efficiency can be optimized if the peristaltic electrode is provided in alignment with the vibration phase inversion boundary.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a perspective view of a piezoelectric transformer according to the present invention. The input peristaltic electrodes 12 and 13 are provided on the front and back surfaces of the piezoelectric ceramic substrate 11 in about half the length direction, and the output electrode 14 is provided on the opposite end surface. In the piezoelectric ceramic substrate 11, the input electrode portion is polarized in the thickness direction, and the output-side power generation unit is polarized in the length direction. The conductor patterns of the input peristaltic electrodes 12 and 13 are formed so as to coincide with the vibration phase inversion boundary. In this case, since all the drive unit (or output unit) regions vibrate at the same phase, waste of power can be minimized. The actual vibration phase inversion boundary can be obtained by the finite element method. The theoretical vibration phase inversion boundary line can be obtained by connecting the points where the displacement in the length direction is maximum in the vicinity of each point on the center axis in the width direction of the element.

  FIG. 2 shows examples of various characteristics in the operating frequency range when the output current of the piezoelectric transformer according to the present invention is constant. When the peristaltic electrode is curved along with the vibration phase reversal boundary, it indicates that the efficiency and heat generation characteristics of the piezoelectric transformer are improved compared to the conventional electrode structure (linear boundary).

  FIG. 3 shows a perspective view of an obliquely symmetric surface-spread vibration mode piezoelectric transformer with an auxiliary peristaltic electrode. The main peristaltic electrodes 32 and 33 are formed on the front and back surfaces of the half-section of the piezoelectric ceramic substrate 31 whose main surface is rectangular in the length direction, and the auxiliary peristaltic electrodes 36 and 37 are formed in the vicinity of the main peristaltic electrode. The length of the auxiliary peristaltic electrode is L2. An output electrode 34 is formed on the opposite end face, and the length of the power generation unit is L3. The main peristaltic electrode and the auxiliary peristaltic electrode are polarized in the thickness direction, and the power generation part is polarized in the length direction. Since the main peristaltic electrode and the auxiliary peristaltic electrode are in the region of antiphase vibration, it is necessary to peristate in the opposite phase. For this purpose, as shown in FIG. 3, the electrodes are polarized in the opposite direction or the polarization directions are the same, and the input electrodes are connected to the electrodes 33 and 36 on the same side and the electrodes 32 and 37 are connected to the other side. You may make it do. By providing the auxiliary peristaltic electrode, the length of the output portion becomes uniform, so that polarization can be facilitated. Further, the length L3 of the output unit can be adjusted if adjustment of the coupling coefficient or the like is necessary.

  Even if the auxiliary peristaltic electrode is provided, the vibration mode is the same as the conventional oblique symmetry mode structure. That is, when the area of the main peristaltic electrode section L1 expands, the areas of the auxiliary peristaltic electrode section L2 and the output section L3 contract, and then when the main peristaltic electrode section contracts, the other expands. Yes.

In the conventional oblique symmetry mode as shown in FIG. 5, the ratio of the length L to the width W is optimally about twice. However, when the auxiliary electrode is provided, the coupling coefficient related to the step-up ratio and the spurious around the main vibration are obtained. Since the state changes greatly, the optimum dimension range must also be changed. FIG. 4 is an example of the impedance characteristics of the input portion of the piezoelectric transformer when L / W = 1.5. As can be seen, the main vibration is sandwiched between the spurious (SP1) on the low frequency side and the spurious (SP2) on the high frequency side. When the ratio (L / W) between the length L and the width W of the element decreases, SP1 approaches, but when L / W increases, SP2 approaches. From the experimental results, the range of L / W ratio in which the piezoelectric transformer can operate is 1.3 to 1.7. Also, if the output portion length L3 is too short (or the auxiliary electrode length L2 is too long), the output side mechano-electric coupling coefficient will be small, the conversion efficiency will deteriorate, and heat will be generated. The range is 0.25 to 0.45.

  Conventional piezoelectric transformers having auxiliary peristaltic electrodes are known. However, since it is a simple secondary longitudinal vibration mode in the longitudinal direction, the width of the element is very narrow, and the expansion / contraction boundary may be regarded as a straight line. In addition, in the case of simple longitudinal vibration, there is little spurious and any dimension is possible in the range of L / W = 4.5 to 7.0. However, the piezoelectric transformer according to the present invention using the combined vibration in the length direction and the width direction has no spurious. There are many, and the range of practical dimensions is very narrow.

  The piezoelectric transformer according to the present invention can improve heat efficiency and suppress heat generation, and is optimal for an inverter such as a liquid crystal display device.

The perspective view which shows the Example of this invention Illustration of its characteristics The perspective view which shows the other Example of this invention. Illustration of its impedance characteristics A perspective view showing a conventional piezoelectric transformer

Explanation of symbols

1, 11, 31: Piezoelectric ceramic substrate 2.3, 12, 13: Input (peristaltic) electrodes
32, 33: Main input (peristaltic) electrodes
36, 37: Auxiliary input (peristaltic) electrodes 4, 14, 34: Output electrodes

Claims (3)

In a piezoelectric transformer having an input electrode on the front and back surfaces on one end side in the length direction of the piezoelectric ceramic plate having a rectangular main surface, and an output electrode on the opposite end surface,
The input electrode is formed approximately half the length of the piezoelectric ceramic plate,
A piezoelectric transformer, characterized in that a central portion in the width direction is formed to be curved and protrude toward the output electrode side. In a piezoelectric transformer comprising a piezoelectric ceramic plate having a rectangular main surface with one end in the length direction as a drive unit and input electrodes on the front and back surfaces, and an output electrode on the other end as an output unit,
The drive part is polarized in the thickness direction of the piezoelectric ceramic plate, the output part is polarized in the length direction of the piezoelectric ceramic plate,
The input electrode is formed approximately half the length of the piezoelectric ceramic plate, and operates in an obliquely symmetric surface-spread vibration mode in which vibrations in the length direction and the width direction are combined.
A piezoelectric transformer, characterized in that a central portion in the width direction is formed to be curved and protrude toward the output electrode side. In a piezoelectric transformer having an input electrode on the front and back surfaces on one end side in the length direction of the piezoelectric ceramic plate having a rectangular main surface, and an output electrode on the opposite end surface,
The main input electrode is formed to be approximately half the length of the piezoelectric ceramic plate, and the center part in the width direction is formed to be curved and protrude toward the output electrode side,
A piezoelectric transformer characterized in that an auxiliary input electrode that is driven in a phase opposite to that of the main input electrode is formed so as to be opposed to the main input electrode at a predetermined interval on the main surface.

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