JP2000294850A - Piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a small-sized, high-efficiency piezoelectric transformer having a high boosting ratio specifying the ratio of the resonance resistance of a generating part to the resonance resistance of a driving part. SOLUTION: A piezoelectric transformer has construction provided with input electrodes 3, 4 for applying voltage to a lead zirconate titanate family piezoelectric ceramic 1 for example, and an output electrode 5 for taking power out. The input electrodes 3, 4 are formed in a driving part where vibration by an inverse piezoelectric effect of the piezoelectric ceramic 1 is excited, and the output electrode is formed in the generating part where vibration is taken out as power by a normal piezoelectric effect. In the piezoelectric transformer, the ratio Rg/Rd of the resonance resistance Rg of the generating part to the resonance frequency Rd of the driving part is determined to be 30 or less. If the ratio exceeds 30, the efficiency lowers and becomes lower than 80%, and a calorific value on the occasion of driving the piezoelectric transformer increases, and operation of an inverter for liquid crystal back light becomes unstable. Accordingly, a piezoelectric transformer small-sized, capable of producing large output power, and high-efficiency is obtained, if the ratio Rg/Rd<30 is satisfied.

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.

[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 alternating high voltage of several hundred volts or more is usually required, depending on the length and diameter of the discharge tube used.

[0003] There is a Rosen type piezoelectric transformer as a typical conventional piezoelectric transformer for generating a high voltage. FIG. 1 shows the structure. 1 in the figure is, for example, lead zirconate titanate (PZ
T) piezoelectric ceramics. Input electrodes 3 and 4 formed by, for example, a silver printing method are formed on the upper and lower surfaces of a left half in the drawing of the piezoelectric transformer, and an output electrode 5 is formed on the right half by the same method. Then, the left half driving section of the piezoelectric transformer performs polarization processing in the thickness direction, and the right half power generation section performs the polarization processing in the length direction as indicated by arrows.

When an AC voltage having a frequency substantially equal to the longitudinal resonance frequency of the piezoelectric ceramic 1 is applied between the input electrodes 3 and 4 of the piezoelectric transformer, the piezoelectric ceramic 1 generates strong mechanical vibration in the longitudinal direction. As a result, electric charges are generated by the piezoelectric effect in the right half of the power generation section, and the output electrode 5
An output voltage Vo is generated between the input electrode and one of the input electrodes, for example, the input electrode 4.

[0005] The step-up ratio (Vo / Vi) (where Vi is an input voltage) at no load obtained by the piezoelectric transformer having the above configuration is expressed by the following equation.

[0006]

(Equation 1)

The above-mentioned k ₃₁ , k ₃₃ , and Q _M are material constants determined by the piezoelectric material, and L and T are determined by the dimensions and shape of the element.

[0008]

As described above, a piezoelectric transformer used for a backlight requires a high AC high voltage of several hundred volts or more, and therefore requires a high step-up ratio. For that purpose, it is effective to reduce the thickness T of the piezoelectric transformer or increase the length L as derived from Equation 1. However, the thickness T of the piezoelectric transformer
When the thickness is reduced, the output impedance of the piezoelectric transformer increases in inverse proportion to several tens of kΩ. Since the cold-cathode tube has a substantially infinite impedance until discharge breakdown occurs, a sufficiently large boost ratio can be obtained even when the output impedance of the piezoelectric transformer is high.
The impedance becomes about 00kΩ, and the boost ratio sharply decreases.
Further, the output power also decreases, and the efficiency of the piezoelectric transformer decreases. The present invention has been made to solve the above-described problems, and has as its object to obtain a small-sized piezoelectric transformer with a high boosting ratio and high efficiency.

[0009]

According to a first aspect of the present invention, there is provided a piezoelectric transformer having a driving unit for exciting vibration of a piezoelectric body due to an inverse piezoelectric effect, and a power generating unit for extracting the vibration as electric power by a positive piezoelectric effect. And the ratio Rg / Rd of the resonance resistance Rd of the drive unit to the resonance resistance Rg of the power generation unit is 30 or less.

According to a second aspect of the present invention, there is provided a long plate-shaped device including a driving unit for exciting vibration of a piezoelectric body due to an inverse piezoelectric effect, and a power generating unit for extracting the vibration as electric power by a positive piezoelectric effect. A piezoelectric transformer, wherein the thickness of the piezoelectric transformer is 1 mm to ³ mm, the cross-sectional area in the width direction is 4 mm ² or more, and the resonance resistance of the power generation unit is 100 mm.
This is a piezoelectric transformer having a resistance of 00Ω or less.

According to a second aspect of the present invention, there is provided a piezoelectric transformer in which the piezoelectric body of the drive unit is polarized in the thickness direction and the piezoelectric body of the power generation unit is polarized in the length direction.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A piezoelectric transformer according to the present invention has a structure provided with an input electrode for applying a voltage to, for example, a lead zirconate titanate (PZT) -based piezoelectric material and an output electrode for extracting electric power. The input and output electrodes are made of Ag or A
A conductive paste made of an alloy containing g is formed on the surface of the piezoelectric body by a thick film printing method or the like, or formed inside the piezoelectric body by laminating a thin piezoelectric body by a lamination technique used for a multilayer capacitor. Good. In this case, external electrodes are formed on the outer surface of the piezoelectric body so as to be electrically connected to input electrodes and output electrodes formed inside. Further, the input electrode is formed on a drive unit that excites vibration of the piezoelectric body due to the inverse piezoelectric effect, and the output electrode is formed on a power generation unit that extracts the vibration as electric power by the positive piezoelectric effect. In the piezoelectric transformer, it is preferable that the ratio Rg / Rd of the resonance resistance Rd of the drive unit to the resonance resistance Rg of the power generation unit be 30 or less. The Rg
If / Rd is more than 30, the efficiency drops to less than 80%, the amount of heat generated when driving the piezoelectric transformer increases, and the operation of the inverter for the liquid crystal backlight becomes unstable, which is not preferable. Further, it is preferable that the piezoelectric transformer has a long plate shape and the thickness thereof is 1 mm to 3 mm. If it is less than 1 mm, the resonance resistance in the power generation unit is significantly increased, so that the efficiency of the piezoelectric transformer is greatly reduced. Further, the cross-sectional area of the piezoelectric transformer in the width direction is 4 mm ² or more, and the resonance resistance of the power generation unit is 1000
It is preferable to be 0 Ω or less.

The resonance resistor is an equivalent circuit of the piezoelectric transformer in a resonance state, and is a resistance component corresponding to the equivalent resistor R1.
FIG. 2 is an equivalent circuit of a piezoelectric transformer in a resonance state.
o, L1 and C1 are respectively the capacitance of the piezoelectric transformer,
Equivalent inductance and equivalent capacitance. This resonance resistance is equivalent to the impedance at the time of resonance measured by the impedance analyzer.

A cold cathode tube used for a backlight for a liquid crystal display is usually provided with a metal reflector so as to cover the cold cathode tube. For this reason, a stray capacitance is generated between the cold cathode tube and the metal reflector, but when the driving frequency of the piezoelectric transformer is high, a high-frequency current flows through the surrounding stray capacitance, and a sufficient tube current flows through the cold cathode tube. As a result, the efficiency is lowered and sufficient brightness of the cold cathode fluorescent lamp cannot be obtained. Further, even if the driving frequency is low, the tube current is small and sufficient brightness cannot be obtained. Therefore, the driving frequency is practically set to 40 kHz to 110 kHz. Here, the driving frequency of the piezoelectric transformer has the following relationship with the length of the piezoelectric transformer.

[0015]

(Equation 2)

Here, the frequency constant N is a constant inherent to the material.

In a lead zirconate titanate (PZT) -based piezoelectric material, the frequency constant represented by N is 1700 to 1700.
The driving frequency is 40 k
In order to set the frequency in a range from Hz to 110 kHz, the length of the piezoelectric transformer is preferably set in a range from 17 mm to 42 mm.

The piezoelectric transformer vibrates not only in the longitudinal direction but also in the width direction. Therefore, the vibration in the width direction and the vibration in the length direction may affect each other to cause a decrease in output power. In order to prevent such a decrease in output power, it can be prevented by optimizing the ratio between the width and the length of the piezoelectric transformer. Specifically, the ratio L of the length L to the width W is L
/ W is preferably 4.2 or less.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a method for preparing a sample will be described. Lead oxide, titanium oxide, zirconium oxide, strontium carbonate, piezoelectric ceramics (Pb _0.95 Sr
_0.05 ) (Zr _0.52 Ti _0.48 ) O ₃ was weighed to have a composition, mixed with a wet ball mill, dried, crushed, and calcined at 850 ° C. for 2 hours. To prepare a calcined powder.

Further, PVA as a binder was added to the calcined raw material powder, mixed and pulverized, and sized through a 60-mesh nylon sieve. The sized raw material powder was filled in a mold and molded at a molding pressure of 350 kg / cm ² to produce a molded body. After degreasing the molded product, it is arranged in a firing jig made of alumina, fired, printed and baked with input / output electrodes, and subjected to a polarization treatment at 140 ° C. by applying a polarization electric field of 2 kV / mm to form a long plate. Was manufactured.

The efficiency of the piezoelectric transformer was measured under the following conditions. A 100 kΩ resistor as a load resistor was connected to the output side of the transformer, a current was amplified by a power amplifier at a drive frequency from an oscillator, and power of 3.0 W was supplied to an input portion of the piezoelectric transformer. The output current was measured by an AC ammeter, and the ratio of the output power to the input power was evaluated as the efficiency of the piezoelectric transformer. The resonance resistance was measured using an impedance analyzer (4194A, manufactured by Yokogawa Hewlett-Packard Company).
Was used to measure the magnitude of impedance at resonance. Table 1 shows the results obtained as described above.

[0022]

[Table 1]

No. 1 in Table 1. Nos. 1 to 6 are examples of the present invention. 7 to 10 show comparative examples. No. In Nos. 1 to 6, an excellent piezoelectric transformer having an efficiency of 80% or more can be obtained. In the case of 7 to 10, the efficiency is less than 80%, which is not practical for a piezoelectric transformer. Above,
Although the embodiment using the single-plate piezoelectric transformer has been described,
The same effect can be obtained by configuring the piezoelectric transformer of the laminated type as in the present invention.

[0024]

As described above, according to the present invention, a piezoelectric transformer having a small size, a large output power and a high efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating the structure of a Rosen-type piezoelectric transformer.

FIG. 2 is an equivalent circuit diagram of a piezoelectric transformer in a resonance state.

[Explanation of symbols]

 1 Piezoelectric ceramics 3, 4 Input electrode 5 Output electrode

Claims (3)

[Claims] 1. A piezoelectric transformer comprising: a driving unit for exciting vibration of a piezoelectric body due to an inverse piezoelectric effect; and a power generation unit for extracting the vibration as electric power by a positive piezoelectric effect, wherein the resonance resistance Rd of the driving unit and the power generation unit are included. Ratio Rg / Rd with the resonance resistance Rg
Is 30 or less. 2. A long plate-shaped piezoelectric transformer comprising: a driving section for exciting vibration of a piezoelectric body due to an inverse piezoelectric effect; and a power generating section for extracting the vibration as electric power by a positive piezoelectric effect, adjacent to each other in a longitudinal direction. The thickness of the piezoelectric transformer is 1 mm to 3 mm.
mm, the cross-sectional area in the width direction is 4 mm ² or more, and the resonance resistance of the power generation unit is 10000 Ω or less. 3. The piezoelectric transformer according to claim 2, wherein the piezoelectric body of the drive unit is polarized in the thickness direction, and the piezoelectric body of the power generation unit is polarized in the length direction.