JPH0620874A - Variable capacitor - Google Patents

Abstract

PURPOSE:To obtain a variable capacitor which is low in loss, wide in variable range of capacity, and stable in temperature change. CONSTITUTION:A capacitor comprises two parallel metal plates 3 and a piezoelectric element 4 for varying the distance between the plate to vary its capacitance. One plate 3a out of the metal plates 3 is fixed to a structure 5, and the other metal plate 3 is fixed to a piezoelectric element 4 for the formation of a variable electrostatic capacitive element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention responds to downsizing, high frequency, and low power consumption of electronic equipment in a high frequency band, thereby reducing the loss and increasing the wide range of a variable capacitance element which is a passive component used for the electronic equipment. Regarding variable width and temperature stabilization.

[0002]

2. Description of the Related Art With the progress of integrated circuits, miniaturization, higher frequency, lower power consumption, and higher performance of electronic devices in the high frequency band are rapidly advancing. Among them, it can be said that the performance improvement of the variable capacitance element is delayed as compared with other elements. In particular,
In the 10 MHz to 5 GHz band, it is strongly desired to develop a variable capacitance element having low loss and a wide variable width and stable in temperature. As a solution to these demands, as shown in FIG. 8, a variable capacitor using a PN junction of a semiconductor diode is widely used. This variable capacitance element is characterized by being small and inexpensive.

[0003]

However, the variable capacitor is realized by applying a voltage to the semiconductor diode in the opposite direction to form a depletion layer without carriers and controlling the thickness thereof by the magnitude of the voltage. However, since the material is a semiconductor, there are disadvantages that the loss in the high frequency band due to the conductor loss is large and the temperature change is large. Therefore, an object of the present invention is to improve the above-mentioned drawbacks of the prior art and provide a temperature-stable variable capacitance element having a low loss and a wide variation range.

[0004]

The variable capacitance element of the present invention is an element that changes the capacitance by changing the distance between two metal flat plates, and is a piezoelectric element for changing the distance between the flat plates. It is characterized by using an element.

[0005]

According to the above structure, the basic structure is a parallel plate capacitor using air, which is an insulator having a small loss, as a medium, has extremely low loss, and the distance between the parallel plates is delicately changed by the displacement of the piezoelectric element. Since it can be changed, a variable capacitance element having low loss and a wide change range can be realized.

[0006]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a diagram for explaining the basic configuration of the present invention. If the distance between two parallel metal plates 3 and 3a is d, their surface area is S, and the relative permittivity of the insulating medium 10 is ε, the capacitance C between the terminals 2 and 2a is

## EQU1 ## C = K ₁ εε _o S / d Here, ε _o is the dielectric constant of vacuum, and K ₁ is a proportional coefficient smaller than 1. One metal flat plate 3 is fixed to the piezoelectric element 4, and the other metal flat plate 3a is fixed to the structure 5. The piezoelectric element 4 is fixed to the structure 5 and
An interval d between the metal flat plates 3 and 3a is formed. When the external voltage V is applied, the piezoelectric element 4 extends by Δd in the direction of the arrow. At this time, the capacitance C is

## EQU2 ## C = K ₁ εε _o S / (d−Δd). In general, the displacement amount Δd is proportional to the external voltage, such as Δd = K ₂ V due to the characteristics of the piezoelectric element 4, so
Is as follows.

## EQU3 ## C = K ₁ ε ε _o S / (d- (K ₂ V) ^-1 ) Thus, by changing the external voltage V, the capacitance C can be changed. In particular, the electrostatic capacitance C changes greatly near V = d / K ₂ . FIG. 2 shows one embodiment of the present invention. This is 5mm x 5 as a piezoelectric element
This is a case of using the laminated piezoelectric element 4a in which 100 layers having a sectional area of mm are laminated. A voltage of maximum 150 V is applied to the terminals 11 and 11a of the laminated piezoelectric element 4a.
As a result, the maximum value of Δd was 10 μm. As the metal flat plates 3 and 3a, a copper plate having a cross-sectional area of 5.5 mm × 5 mm was used. As the structure 5a, PZT, which is the same material as the piezoelectric material of the laminated piezoelectric element, was used. The thickness of the metal flat plate 3a is slightly changed by polishing so that the distance d between the two metal flat plates is about 2
It was set to 0 μm. FIG. 3 shows the measurement results of the embodiment of FIG. The horizontal axis represents the external voltage V and the vertical axis represents the capacitance between the terminals 2 and 2a. By changing the external voltage V from 0 to 150 V, the capacitance C could be changed from 10 pF to 20 pF. Generally, electronic parts are -20 to 6
Used in a wide temperature range of 0 ° C. The variable capacitance element having the structure of the embodiment shown in FIG. 2 also needs to operate stably in this temperature range. When the temperature changes, the distance d between the two metal flat plates 3 and 3a changes. This directly appears as a change in capacitance. The thermal expansion coefficient of PZT, which is a typical piezoelectric material, is about 3 × 10 ⁻⁶ (% / ° C.), and when the length of the piezoelectric element 4 is 10 mm, the length changes by about 3 μm in the above operating temperature range. This is d-as in the embodiment of FIG.
When δd = 10 μm is set, it corresponds to about 30% thereof. This is not preferable because it acts as it is to narrow the operating temperature range of the device of the present invention. The temperature change of d-δd is 1
If it is 0% or less, the variation range of the external voltage V can be enlarged to some extent. In order to realize this, it is necessary to make the coefficient of thermal expansion of the structure 5a and the coefficient of thermal expansion of the laminated piezoelectric element 4a as equal as possible. In the embodiment of FIG. 2, the structure 5a
As PZT, which is the same material as the piezoelectric material forming the laminated piezoelectric element 4a, is used as the piezoelectric material. However, the material is not necessarily the same, and the difference in the coefficient of thermal expansion from the laminated piezoelectric element 4a is 1
It was found that if it was not more than × 10 ⁻⁶ (% / ° C.), it could withstand sufficient use. FIG. 4 shows another embodiment of the present invention. This is a case where a unimorph type piezoelectric element 4b having an area of 5 mm × 20 mm is used as the piezoelectric element 4. The unimorph type piezoelectric element 4b has a structure in which a single piezoelectric body 6 having electrodes 7, 7'adhered on both sides is fixed to a metal plate 8 with an adhesive or the like. Both ends of the metal plate 8 have the structure 5
Semi-fixed to a. Maximum of 10 for electrode 7 and electrode 7 '
A voltage of 0V is applied. Therefore, the maximum value of Δd is 1
0 μm was obtained. Copper plates having an area of 5.5 mm × 5 mm were used as the metal flat plates 3 and 3a. The structure 5a is made of PZT, which is the same material as that of the embodiment of FIG. 2 using the laminated piezoelectric element. The thickness of the metal flat plate 3a was slightly changed by polishing to set the distance d between the two metal flat plates to about 20 μm. FIG. 5 shows another embodiment of the present invention. This is a case where the bimorph type piezoelectric element 4c having an area of 5 mm × 20 mm is used as the piezoelectric element. The bimorph type piezoelectric element 4c is composed of two piezoelectric bodies 6 and 6a in which double-sided electrodes 7, 7 ', 7a and 7'a are added around a single metal plate 8.
Is fixed by an adhesive or the like. Both ends of the metal plate 8 are semi-fixed to the structure 5. A maximum voltage of 100 V is applied to the electrodes 7 and 7 ', and a maximum negative voltage of 100 V is applied to the electrodes 7a and 7'a. As a result, the maximum value of Δd was 10 μm. Copper plates having an area of 5.5 mm × 5 mm were used as the metal flat plates 3 and 3a. The structure 5a is made of PZT, which is the same material as that of the embodiment of FIG. 2 using the laminated piezoelectric element. The thickness of the metal flat plate 3a was slightly changed by polishing to set the distance d between the two metal flat plates to about 20 μm. FIG. 6 is a diagram showing the structure of another embodiment of the present invention. When two metal flat plates are displaced and come close to each other, both electrodes do not come into direct contact,
The case where the surfaces of the electrodes are coated with insulating films 9 and 9a such as SiO ₂ is shown. FIG. 7 is a diagram showing another structure of the embodiment of the present invention. The insulating liquid 10 is placed between the two metal flat plates 3, 3a.
The case where a is immersed is shown. As a result, it is possible to prevent water from adhering to an extremely narrow space of about 20 μm.
Further, since the dielectric constant of the insulating liquid 10a is generally larger than that of air, there is an advantage that a large capacitance can be realized. Since the medium is a liquid, it is considered that there is a problem in reliability, but with the insulating oil having a low vapor pressure used in a vacuum device or the like, the characteristics hardly changed with time. Further, by modifying the surfaces of the two metal flat plates 3 and 3a so that the wettability with the insulating oil 10a is improved, the insulating oil 10a can be stably confined in a narrow region by a capillary phenomenon. It was possible. As a result, as a result of a normal vibration test, almost no reliability problems such as characteristic deterioration occurred. Further, in some of the above-described embodiments, only one piezoelectric element is used, but even if the two plate electrodes 3 and 3a are fixed to two piezoelectric elements, respectively, the effect of the present invention can be obtained. It is clear that does not change. From this, it can be easily understood that a variable capacitance element using a plurality of piezoelectric elements also falls within the scope of the present invention.

[0007]

According to the present invention, as compared with the prior art, since the parallel plate capacitor using an insulator having a low loss as a medium is a basic structure, the variable capacitor having a low loss, a wide variable width, and a temperature stable is provided. A capacitive element can be provided.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is a perspective view showing the structure of the embodiment of the present invention.

FIG. 3 is a characteristic diagram of an example of the present invention.

FIG. 4 is a sectional view showing the structure of the embodiment of the present invention.

FIG. 5 is a sectional view showing the structure of the embodiment of the present invention.

FIG. 6 is a sectional view showing the structure of the embodiment of the present invention.

FIG. 7 is a sectional view showing the structure of the embodiment of the present invention.

FIG. 8 is a component symbol diagram showing a conventional technique.

[Explanation of symbols]

 2 terminals 3 metal flat plate 4 piezoelectric element 5 structure 6 piezoelectric body 7 electrode 8 metal plate 10 medium

Claims (7)

[Claims] 1. A variable capacitance element, which is an element for changing the capacitance by changing the distance between two metal flat plates, wherein a piezoelectric element is used as a means for changing the distance between the flat plates. 2. The variable capacitance element according to claim 1, wherein one of the two metal flat plates is fixed to the entire structure and the other is fixed to the piezoelectric element. 3. The variable capacitance element according to claim 2, wherein the difference between the coefficient of thermal expansion of the piezoelectric element and the coefficient of thermal expansion of the structure is 1 × 10 ⁻⁶ (% / ° C.) or less. . 4. The variable capacitance element according to claim 1, wherein the piezoelectric element is a laminated piezoelectric element. 5. The variable capacitance element according to claim 1, wherein the piezoelectric element is a unimorph type piezoelectric element or a bimorph type piezoelectric element. 6. The variable capacitance element according to claim 1, wherein insulating thin films are coated on respective surfaces of the two metal flat plates facing each other. 7. The variable capacitance element according to claim 1, wherein an insulating liquid is immersed between the metal flat plates.