Piezoelectric transformer

Details Piezoelectric transformer Piezoelectric transformer

2000-02-02

2001-08-21

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S366000

Reexamination Certificate

active

06278227

ABSTRACT:

TECHNICAL FIELD
This invention relates to a piezoelectric transformer which utilizes a piezoelectric vibrator using a rectangular plate of piezoelectric ceramics.
BACKGROUND ART
As a transformer (voltage transformer), there is well known an electromagnetic transformer comprising windings wound around an iron core. The electromagnetic transformer is unsuitable for use in a power supply for a small-sized electric apparatus because it is bulky in size, is large in power consumption, and generates electromagnetic noise and heat. For example, for use in a high-voltage power supply in an electrostatic generating device or a back-lighting lamp of a liquid-crystal display, the transformer does not require a large output current but requires an output voltage between 1 kV and about several watts. In addition, it is required to reduce the electromagnetic noise, the power consumption, and the size.
On the other hand, since a piezoelectric transformer utilizing a piezoelectric phenomenon generates little electromagnetic noise and can be reduced in size, practical use is considered as a power supply transformer for a small-sized apparatus.
Referring to FIGS.
1
(
a
) and (
b
), a conventional piezoelectric transformer
11
comprises a piezoelectric-ceramics rectangular plate
13
, two surface electrodes
15
and
15
formed on the piezoelectric-ceramics rectangular plate
13
opposite to each other in a thickness direction at a part (hereinafter referred to as a first part) extending from one end to an approximate half in a longitudinal direction, and a plurality of internal electrodes
16
and
17
formed in the interior of the above-mentioned first part between the both surface electrodes with a space kept from one another in the thickness direction. Side electrodes
18
and
19
formed on confronting side surfaces of the above-mentioned first part, respectively, are connected to the surface electrodes
15
and
15
, respectively, and to the alternate internal electrodes
16
and the remaining internal electrodes
17
, respectively. Moreover, an end electrode
20
for output extraction is formed on the piezoelectric-ceramics rectangular plate
13
over an end surface of a half part (hereinafter referred to as a second part) opposite to the above-mentioned first part.
The above-mentioned first part of the piezoelectric-ceramics rectangular plate
13
is polarized by applying a DC voltage between the side electrodes
18
and
19
. Specifically, the piezoelectric-ceramics rectangular plate
13
is polarized between adjacent electrodes of the surface electrodes
15
and
15
and the internal electrodes
16
and
17
. The polarization directions are opposite to each other at both sides of each of the internal electrodes
16
and
17
, as depicted by small arrows in FIG.
2
(
b
). Furthermore, by applying a DC voltage between the both surface electrodes
15
and the end electrode
20
, the second part of the piezoelectric-ceramics rectangular plate
13
is polarized in the longitudinal direction, as depicted by a large arrow in FIG.
1
(
b
).
The above-mentioned type including the plurality of internal electrodes will be referred to as a stacked type because it is actually formed by alternately stacking the internal electrodes and piezoelectric members in manufacture. On the other hand, another type is also known in which the polarization in the thickness direction is only one direction between the confronting surface electrodes
15
and
15
without any internal electrodes formed. This type will be referred to as a single plate type because no stacking is required during manufacture and it is implemented by a single piezoelectric member with electrodes formed on its surfaces.
Description will be made as regards an operation of the piezoelectric transformer illustrated in FIGS.
1
(
a
) and (
b
).
Now, one of the side electrodes
18
and
19
is used as an ground terminal and the other is applied as an input voltage with an AC voltage having a frequency equal to a resonant frequency of the piezoelectric-ceramics rectangular plate
13
in a one-wavelength resonance mode of a longitudinal vibration. Then, the stacked-type piezoelectric transformer acts as a piezoelectric vibrator to vibrate with a displacement distribution and a strain distribution illustrated in FIGS.
2
(
a
) and (
b
), respectively. At this time, an AC voltage is produced between each of the surface and the internal electrodes
15
,
16
and
17
and the end electrode
20
due to the piezoelectric effect. The level of the voltage thus produced is generally determined by distances between adjacent ones of the surface electrodes
15
and the internal electrodes
16
and
17
, a distance between the surface electrodes
15
and the end electrode
20
, and the input voltage.
Specifically, in the piezoelectric transformer, a transformed voltage can be obtained by energy conversion utilizing the piezoelectric effect, that is, electric-mechanical-electric conversion. In the meanwhile, in the piezoelectric transformer having the above-mentioned transforming system, the thickness of the piezoelectric plate is reduced and/or the thickness between the input electrodes (the surface electrodes and the internal electrodes) is reduced in order to satisfy the demands for low voltage driving, reduction in size, and a large step-up ratio (output voltage/input voltage). As a result, an input impedance is decreased so that an input current (motional current) is increased. The input current is converted into a vibration rate by mechanical conversion. Thus, in case where the vibration rate and an amplitude exceed a vibration level limit (this means the vibration rate at which the temperature (&Dgr;T) of the vibrator reaches a predetermined level due to heat generation at a high vibration rate and a large amplitude; the predetermined level can be selected, for example, as &Dgr;T=25° C.) inherent to the used piezoelectric ceramics by the increase of the input current, there arises a disadvantage that the heat generation increases and an efficiency is decreased.
Therefore, in order to solve the above-mentioned drawback in the prior art, it is a technical object of this invention to provide a structure of a piezoelectric transformer which satisfies the demands for low power consumption, low voltage driving, reduction in size, and a large step-up ratio and which is low in vibration rate, small in heat generation, and high in efficiency.
DISCLOSURE OF THE INVENTION
According to this invention, there is provided a piezoelectric transformer comprising a piezoelectric rectangular plate having a plate length extending from a first end to a second end opposite thereto, a plate width, and a plate thickness and comprising four regions virtually divided into quarters along the plate length and continuously arranged from the first end to the second end; input electrode means formed on at least two regions selected as input regions from the four regions to drive two-wavelength resonance of the piezoelectric plate in response to an AC input voltage having a frequency corresponding to a wavelength equal to a half of the plate length; and output electrode means formed at one position of at least one of the remaining regions among the four regions to output an AC output voltage produced from the two-wavelength resonance of the piezoelectric plate; the input regions being polarized in a direction of the plate thickness, the above-mentioned at least one of the remaining regions being polarized in a direction of the plate length.
According to this invention, there is also provided a piezoelectric transformer claimed in each of claims
2
to
11
.
According to this invention, the electric voltage having the frequency corresponding to the two-wavelength resonance mode of the piezoelectric rectangular member is applied to the input electrodes to vibrate the piezoelectric rectangular member in the two-wavelength resonance mode. The voltage generated by the vibration as a result of a piezoelectric effect is obtained from an output terminal. As compared with a one-wave

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