Piezoelectric transformer

Details Piezoelectric transformer Piezoelectric transformer

2002-09-23

2004-11-02

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S364000, C310S365000, C310S366000, C310S367000, C310S368000, C315S224000, C315S291000, C315S2090PZ, C345S074100, C345S211000, C252S06290R

Reexamination Certificate

active

06812623

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric transformer which transforms the amplitude of an alternating voltage by the piezoelectric effect of a piezoelectric material such as a piezoelectric ceramic.
2. Description of the Related Art
A piezoelectric transformer which was designed for a step-up transformer of a high voltage power supply has not been commercialized because of limited properties of a piezoelectric ceramic material such as breaking strength. However, with the advance of high strength piezoelectric ceramics, attention has been recently paid again to the piezoelectric transformer as the step-up transformer for an inverter of a backlight source of the liquid crystal display (LCD) panel installed on a portable information equipment, in the face of the increasing demand for the thin and compact equipment such as a notebook personal computer and a portable terminal.
In that information equipment, the inverter for the LCD is used, for example, as a power supply for lighting a cold cathode fluorescent lamp (CCFL) which is employed as a backlight source. This inverter must be a kind capable of converting a direct current voltage of about 3V to 12V from a battery or the like to a high frequency high voltage of 1 kVrms when starting lighting of the backlighting elements and about 500 Vrms when constantly lighting the backlighting elements and a frequency of about 60 to 80 kHz. An electromagnetic transformer which is used at present for the inverter for the backlight source satisfies the demand for making the equipment thin as a horizontal type transformer using a core of special shape. However, there is a limit to making the electromagnetic transformer small in size and thin because it needs to have a withstand voltage against the voltage as high as several kVrms. Further, winding loss disadvantageously increases and transform efficiency disadvantageously decreases because a thin copper wire is employed to increase the number of turns for stepping up voltage. Besides, the loss disadvantageously occurs which is caused by the material of the core.
The piezoelectric transformer is produced by forming primary (input) electrodes and secondary (output) electrodes on a piezoelectric ceramic material such as lead zirconate titanate (PZT) or piezoelectric crystal material such as lithium niobate. If an alternating voltage having a frequency near the resonance frequency of the piezoelectric transformer is applied to the primary electrodes to mechanically vibrate the piezoelectric transformer, the mechanical vibrations are transformed to a voltage by the piezoelectric effect, which makes it possible to obtain a high voltage from the secondary electrodes in accordance with the impedance ratio between the primary and secondary electrodes. Thus, the piezoelectric transformer can be made smaller in size and thinner than the electromagnetic transformer and can achieve high transform efficiency.
A conventional piezoelectric transformer will next be described with reference to the drawings.
FIG. 31
is a perspective view of a conventional piezoelectric transformer
100
. The piezoelectric transformer
100
includes an electrode
104
and an electrode
106
serving as primary (input) electrodes which are formed opposed to each other on almost the left halves of main surfaces of a rectangular plate
102
made of a piezoelectric material perpendicular to the thickness direction thereof, and an electrode
108
serving as a secondary (output) electrode which is formed on one end face of the rectangular plate
102
in the longitudinal direction thereof. If the rectangular plate
102
is made of a piezoelectric ceramic such as lead zirconate titanate (PZT), as indicated by arrows in
FIG. 31
, the rectangular plate
102
is polarized in advance in the thickness direction thereof on the left half thereof by using the electrodes
104
and
106
and is polarized in advance in the longitudinal direction thereof on the right half thereof by using the electrodes
104
,
106
and
108
. If an alternating voltage having a frequency near the resonance frequency of the piezoelectric transformer
100
for exciting mechanical vibrations to expand and contract the rectangular plate
102
in the longitudinal direction thereof is applied between the electrodes
104
and
106
(the electrode
106
is a common electrode), the longitudinal extensional vibrations are excited in the piezoelectric transformer
100
. These mechanical vibrations are transformed to a voltage by the piezoelectric effect. As a result, it is possible to fetch a high voltage between the electrodes
108
and
106
which serve as the secondary electrodes in accordance with the impedance ratio between the electrodes
104
and
106
serving as the primary electrodes, and the electrodes
108
and
106
serving as the secondary electrodes.
FIG.
32
(
1
) is a side view of the piezoelectric transformer
100
shown in FIG.
31
. In FIG.
32
(
1
), arrows indicate the directions in which the rectangular plate
102
is polarized in advance. FIG.
32
(
2
) shows the displacement distribution of the piezoelectric transformer
100
in the longitudinal direction thereof at a certain point of time while extensional vibrations of a half wavelength are generated in the piezoelectric transformer
100
in the longitudinal direction thereof. In FIG.
32
(
2
), the horizontal axis indicates the position in the piezoelectric transformer
100
in the longitudinal direction thereof. The vertical axis indicates the displacement of the piezoelectric transformer
100
in the longitudinal direction thereof caused by the mechanical vibration at a certain instance. On the vertical axis, +direction indicates the right displacement of the piezoelectric transformer
100
in the longitudinal direction thereof and direction indicates the left displacement thereof in the longitudinal direction thereof. Further, FIG.
32
(
3
) shows the internal stress distribution in the rectangular plate
102
when the piezoelectric transformer
100
has the displacement distribution shown in FIG.
32
(
2
). FIG.
32
(
4
) shows the vibration-induced electric charge distribution when the piezoelectric transformer
100
has the displacement distribution shown in FIG.
32
(
2
). In FIG.
32
(
3
), the horizontal axis indicates the position in the piezoelectric transformer
100
in the longitudinal direction thereof and the vertical axis indicates the magnitude of the internal stress in compression/expansion direction along the longitudinal direction thereof. In FIG.
32
(
4
), the horizontal axis indicates the position in the piezoelectric transformer
100
in the longitudinal direction thereof and the vertical axis indicates the positive
egative polarity and quantity of the electric charges induced by the vibrations. As is obvious from FIGS.
32
(
3
) and
32
(
4
), in the central portion of the rectangular plate
102
, that is, in the portion in which the rectangular plate
102
has a vibration displacement of 0, the rectangular plate
102
has the maximum internal stress and the largest quantity of the induced electric charges. Such a piezoelectric transformer in which the mechanical vibrations of a half wavelength are excited, for example, having the displacement distribution shown in FIG.
32
(
2
) is normally referred to as a “piezoelectric transformer having a &lgr;/2 longitudinal extensional vibration mode (where &lgr; indicates one wavelength)”.
Generally, when extremely large strains are caused by the mechanical vibrations in a piezoelectric transformer, the possibility that a piezoelectric transformer will break is high, which leads to a deterioration in reliability. It is, therefore, necessary to hold down the amplitudes of the mechanical vibrations of the piezoelectric transformer as much as possible. Even if the piezoelectric transformer handles high electric power, it is possible to decrease the amplitudes of the mechanical vibrations of the piezoelectric transformer by making the piezoelectric transformer thicker

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