Monolithic piezoelectric transformer

Details Monolithic piezoelectric transformer Monolithic piezoelectric transformer

1999-03-15

2001-05-08

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S359000

Reexamination Certificate

active

06229251

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric transformer for use in an inverter for the backlight of a liquid crystal display, an inverter for starting a fluorescent tube, or a similar device.
2. Description of the Related Art
Conventionally, a Rosen-type piezoelectric transformer includes a rectangular, flat piezoelectric substrate, having a drive portion polarized in the thickness direction and a power generation portion polarized in the longitudinal direction. In order to achieve a reduction in size and a higher step-up voltage ratio, a monolithic piezoelectric transformer has been proposed. Piezoelectric ceramic sheets, each containing an internal electrode, are layered and fired into a single unit, thereby obtaining a monolithic piezoelectric transformer.
FIG. 7
is a longitudinal sectional view of a conventional monolithic piezoelectric transformer.
The conventional piezoelectric transformer uses a primary (basic) mode or a secondary mode and includes a rectangular, flat piezoelectric substrate
1
. A plurality of piezoelectric ceramic sheets
5
shown in
FIG. 7
are layered and fired into a single unit, to thereby obtain the piezoelectric substrate
1
. Pairs of input electrodes
21
and
22
are formed within and on the upper and lower surfaces of a longitudinal half portion (left-side half in
FIG. 7
) of the piezoelectric substrate
1
such that the input electrodes
21
and
22
face each other and extend over substantially the entire area of the longitudinal half portion of the substrate
1
. An output electrode
3
is formed on the longitudinal end surface of the other longitudinal half portion of the substrate
1
. The half portion including the input electrodes
21
and
22
functions as a drive portion A and is polarized in the thickness direction of the substrate. The other half portion including the output electrode
3
functions as a power generation portion B and is polarized longitudinally.
The input electrode
21
includes internal electrodes
21
i
formed within the piezoelectric substrate
1
and an external electrode
21
e
formed on the upper surface of the piezoelectric substrate
1
(as viewed in FIG.
7
). The input electrode
22
includes internal electrodes
22
i
formed within the piezoelectric substrate
1
and an external electrode
22
e
formed on the lower surface of the piezoelectric substrate
1
(as viewed in FIG.
7
). The electrodes
21
i
and
22
i
are formed alternately and between piezoelectric ceramic layers. The internal electrodes
21
i
and
22
i
have a length that is equal to that of the external electrodes
21
e
and
22
e.
The internal electrodes
21
i
are connected in common to the external electrode
21
e,
and the internal electrodes
22
i
are connected in common to the external electrode
22
e.
An input line is connected to the input electrodes
21
and
22
at the node point thereof, whereas an output line is connected to the output electrode
3
. An input voltage is applied to the piezoelectric transformer
1
through the input line and is stepped up by the piezoelectric effect and the reverse piezoelectric effect. The stepped-up voltage is obtained from the output electrode
3
through the output line.
A piezoelectric transformer having the above-described structure is used in a starting circuit for a cold-cathode tube serving as a backlight for a liquid crystal display. Light quality of the cold-cathode tube is adjusted by varying the output voltage of the piezoelectric transformer. The piezoelectric transformer is required to output a stable output voltage even when the input voltage varies. Usually, the output voltage is adjusted or regulated by varying a drive frequency for the piezoelectric transformer. Conventionally, the longitudinal dimension or length of the input electrodes (the length of the drive portion) is made longer than half the length of the piezoelectric substrate so as to make the frequency characteristic of a step-up ratio steep, thereby obtaining a desired output voltage without impairing efficiency. Such a piezoelectric transformer is proposed in, for example, Japanese Patent Application Laid-Open (kokai) No. 9-21362.
According to the above-described conventional monolithic piezoelectric transformer, the internal and external electrodes, which constitute the input electrodes, have the same length. Whenever the conditions of the step-up ratio and efficiency are to be modified by altering the length ratio between the drive portion and the power generation portion, jigs and tools for forming the external electrodes must be manufactured accordingly. Such jigs and tools include a screen mask for printing and a metallic mask for vapor deposition. Also, electrode patterns formed on a substrate on which the piezoelectric transformer is mounted must be modified accordingly.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a monolithic piezoelectric transformer which achieves excellent characteristics by arranging the length of internal electrodes to be greater than the length of the external electrodes and enabling common use of jigs and tools for formation of the external electrodes and common use of mounting substrates.
To achieve the above object, one preferred embodiment of the present invention provides a monolithic piezoelectric transformer including a piezoelectric substrate having a laminated body made of a plurality of piezoelectric ceramic members, the piezoelectric substrate including a drive portion and a power generation portion which are arranged adjacent to each other in the longitudinal direction of the piezoelectric substrate, the drive portion being polarized in the thickness direction of the piezoelectric substrate and further including an input electrode which has internal electrodes provided in the drive portion and external electrodes provided on a surface of the drive portion, and the power generation portion being polarized in the longitudinal direction of the piezoelectric substrate and further having an output electrode associated with the power generation, wherein a length of the external electrodes is different from that of the internal electrodes.
According to the above described structure and arrangement, excellent characteristics are achieved by modifying the length of the internal electrodes while holding the length of the external electrodes constant. This enables common use of jigs and tools during formation of the external electrodes and common use of mounting substrates. More specifically, when piezoelectric transformers of the same size and different characteristics are to be manufactured, common and standard jigs and tools for formation of the external electrodes and mounting substrates can be used for all components, since the external electrodes can be made to have a predetermined length. Therefore, the manufacturing cost and the mounting cost can be reduced.
Preferably, the length of the internal electrodes is at least approximately 50% that of the piezoelectric substrate but less than the full length of the piezoelectric substrate.
According to the above described structure and arrangement, the length of the effective drive portion becomes longer than that of the power generation portion, thereby obtaining a steeper frequency characteristic of step-up ratio. Thus, a desired output voltage can be obtained through a slight adjustment of driving frequency such that efficiency does not decrease.
Preferably, the length of the internal electrodes is about 55% to about 70% that of the piezoelectric substrate.
According to the above described structure and arrangement, the output voltage can be adjusted easily, and the piezoelectric transformer operates stably.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.


REFERENCES:
patent: 5440195 (1995-08-01), Ohnishi et al.
patent: 5463266 (1995-10-01), Fukuoka et al.

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