Piezo ceramic transformer and circuit using the same

Details Piezo ceramic transformer and circuit using the same Piezo ceramic transformer and circuit using the same

1999-10-20

2001-08-21

Budd, Mark J. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S366000

Reexamination Certificate

active

06278226

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezo ceramic transformer and a circuit which drives two CCFLs(cold cathode fluorescent lamps) and more particularly to a step-up piezo ceramic transformer with two output sections and a circuit which drives two CCFLs for LCD backlight unit, working at a full wavelength vibration mode.
2. Description of the Prior Art
U.S. Pat. No. 2,974,296 discloses a piezo ceramic transformer which was first developed in early 1960s and has regained strong interest recently as a step-up transformer in inverter circuit for CCFL of LCD backlight unit.
Based on the original Rosen type design, several variations of piezo ceramic transformer, which are disclosed in U.S. Pat. Nos. 5,440,195, 5,463,266 and 5,365,141, and Japanese Patent No. 6-224484 and 8-107240, have been developed and applied successfully in inverters for LCD backlight system. Piezo transformer has many advantages compared to conventional magnetic transformer: a low profile, a narrow width, a high efficiency, less EMI, etc. In addition, unique dependence of step- up ratio on load resistance is a perfect match for ignition and regulation of CCFL, i.e. a very high step-up ratio at no load for a start of CCFL and a reduced step-up ratio for regulation of lamp current with a decreased impedance of CCFL after ignition. Most of the commercial application requires a low input voltage and thus input section of piezo transformer is multilayered to further increase step-up ratio for inverters of notebook PC, etc.
Although a single CCFL might be used as a backlight source in LCDs no bigger than 13.3″, two or more CCFLs are generally required for bigger LCD than 14″ to provide an increased light output and to enhance brightness, especially in LCD monitors. It is possible by increasing working frequency to increase power density of piezo transformer per unit weight, to reduce its size and to increase light efficacy of CCFL, but its working frequency is limited by characteristics of CCFL. Maximum working frequency of CCFL guaranteed by lamp manufacturer is usually no more than 80~90 kHz. In order to further reduce the size, especially thickness and width, of inverters for LCD, it is required to maximize working frequency within the limit and to design a piezo transformer with multiple output electrodes to drive multiple CCFLs.
Most of piezo transformers in the prior arts have only one output electrode and thus it is not possible to drive two CCFLs separately. Two CCFLs might be connected in parallel to a piezo transformer with a single output electrode, but then load impedance becomes half from parallel connection of two CCFLs, which makes it very difficult to design a piezo transformer with a matching output impedance to a load to get a high conversion efficiency.
Piezo transformer with two output electrodes
101
which is working at a half wavelength mode, &lgr;/2-mode could be built with a driving section in center
103
as shown in
FIG. 1
, including a generating section
104
and an input electrode
102
, but it has the following disadvantages: 1) a full wavelength resonance, &lgr;-mode is not working due to its geometry of input electrode and thus its working frequency is low, and 2) elemental holding is not stable because it has only a single nodal point at the center.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a piezoceramic transformer working at a full wave length mode, &lgr;-mode, with two output electrodes each of which drives a CCFL separately.
Another object is to provide a circuit to drive two CCFLs separately with the piezo ceramic transformer invented.
These and other objects will appear upon reading the following specification and claims and upon considering in connection with the attached drawing to which they relate.
SUMMARY OF THE INVENTION
A first piezoceramic transformer in accordance with the present invention comprises a rectangular piezo ceramic plate
210
working at a full wave length mode(&lgr;-mode), which is composed of a driving section and two generating sections
213
and
214
, wherein said driving section is located in the middle of said piezo ceramic plate and divided into two input sections
211
and
212
, and said two generating sections are located at the ends of said piezo ceramic plate; two sets of multiple internal electrodes
219
and
220
which are built into said input sections
211
and
212
having a multi-layered structure; two external electrodes
215
,
216
,
217
and
218
for each input section on the surface of said piezo ceramic plate
210
connecting two sets of alternating internal electrodes
219
and
220
in parallel; and output electrodes
221
and
222
which are formed on the ends of two generating sections
213
and
214
, wherein each layer of input multi-layered structure is poled along the thickness direction with polarization in the neighboring layers opposite to each other, and polarization in the same layer of said two input sections
211
and
212
is disposed in the same direction, and polarization in two generating sections is along the length of said piezo ceramic plate
210
but the respective direction is in opposite way.
Each of two CCFLs is connected directly to one of two output electrodes formed at the end of the piezo ceramic plate, respectively, and two external electrodes of two input sections on the same side of the piezo ceramic plate are connected 180 degree out of phase to each other to a driving circuit.
In a second piezo ceramic transformer in accordance with the present invention, polarization in the same layer of the two input sections is disposed in the opposite direction, and two external electrodes of two input sections on the same side of the piezo ceramic plate are connected in phase to each other to a driving circuit so as to make electrical connection easy, and all the other features are the same as in the first invention.
The above and other objects, features and advantages of the present invention will become clear from the following description based upon the accompanying drawings which illustrate examples of preferred embodiment of the present invention.


REFERENCES:
patent: 5278471 (1994-01-01), Uehara et al.
patent: 5365141 (1994-11-01), Kawai et al.
patent: 5440195 (1995-08-01), Ohnishi et al.
patent: 5463266 (1995-10-01), Fukuoka et al.
patent: 5504384 (1996-04-01), Lee et al.
patent: 5736807 (1998-04-01), Hakamata et al.
patent: 5751092 (1998-05-01), Abe
patent: 5929554 (1999-07-01), Kanayama et al.
patent: 5962954 (1999-10-01), Leers et al.
patent: 8-107240 (1990-04-01), None
patent: 6-224484 (1994-08-01), None

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