Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2001-03-30
2004-01-06
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S246000, C362S260000, C362S265000, C362S267000, C313S624000, C313S607000
Reexamination Certificate
active
06674250
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a fluorescent lamp with external electrodes and a backlight luminaire, and more particularly, to an improved backlight including an external fluorescent lamp in which the external electrodes are installed at both ends of the fluorescent lamp, and a method for driving the backlight.
2. Background of Invention
In general, a flat panel display is categorized into two types: an active lighting type and a passive lighting type. The active lighting type includes a flat panel cathode-ray tube, a plasma display panel, an electronic active lighting element, a fluorescent display, an active lighting diode, etc., where as the passive lighting type includes a liquid crystal display.
In the liquid crystal display, an image is formed not by a self-illumination provided by the incident light from the outside of the liquid crystal panel. To accomplish this, a backlight luminaire is typically installed at the rear end of the liquid crystal panel to enable the illumination. Hence, the image formed on the liquid crystal display can be seen even in dark places. It is highly desirable to produce a thin, light weight, and low-cost backlight luminaire that has high luminance, high efficiency, uniform luminance, and longer operation life. Highly efficient and long lasting lamp is desirable for use in notebook PCs to reduce the electrical power consumption, whereas high luminant lamp is desirable for use in regular PC monitors and TVs.
Two widely used methods as a backlight luminaire are a cold cathode fluorescent lamp (CCFL) and a flat fluorescent lamp. The CCFLs can be categorized into two types: (i) an edge light arrangement utilizing a plastic light guide, and (ii) a direct light arrangement in which repeated light sources are disposed on a plane in accordance with the arrangement of the light source with respect to the display face.
The above CCFL operates at a high luminance of about 30,000 cd/m
2
, and as a result has a shorter lamp life span. In particular, the edge light type is not suitable for a large screen panel as the luminance of the panel is weak even though the CCFL itself is highly luminescent. In the direct light type, it is impossible to connect the CCFLs in parallel arrangement and drive the backlight using a conventional inverter as the distance between the CCFLs has to be provided within a limited screen space to achieve desired illumination.
Meanwhile, the conventional flat-fluorescent lamp requires sufficient thickness to prevent the substrate made of glass from being damaged as the pressure between the upper and lower substrates is lower than the atmospheric pressure. As a result, the weight of the lamp tend to increase. In addition, in the conventional flat-fluorescent lamp, partitions and spacers in the form of a bead or cross are typically interposed between the upper and lower substrates in order to enlarge a screen area; thus, a uniform luminance cannot be achieved as the striped patterns of the partitions appear on the screen.
Accordingly, there is a need to develop a backlight source that is capable of ensuring high luminance and efficiency when placed in the back of a liquid crystal display.
Currently, there are various external electrode fluorescent lamp (EEFL) that are available as shown in FIG.
11
. Although the EEFL tends to have a longer operation life than the CCFL, it has not been widely accepted as a backlight source due to the EMI and low efficiency. Moreover, the EEFL requires a larger power source using a high frequency of about several MHz. Furthermore, the EEFL has not been employed as a backlight source as its luminance and efficiency tend to be low as the LC-resonance type inverter designed for driving the CCFL is used for driving the EEFL.
FIG. 11
shows different types of the conventional external electrode fluorescent lamps. In particular, FIG.
11
(
a
) illustrates a belt type external electrode with a pairs of the belt type electrodes installed on the cylinder of the glass tube driven typically at a high frequency of several MHz. The belt type EEFL (a) has an advantage in that additional electrodes can be installed even at an intermediate portion of the glass tube. This type of external electrode fluorescent lamps can attain a high luminance of several 10,000 cd/m
2
by driving the lamps at a high frequency of several MHz. Moreover, the installation of the belt type electrodes in the intermediate portion of the glass tube is helpful to operate even at a higher frequency. However, there are some drawbacks in that a uniform and thin panel cannot be realized due to a decrease in the luminance of the electrode portion. In addition, the high frequency driving causes the undesirable EMI to be emitted, thus the efficiency of the electrodes becomes low. Furthermore, the high frequency power source is undesirable in designing a compact device using such a power source.
FIG.
11
(
b
) illustrates a conventional external electrode in which metal capsules are bonded at the ends of the glass tube, and ferrodielectrics are applied to the inside of the metal capsules. This type of electrode is disclosed in U.S. Pat. No. 2,624,858 (Jun. 6, 1953) and typically employed to prevent the electric capacitive voltage drop caused by the thickness of the glass tube. However, the bonded portions of the electrodes can be easily damaged since coefficient of the thermal expansion of the glass tubes is different from that of the metal. However, if a fine glass tube, i.e., a cold cathode-ray tube, is used as the backlight source with an outer diameter of 2.6 mm and thickness of 0.5 mm or less, the metal capsules bonded to the glass tubes, as shown in FIG.
11
(
b
), does not have to be used since the electric capacitive voltage drop due to the thickness of the glass tube is small.
FIGS.
11
(
c
) and (
d
) illustrate lamps where the spaces at both ends of the glass tube are larger for achieving high luminance and efficiency. This type of external electrode is disclosed in U.S. Pat. Nos. 1,612,387 (Nov. 28, 1926) and 1,676,790 (Jul. 10, 1928). When the spaces at both ends of the glass tube are configured as shown in
FIG. 11
, the luminance and efficiency of the lamp increase. However, it is difficult to apply this type of structure to manufacture a fine tube to be used in a compact device.
FIG. 12
is a prior art circuit diagram showing an IC for driving the CCFL for use in the LCD panel. The circuit includes a lamp driving IC
100
having a plurality of I/O pins, a main electrical power circuit portion
120
having a half bridge circuit, and a lamp
140
. The lamp driving IC
100
comprises a first pin
1
connected to an input voltage terminal; a second pin
2
connected to a predetermined minimum frequency terminal; a third pin
3
connected to a predetermined maximum frequency terminal; a fourth pin
4
connected to a ground voltage terminal; a fifth pin
5
connected to a feedback ground terminal; a sixth pin
6
connected to a predetermined comparative terminal; a seventh pin
7
connected to a predetermined internal high voltage terminal; and, a eighth pin
8
connected to a predetermined external control signal terminal for determining ON/OFF of the IC circuit. The main electrical power circuit portion
120
comprises a half bridge circuit which responds to the output signal of the predetermined pin of the lamp driving IC
100
and includes a plurality of passive elements. The lamp
140
is driven in response to a predetermined output signal of the main electrical power circuit portion
120
.
As shown in
FIG. 12
, the power is supplied to the CCFL employed in the LCD backlight by a means of an inverter. The function of an inverter is to obtain a high voltage required for initiation and maintenance of the CCFL discharge from a low alternating voltage of several ten kHz obtained from the LC-resonance type inverter by a boosting transformer. Here, the waveform outputted from the inverter takes the shape of sine wave. This LC-resonance type inverter is helpful in designing a simple and high
Cho Guang-Sup
Choi Eun-Ha
Cha & Reiter
Guang-Sup Cho
Philogene Haissa
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