Download EEFL Backlight, Present and Future

EEFL Backlight, Present and Future
Jae-Hyeon Ko
Department of Physics, Hallym University, Hallymdaehakgil 39, Chuncheon, Gangwondo 200-702, Korea
ABSTRACT
The EEFL backlight has recently been applied
to LCD TV as a new light source. The present
status of EEFL backlight for LCD TV applications
will be reviewed in this contribution. The performances of the present EEFL backlight technology
applied to 32” LCD TV will be introduced, and advantages and disadvantages of EEFL backlight will
be summarized. In particular, the energy balance
in EEFL and the energy flow in EEFL backlight will
be focused on from the viewpoint of efficacy and
power consumption. In addition, detailed electric/optic properties, driving methods, reliability issues will be discussed, based on which the future
prospect of EEFL backlight technology will be derived in comparison with the CCFL backlight technology.
INTRODUCTION
An external electrode fluorescent lamp (abbreviated as EEFL henceforth) has become one of the
light sources for the backlight unit application of
liquid crystal display (LCD). EEFL backlight technology has been adopted in 30” LCD TV in 2003
for the first time, and the available size has been
extended in the range 23 ~ 37” LCD TVs. EEFL
has been used mainly in the direct-lit backlight for
LCD TV applications because multiple EEFLs can
be operated parallel by a single inverter, which is a
very attractive point for cost competitiveness of
LCD among various FPD (flat panel display)s. In
addition to the cost merit, EEFL has other advantages such as a lower power consumption, longer
lifetime, simpler backlight structure, etc., than the
conventional CCFL (cold cathode fluorescent
lamp) backlight technology. The basic characteristics of EEFL and recent technological trends and
issues of EEFL backlight unit will be summarized
in the present section. The technological directions
for future development of EEFL backlight will also
be briefly mentioned.
BASIC CHARACTERISTICS OF EEFL
Lamp Structure
The structure of EEFL is shown in Figure 1. It is
basically identical to the structure of CCFL except
electrode parts. Hollow metal electrodes are in-
serted at both ends in the discharge space of
CCFL, while electrodes of EEFL are located on the
outer surface of the glass tube. The external electrode may be formed by using a metal cap, by
laminating a metal tape or foil beneath which a
conductive adhesive layer is attached, or by dipping the lamp into metal paste, for example, silver
paste consisted of silver power and binder material,
etc. The lamp tube is normally made by borosilicate glass containing low alkali content, and tricolor phosphor is coated on the inner wall of the
tube glass. The discharge space is filled with a
mixture of inert gases such as neon and argon for
Penning effects at a pressure normally in the
range between 30 ~ 100 torr in addition to a minuscule amount of mercury as a source element of
ultraviolet (UV) light. For the purpose of reducing
mercury consumption, glass blackening and
phosphor degradation and thus extending lifetime,
protection layer such as yttrium oxide or aluminum
oxide might be inserted between the inner surface
of the tube glass and the phosphor layer or be
dispersed on the surface of phosphor powders.
Figure 1. The structure of conventional tubular EEFL.
Discharge Characteristics
The discharge induced in the EEFL is basically
a dielectric barrier discharge (DBD). The lamp
glass plays the role of the dielectric barrier across
which displacement current is passed by alternating electric field. The capacitance of the dielectric
layer, which limits the current in the discharge is
determined by the dielectric constant, thickness of
the glass and electrode area. Thanks to the current-limiting role of the glass tube, EEFL needs no
ballast elements and many EEFLs can be driven
parallel by a single inverter.
The glow discharge induced in EEFL is sustained by the creation of charged particles due to
ionization collisions in the discharge and by the
secondary electron emission due to the colliding
ions onto the inner glass wall underneath the electrodes. However, wall charges are accumulated
during the main discharge on the inner surfaces
underneath electrodes. The electric field formed by
these wall charges partly cancels the externally
applied field and thus self-terminates the discharge.
In addition, self-discharge is induced due to the
recombination of the wall charges when the driving
voltage falls to zero. These wall-charge effects are
more effective in the case when the EEFL is driven
by a square voltage wave having sharp rising and
falling edges.
The UV light is generated in the glow discharge
by electron/ion-impact excitations of mercury atoms. Generated UV light is transformed into visible
light by phosphor layer which plays the role of a
wavelength converter. The output spectral characteristics are determined by the tri-color phosphor
materials. The color coordinates, the correlated
color temperature and the color gamut of LCD can
be easily tuned by changing the compositions and
the mixing ratio of the tri-color phosphor materials.
3.5
EEFL
CCFL
Lamp Voltage (kV)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2
3
4
5
6
7
Lamp Current (mA)
8
Figure 2. The relationships between the lamp
voltage and the lamp current of CCFL and
EEFL[1]. Both lamps have the same outer
diameter, tube length and gas pressure of
Ne:Ar with a ratio of 9:1, which are 2.6 mm,
420 mm and 60 torr, respectively.
In EEFL, the lamp current(IL) increases with the
lamp voltage(VL) showing a positive IL-VL relationship. Figure 2 shows the lamp voltage as a function of current of an EEFL whose outer diameter,
tube length and a gas pressure of Ne:Ar with a
ratio of 9:1 are 2.6 mm, 420 mm and 60 torr, respectively[1]. The IL-VL relationship of CCFL with
the same dimensions and discharge gas condi-
tions is also plotted for comparison. Contrary to the
positive current-voltage characteristic of EEFL, the
CCFL shows a negative IL-VL characteristic, which
needs a ballast element for the stabilization of the
discharge. The increasing current induces a higher
density of excited atoms in the discharge and thus
a higher ionization rate per electron, which in turn
induces a lower axial electric field, low voltage
drop in the discharge for achieving steady state[2].
However, if the CCFL is connected to two ballast
capacitors at both ends and if the voltage applied
to the ballast capacitors is included into the lamp
voltage, the IL-VL characteristics of CCFL show
similarity with those of EEFL including the slope of
the dark current, magnitude of firing voltage of
Townsend discharge, and current increment at the
firing voltage[3]. These results imply that discharge
mechanism of EEFL is similar to that of the CCFL
having ballast capacitors at both ends.
Efficacy
The light-generating efficacy of light sources is
defined by the total luminous flux divided by the
power consumption. The efficacy of EEFL has
been shown to be comparable to that of CCFL[1].
For EEFLs with an outer diameter of 2.6 mm and a
tube length of 420 mm, the gas pressure for optimum efficacy was in the range between 40 and 90
torr at a normal operating condition. From now on,
several parameters affecting the efficacy of EEFL
in addition to the energy balance in EEFL will be
shortly discussed.
Lamp current
The luminance rises as the lamp current increases in the low-current range but the slope becomes smaller with increasing current and then
becomes saturated above some particular current
values [1,3]. This result indicates that the efficacy
becomes lower with increasing current density.
This is due to the fact that elastic scattering losses
and excitation of useless non-ultraviolet radiation
increase with increasing current density. The rate
of quenching collisions of the second kind by slow
electrons, which takes away the excitation energy
from excited mercury as kinetic energy, becomes
larger than the rate of radiating transitions under
the condition of high current density [2].
Capacitance
Since the discharge in EEFL is induced and
maintained by capacitive coupling through the
glass wall, it is expected that the efficacy will depend on the capacitance of the glass wall at the
external electrodes, which will in turn affect the
amount of wall charges and the voltage distribution
applied to EEFL. The applied voltage to EEFL will
be divided into two parts, the voltage across the
glass tube at the electrode parts (VG) and the volt-
age in the glow discharge (VP). As the capacitance
increases, the ratio of VP to the total applied voltage (VG + VP) will increase[4], which will in turn
change the discharge energy and efficacy to some
degree. This was demonstrated by investigating
the effect of electrode length on the efficacy, which
showed that the efficacy first increased and then
saturated with increasing electrode length [5,6].
The same effects may be accomplished by
changing the thickness or dielectric constant of the
glass tube. However, the relationship between the
capacitance and efficacy is not straightforward
because the capacitance has also an effect on the
ion heating losses in the cathode fall region in
DBD [7].
Driving conditions
There are several studies reporting the effect of
driving waveform on the efficacy, in particular between driving voltages of the sine wave and the
square wave forms. Some studies reported that
the efficacy becomes much higher when EEFL is
driven by square wave voltage forms with a duty
ratio smaller than 100% than the case driven by
the sine wave at the same input power and frequency [8,9]. Here, the duty ratio is defined by the
ratio of the voltage-on time to the period of the
driving voltage. If the duty ratio is smaller than
100%, double lightings in half-period can be obtained from the self-discharge occurring from the
recombination of the wall charges when the voltage becomes zero. On the other hand, if the duty
ratio of the square wave becomes 100%, the efficacy of EEFL becomes lower than the case driven
by sine wave in the low-frequency range below
150 kHz [1]. Combining these results, it is recommendable that for higher efficacy the EEFL be
driven by a voltage of the square wave form with
the duty ratio smaller than 100% and a sharp rising
edge for efficient double lightings in half period.
The efficacy increases with increasing driving frequency in both square wave and sine wave driving
conditions [1]. Since the impedance of the glass
wall at the electrode parts is inversely proportional
to the driving frequency, the voltage applied on the
glass tube will decrease with increasing frequency.
The lamp voltage was shown to decrease with increasing frequency. In addition, it is expected that
ionizing collision rate of electrons will increase with
increasing frequency, resulting in efficient production of charged particles and thus reduced cathode
fall [10]. However, driving conditions such as
waveform and frequency should be optimized in
the backlight structure considering the effect of the
existence of a metal chassis on the leakage current
and
the
possible
generation
of
EMI(electromagnetic interference) from the
high-frequency Fourier components included in the
square waveform.
Energy balance in EEFL
The reported efficacy of EEFL is in the range of
38 ~ 58 lm/W [11,12]. These values are comparable to those of CCFL. Efficacy can simply be obtained from the total luminous flux and the power
consumed in the lamp. However, for more complete understanding of the energy balance in EEFL,
it is necessary to measure the total UV output,
which can be estimated indirectly from the procedure described in [13]. The luminous efficacy η L is
described by the following equation.
η L = ηUV ⋅η E ⋅ QL ⋅ Le
where
ηUV
,
ηE
( +η PLASMA−VIS )
(1)
, QL , Le are UV generating
efficiency, energy transfer ratio in the conversion
from UV to visible light, average quantum efficiency of phosphors and lumen equivalence of the
emitting spectrum, respectively [13]. η PLASMA−VIS
is the luminous efficacy of visible light emitted from
the discharge plasma, which is of the order of 1~2
lm/W in typical low-pressure mercury fluorescent
lamps.
For the estimation of ηUV , electro-optic characteristics of a standard EEFL for 32” backlight unit
was investigated. The outer diameter was 4 mm
with a glass thickness of 0.5 mm, and the total
length was 715 mm. The electrode length was
fixed to be 33 mm. The gas pressure and composition were 50 torr and Ne:Ar with a ratio of 90:10,
respectively. A square-wave driving voltage with a
frequency of 50 kHz and a duty of 60% was used
to ignite and maintain the glow discharge in EEFL
using the high-voltage power supply (PDS4000,
FTLAB). During the measurement, the discharge
current was fixed to 10 mA using the current probe
(Tektronics P6022), and the lamp voltage was
monitored by the high voltage differential probe
(Tektronics P5210). From this experiment, luminous efficacy η L was obtained to be 52 lm/W [14].
Other parameters were obtained from the measured emitting spectrum and the reported quantum
efficiency of phosphors [15], and the estimated
values are η E ~0.488, QL ~0.8, Le ~220 lm/W at
the color coordinates of (x=0.26, y=0.23). The final
UV efficiency ηUV could be obtained from equation (1) to be ~0.58 at the driving conditions used
in the present measurement. This value is smaller
than the value of hot-cathode fluorescent lamp,
which is normally 0.65 [2]. This is a natural result
considering the higher electrode losses in EEFL.
More detailed experimental data and analysis will
be reported elsewhere [14].
Lifetime
One of the definitions of the lifetime of fluorescent lamps for the purpose of backlight is the time
when the luminance of the lamp becomes 50% of
the initial luminance. The luminance decrease
during the operation usually occurs from the
phosphor deterioration by ion and UV bombardment in addition to the mercury penetration into the
glass resulting in the glass blackening and the decrease of glass transmission. The luminance
maintenance curve of EEFL is expected to be
similar to that of CCFL considering the similarities
of lamp structure and discharge characteristics
between these two type of fluorescent lamps. On
the other hand, complete consumption of mercury
in the lamp will terminate the lamp life irrespective
of the condition of the luminance maintenance.
From the viewpoint of mercury consumption
mechanism, EEFL is superior to CCFL. In case of
CCFL, internal electrodes will be continuously
sputtered and eroded by bombarding ions during
normal operation, and sputtered metal atoms will
be combined by mercury atoms forming mercury-metal amalgam near the electrodes, which is
the main mechanism of mercury consumption in
CCFL. In contrast, there is no electrode sputtering
in EEFL, which is a very favorable condition for
reducing the mercury consumption. The amount of
consumed mercury of EEFL was shown to be
much smaller than the amount of CCFL at both 25
and 0 oC [16]. Moreover, the speed of mercury
consumption in EEFL was barely accelerated at
low temperatures.
Another issue related to the lifetime of EEFL is
the so-called “pinhole formation” in the glass near
or beneath the external electrodes. It refers to the
phenomenon of dielectric breakdown and formation of a small hole in the tube glass at the external
electrodes when the EEFL is operated at much
higher lamp currents and powers than normal values, usually more than 3 times [17,18]. The diameter of the pinhole was approximately 0.3 mm.
If the current density at external electrodes increases significantly, the temperature at the electrode will increase due to increased ionic bombardment and possible occurrence of corona discharges. These will in turn decrease the breakdown voltage and the resistance of the glass resulting in further increase of the glass temperature
due to the Joule heating. If the effective voltage
applied to the glass becomes larger than the
breakdown voltage or if the local temperature of
some area of the glass becomes higher than the
melting point, the pinhole will be formed and the
life of EEFL will be terminated. Experiments on
various EEFLs have empirically shown that the
possibility of pinhole formation becomes very high
if the electrode temperature approaches approximately 200 oC during the operation. In order to
decrease the possibility of pinhole formation, it is
important to reduce the current density at the electrodes by increasing the electrode area and also to
maintain the temperature near electrodes as low
as possible by optimizing the backlight structure
for effective heat release from the electrodes.
EEFL BACHLIGHT TECHNOLOGY
Power Consumption
The most attractive characteristic of EEFL backlight is that multiple EEFLs can be operated by a
single inverter, in contrast to CCFL that should be
operated individually by an independent inverter.
In this respect, EEFL is a more suitable light
source for the direct-lit backlight which usually
needs a few tens of lamps in one unit. As has
been mentioned in the previous section, the parallel driving of EEFLs could become possible owing
to the positive resistance characteristic in the lamp
voltage-current relationship, inherent in the capacitively coupled discharge. The light-generating
efficacy of EEFL itself is comparable to that of
CCFL if physical parameters of both lamps are the
same. However, the power dissipation in the driving circuit becomes lower due to the decrease of
the number of circuit elements, in particular, transformers in the driving circuit. In addition, the leakage current flowing through the back chassis via
the stray capacitance would become lower in
EEFL backlight thanks to the half-ground driving
method, which is also called center-balance operation [11]. Table 1 shows the comparison of the
performances of 26” and 32” CCFL and EEFL
backlights [19], which exhibits that the power consumption of 32” EEFL backlight can become lower
than CCFL backlight of the same size by more
than 15% at the same luminance level. Recently-exhibited 32” EEFL backlight at IMID and
SID showed a power consumption of only 68 W at
the panel luminance of 500 cd/m2.
Table 1. The comparison of the performances of
26” and 32” CCFL and EEFL backlights [19]
Luminance on modules
(cd/m2)
CCFL
EEFL
Product
Size (inch)
Power consumption (W)
26”
67.2
480
510
32”
85.0
Efficacy comparison
(cd/m2/W, %)
450
530
100%
106~120 %
Cost
The decrease of the number of transformers
and electric lead wires in EEFL backlight simplifies
the structures of the inverter and the backlight unit.
In addition, the manufacturing process might also
be simplified because EEFLs can be simply inserted into the capping components at both ends
as can be seen from Figure 3, instead of being
soldered to each lead wire. Owing to these advantages, not only the power consumption but also the
cost of the EEFL backlight can become lower than
those of CCFL backlight. The inverter cost of 32”
EEFL backlight is expected to be smaller than that
of 32” CCFL backlight by more than 50% [20].
However, the total material cost of 32” EEFL BLU
is estimated to be lower than that of 32” CCFL
BLU by 5~11 % [11,20], which seems to be in part
due to the more expensive lamp price at the moment. Accordingly, the cost merit of EEFL BLU
seems to have become less attractive than expected due to the relatively expensive lamp cost
and continuing improvements of CCFL technology.
ter-balance operation was adopted for reducing
electrode temperature. These modifications have
enlarged the driving margin for EEFL, which made
EEFL backlight become commercialized successfully.
Comparison between CCFL and EEFL Backlights
Table 2 summarizes the comparison of detailed
specifications and performances of 32” CCFL and
EEFL backlights. This summary shows that the
EEFL backlight is superior to CCFL backlight in the
structure of the backlight, power consumption and
cost.
Table 2. The comparison of detailed specifications
of 32” CCFL and EEFL backlights.
Item
Structure
Performance
Figure 3. The capping structure of external
electrodes of EEFL backlight
Pinhole Issues
The formation of pinholes at the electrodes
might be a serious problem in EEFL backlight because the occurrence of pinhole will terminate the
lamp life during the operation of LCD TV. Even if
there is a small possibility of pinhole formation
during the operation period of LCD TV, it will decrease the reliability of EEFL technology markedly.
Several improvements have been carried out on
EEFL backlight in order to prevent pinhole formation. First, the EEFL backlight unit is designed to
include larger number of lamps in one unit than
that of the CCFL backlight of the same size. For
example, 32” CCFL backlight includes 16 lamps or
smaller while 32” EEFL backlight includes 18
lamps. It will decrease the lamp current and thus
the current density at the electrode area of each
EEFL. Second, the capacitance at the external
electrodes has been increased by using a large
electrode area in order to reduce the voltage applied to the glass and also to reduce the current
density at the electrode area. Third, special driving
method so called half-ground method or cen-
CCFL
EEFL
(12~)16
18
(12~)16
2
(24~)32
2
Lamp lifetime (Hr)
~ 50,000
>60,000
Lamp voltage (V)*
~ 1,050
~ 1,450
Lamp efficacy
Good
Good
Backlight efficiency
Good
Better
Power consumption
(W)**
~ 95
~ 85
Inverter cost ($)
33.9
13.6
Lamp quantity
in a backlight
Transformer quantity
in the inverter
The number of
electric lead wires
Cost[14]
Total materials
120.7
113.3
cost ($)
* The outer diameter of CCFL/EEFL is 4 mm.
** With diffuser plate/diffuser sheet/BEF/DBEF [11]
RECENT TECHNOLOGIES
Driving Technology
Center-balance operation or half-ground driving
method has been developed and adopted to the
EEFL backlight [11]. In this method, two synchronized sine (or square) waves, 180 degrees out of
phase to each other, are applied to both electrodes.
The applied voltage is divided into these two driving waves reducing the voltage amplitude on each
electrode and glass wall while keeping the effective lamp voltage across the discharge space high
enough for igniting and sustaining the discharge.
This driving method is much more effective in the
formation of uniform plasma column along the tube
axis owing to the smaller leakage of the electric
field lines through the glass tube [10]. Thereby it
brings about uniform temperature distribution
along the tube and also low leakage current in the
backlight. These conditions are very important for
achieving both luminance and color uniformities in
addition to uniform distribution of mercury along
the tube axis during the long-time operation. The
fact that the temperature and the effective voltage
on the external electrodes can become smaller by
using the half-ground method is also a favorable
condition for the prevention of pinhole formation at
the electrodes.
Higher Brightness and Efficacy
Recently, decreasing power consumption of
LCD TV is in great demand, since low power consumption is one of the key factors enabling the
LCD TV to become more competitive among various flat panel displays. If the luminance of the
lamp is insufficient for special applications like
blinking backlight technology which needs very
high peak luminance, high-frequency operation in
the MHz range might be adopted to obtain a high
luminance level of 100,000 cd/m2 or higher [10].
For reducing power consumption, not only the luminance level but also light-generating efficacy of
the lamp is very important.
One way to improve the lamp efficacy is to reduce the lamp diameter. If the outer diameter is
reduced from 4 to 3 mm, the luminous efficacy increases from 49 to 53 lm/W [12]. The tube diameter dominates the wall loss of ions and electrons at
a fixed gas pressure, which in turn affects the
electron temperature in the discharge. Reduced
diameter increases the ambipolar diffusion loss at
the inner wall of the lamp tube, which needs higher
electron temperature for the compensation of the
loss of charged particles. Higher electron temperature is expected to give rise to higher efficiency in the generation of UV light in EEFL and
CCFL to some degree. However, the lamp impedance increases with decreasing tube diameter, and
thus the lamp voltage changes from 1.4 to 1.7
kV(rms) upon change of the tube diameter from 4
to 3 [12]. This rise of the lamp voltage and current
density will increase the possibility of the pinhole
formation as well as ozone formation, and thus
careful optimization of the electrode structure is
indispensable to lowering the lamp voltage, efficient heat release and prevention of pinhole formation.
Another way to improve the efficacy of EEFL is
to optimize the driving condition such as the driving waveform, duty ratio and frequency. One example is a “self-discharge synchronizing operation”
of EEFL driven by square waves suggested by
Cho et al.[21]. In this operation scheme, the discharge current becomes zero and changes its direction exactly at the synchronizing points at which
the magnitude of the applied voltage begins to de-
crease from its amplitude. It was suggested that
the above condition is necessary for the synchronization of the self-discharge time of the wall
charge with the voltage rising and falling times,
and was shown to lead to effective discharge in
EEFL backlight resulting in a higher brightness and
higher efficiency. If the current becomes zero too
early or if the current still remains at the synchronizing points, the efficacy becomes lower than the
synchronizing operation [21].
EEFL for Narrow-Bezel
One demerit of EEFL backlight is the larger
bezel than that of CCFL backlight owing to the
longer electrode length, which is necessary for
lowering the lamp voltage and preventing pinhole
formation in EEFL. One possible solution on this
problem has recently been suggested, which is
related to the EEFL having enlarge electrode
structure. It is called differential diameter EEFL
(abbreviated as DEFL), which was developed and
suggested by LG-Philips LCD [12]. The tube diameter of electrode parts is much larger than the
diameter of the main tube from which visible light
comes out. Figure 4 shows one example of DEFL.
It was shown that DEFL and EEFL exhibit the
same electric characteristics if the electrode area
is the same [12]. For example, if the electrode diameter becomes twice of that of main tube, the
lamp length can be reduced by approximately
4.6% while other optical and electrical performances remain the same [12]. This result suggested
a possibility of achieving a narrow bezel in the
EEFL backlight. However, lamp cost and mechanical reliability to shock/vibration should be
considered in the practical application of DEFL into
the backlight unit.
Figure 4. Comparison of electrodes of
DEFL (up) with that of conventional tubular
EEFL (down) [12].
FUTURE CHALLANGES
Longer EEFL for Large-size LCD TV
The lamp voltage of EEFL is larger than that of
CCFL in normal operation since the lamp voltage
of EEFL includes the voltage applied to the tube
glass as well as the discharge voltage drop. If the
lamp voltage becomes beyond 2 kV(rms), the possibility of ozone formation from the electrodes due
to the corona discharge becomes high, which will
accelerate the electrode oxidation. This is one of
the main reasons why EEFL backlight technology
cannot be applied to large-size LCDs above 40” at
the moment. For the extension of EEFL backlight
to larger sizes, it is necessary to improve the lamp
performances, in particular, to reduce the lamp
voltage below 2 kV(rms). The lamp voltage of
EEFL consists of two parts, the voltage applied to
the tube glass and the voltage in the plasma. The
voltage in the discharge space is again divided into
the sheath voltage and the voltage in the positive
column [2]. In order to reduce the voltage on the
glass, it is necessary to increase the capacitance
by changing the electrode shape and/or glass dimensions. For decreasing sheath voltage (cathode
fall voltage), secondary electron emitting materials
may be deposited on the inner surface at the electrode parts. Driving methods including driving frequency might be optimized for the same purpose.
In addition, auxiliary igniting materials like carbon
nano-tubes or needle-shaped conducting materials
may be dispersed in the lamp or auxiliary electrodes may be designed on the inner or outer surface of the tube glass for lowering the voltage drop
in the discharge.
In addition to the problem of high lamp voltage
with increasing tube length, occurrence of luminance and color non-uniformities along the tube
axis might be another potential problem of long
EEFL. For achieving appropriate uniformities, it is
important to keep both the thickness of the phosphor layer and the distribution of liquid mercury as
uniform as possible in the discharge space.
Noncircular EEFL
In a conventional direct-lit backlight, a diffuser
plate of 1~3 mm thickness should be put over the
parallel tubular CCFL or EEFLs in order to achieve
appropriate luminance uniformity. Some distance
between the lamps and the diffuser plate should
be kept for adequate propagation and mixing of
light from each lamp owing to the relatively large
distances between the lamps. Recently, fluorescent lamps having various noncircular cross sections have attracted great attention for the applications of backlight technology owing to the possibility of obtaining higher uniformity and/or efficacy
[22,23]. External electrode structure is expected to
be more appropriate for noncircular fluorescent
lamps because there is a great simplicity in forming the electrodes on noncircular glass tubes by,
for example, dipping technique. In contrast, it
would be more complex to design and optimize
noncircular hollow cold cathodes for noncircular
CCFLs. The cross section of multi-channels comprising flat fluorescent lamps having external electrodes is also usually noncircular. The spreading
condition of positive column in the noncircular,
low-pressure discharge lamps is sensitive to the
dimensional characteristics, in particular, the aspect ratio or flatness of the noncircular cross section [23]. It is necessary to optimize the dimensions of the cross-section for diffused positive
column and high efficacy. Moreover, concentrated,
filament-type plasma of high currents should be
avoided for obtaining high efficacy.
Owing to the anisotropy of the distribution of
light output, there are more degrees of freedom in
the arrangement of noncircular EEFLs in the direct-lit backlight than the tubular EEFLs whose
distribution of light output is isotropic. There is a
possibility that the luminance uniformity of direct-lit
backlight can be improved by adopting noncircular
CCFLs and by optimizing their arrangement [22].
Wide-color-gamut EEFL
CCFL technology for wide color gamut has recently become popular. Changes of the conventional red (Y2O3:Eu2+) and green (LaPO4:Ce,Tb)
phosphors to new phosphors such as
Y(P,V)O4:Eu2+ (for red) and BaMgAl10O17:Eu,Mn
(for green) shift the wavelengths of red and green
peaks in addition to removing side peaks in the
emission spectrum, from which color gamut of
92% of NTSC standard has been achieved [24].
The same technique can simply be applied to
EEFL backlight by using the same tri-phosphor
combinations. However, although the color gamut
can be increased with the changes of phosphor
materials, the luminance level of LCD will decrease by more than 20% due to the change of the
spectrum, and the lumen maintenance curve will
be deteriorated due to the use of BAM-series
phosphors for both blue and green colors. The lifetime of wide-color-gamut fluorescent lamps is estimated to be less than 30,000 hours, which is 60%
of that of the normal CCFL.
Mercury-free EEFL
Recently, there has been a high demand for environmentally-favorable light sources. Several
regulations on hazardous materials such as RoHS
(The Restriction of certain Hazardous Substances)
tend to prohibit the usage of mercury in industrial
products. In this respect, it is necessary to develop
mercury-free light sources for general lighting and
backlight applications [25]. Xenon (Xe) has been
considered as the source element for the UV light
owing to its longest wavelength as well as the
lowest ionization energy among the inert gases.
However, the main limitation for wide applications
of Xe-type fluorescent lamps is their low efficacy,
since the wavelengths generated from Xe glow
discharge are 147 and 173 nm, which are much
shorter than the wavelength from mercury, 254 nm.
For example, only a few percent of the input power
was transformed into a visible light in an aperture-type Xe fluorescent lamp [13]. Intensive studies have been carried out on how to increase the
efficacy of Xe discharge lamps during the past
decades. For example, it was shown that to increase the partial pressure of the Xe gas was effective in improving the efficacy of Xe-type tubular
fluorescent lamps because the relative UV output
of 173 nm wavelength increased compared to the
UV output of 147 nm [26].
SUMMARY
EEFL is one of the fluorescent lamps operated
in a dielectric-barrier-discharge mode, based on
which parallel driving of multiple EEFLs by a single
inverter can be realized. The structure of driving
circuit and the backlight, in particular the electrode
parts can be greatly simplified owing to this characteristic. The most outstanding advantages of
EEFL backlight are low power consumption, long
lifetime and a possibility of cost lowering owing to
the external electrodes, simplified backlight structure and driving circuit/method.
However, EEFL technology has been adopted in
sizes below 40” because the lamp voltage and the
possibility of pinhole/ozone formation become high
with increasing tube length. In addition, the technological improvements of CCFL backlight such as
the parallel driving of CCFL, assembly innovation
like socket assembly, etc. have been significant
and have caught up with the EEFL backlight technology. In this respect, further improvements such
as the optimization of materials such as the tube
glass and electrode structure for reducing lamp
voltage, bezel and non-uniformities in luminance
and color are in great demand for extending the
application fields of EEFL technology. Recent
technological efforts including the optimization of
glass tube [27], circuit modeling [28,29] and
deeper understanding of plasma condition [30] in
EEFL might speed up the adoption of EEFL backlight in large-size LCD TVs in 40~47-inches.
ACKNOWLEDGMENTS
We would like to thank Jae-Young Choi,
Young-Youb Kim, and Yun Seong Lee at Hallym
University for their support on obtaining the data of
energy balance in EEFL. In addition, stimulating
discussions with many scientists and engineers at
Samsung Corning Co. (Flat Backlight Unit Business Division) are highly appreciated. This work
was in part supported by the Research Grant from
Hallym University. Part of the present manuscript
has been published in the book titled “LCD Backlight” (Science & Technology, in Japanese) in
2006.
REFERENCES
Y. Takeda, M. Takagi, T. Kurita, Y. Watanabe,
M. Amano, H. Nakano, “The characteristics of
the external-electrode mercury fluorescent
lamp for backlight liquid-crystal-television displays”, J. SID, Vol.11, pp.667-673 (2003).
[2] J. F. Waymouth, Electric Discharge Lamp,
pp.24-29, (The M.I.T. Press, 1971).
[3] G.-S. Cho, J. Y. Lee, D. H. Lee, S. B. Kim, H.
S Song, J. Koo, B. S. Kim, J. G. Kang, E. H.
Choi, U. W. Lee, S. C. Yang, J. P. Verboncoeur, “Glow discharge in the external electrode fluorescent lamp”, IEEE Trans. Plasma
Sci., Vol.33, pp.1410-1415 (2005).
[4] Y. P. Raizer, Gas Discharge Physics,
pp.381-384, (Springer-Verlag, 1991).
[5] S. S. Lee, J. Kim, S. Lim, “Discharge characteristics of external electrode fluorescent
lamps (EEFLs) for LCD backlighting application”, SID ’02 Digest, pp. 343-345 (2002).
[6] T. S. Cho, Y. M. Kim, N. O. Kwon, S. J. Kim, J.
G. Kang, E. H. Choi, G. –S. Cho, “Effects of
electroe length on capacitively coupled external electrode fluorescent lamps”, Jpn. J. Appl.
Phys., Vol. 41, pp.L355-L357 (2002).
[7] G. Oversluizen, M. Klein, S. De Zwart, S. Van
Heusden, T. Dekker, “Discharge efficiency in
plasma displays”, Appl. Phys. Lett., Vol.77,
pp.948-950 (2000).
[8] T. S. Cho, H. S. Kim, Y. G. Kim, J. J. Ko, J. G.
Kang, E. H. Choi, G. –S. Cho, H. S. Uhm,
“Characteristic properties of fluorescent lamps
operated
using
capacitively
coupled
electrodes”, Jpn. J. Appl. Phys., Vol.41,
pp.7518-7521 (2002).
[9] Y.-J. Lee, W.-S. Oh, S.-S. Lee, K.-M. Cho,
G.-W. Moon, M.-S. Park, S.-G. Lee, M.-G.
Kim, “Comparative study on sinusoidal and
square wave driving methods of EEFL for
LCD
TV
backlight”,
Proceedings
of
IDW/AD ’05, pp.1249-1252 (2005).
[10] Y. Baba, M. Izuka, T. Shiga, S. Mikoshiba, S.
Nishiyama, “A 100,000-cd/m2, capacitively
coupled electrodeless discharge backlight
with high efficacy for LC TVs”, SID ’01 Digest,
pp.290-293 (2001).
[11] G. –S. Cho, “EEFL Technology”, Korea Display Conference 2006.
[1]
[12] J.-B. Kim, J.-K. Kang, J.-H. Yoon, J.-W. Hong,
B.-K. Jeong, B.-C. Ahn, S.-D. Yeo,
“High-Performance EEFL Backlight System
for Large-Sized LCD-TV”, SID ’06 Digest,
pp.1246-1248 (2006).
[13] T. Mizojiri, R. Sonoda, T. Okamoto, M. Yoshioka, “Energy Balance and Other Characteristics of Rare Gas Fluorescent Lamps for
Image Illumination”, J. Light & Vis. Env.,
Vol.29, pp.104-115 (2005).
[14] J.-Y. Choi, Y.-Y. Kim, Y. S. Lee, J. –H. Ko,
“Energy Balance in External Electrode Fluorescent Lamps for 32” Backlight”, (in preparation).
[15] K. Narita, “Relative Quantum Efficiency of
Various Lamp Phosphors”, J. Illum. Inst. Jpn.,
Vol.69, pp.65-69 (1985) (in Japanese).
[16] T. Nishihara, “Technical trend of light source
& inverter for large size LCD TV”, Korea Display
Conference
Seminar
Book,
2,
pp.161-180 (2004).
[17] D.-H. Gill, H. S. Song, J.-H. Kim, S.-J. Kim, S.
B. Kim, T.-Y. Kim, D.-G. Yu, J.-H. Koo, E.-H.
Choi, G. –S. Cho, “The lifetime and pinholes
in the external electrode fluorescent lamps”,
SID ’05 Digest, pp.1312-1315 (2005).
[18] G. –S. Cho, J. Y. Lee, D. H. Lee, J. H. Koo, E.
H. Choi, B. S. Kim, S. H. Lee, M. S. Pak, J. G.
Kang, J. P Verboncoeur, “Pinhole formation in
capacitively coupled external electrode fluorescent lamps”, J. Phys. D: Appl. Phys.,
Vol.37, pp.2863-2867 (2004).
[19] S.-J. Han, “Technology of LCD backlight for
TV application”, FPD International 2006
Pre-seminar, (2006).
[20] Displaybank, “Large-size BLU - Analysis and
Market Forecast”, pp.69-76 (2006).
[21] G. S. Cho, N. O. Kwon, Y. M. Kim, S. J. Kim,
T. S. Cho, B. S. kim, J. G. Kang, E. H. Choi, U.
W. Lee, S. C. Yang, H. S. Uhm,
“Self-discharge synchronizing operations in
the external electrode fluorescent multi-lamps
backlight”, J. Phys. D: Appl. Phys., Vol.36,
pp.2526-2530 (2003).
[22] For
example,
refer
to
the
patent
KR-10-2005-0011074 (and related patents
therein) submitted by Harison Toshiba Lighting Corp.
[23] Jae-Hyeon Ko, “A review on fluorescent
lamps having noncircular cross-sections”,
IMID’05 Digest, pp.1165-1168 (2005).
[24] T. Igarashi, T. Kusunoki, K. Ohno, “A new
CCFL for wide-color-gamut LCD-TV”, Proceedings of EuroDisplay 2005, pp.233-235
(2005).
[25] T. Shiga, S. Mikoshiba, “LCD backlights: with
or without Hg?”, J. SID, Vol.10, pp.343-346
(2002).
[26] M. Jinno, H. Kurokawa, M. Aono, “Fundamental research on mercuryless fluorescent
lamps I, II”, Jpn. J. Appl. Phys., Vol.38,
pp.4608-4617 (1999).
[27] J. S. Yoon, K. B. Jung, H. D. Kim, J. K. Kang,
J. B. Kim, B. K. Jeong, B. C. Ahn, S. D. Yeo,
“A Study of the Novel External Electrode
Fluorescent Lamp for High Optical Efficiency”,
IMID’06 Digest, pp.1412-1414 (2006).
[28] J.-M. Jeong, S.-B. Kim, M.-J. Shin, G.-E. Kim,
H.-S. Song, J.-H. Kim, S.-J. Kim, M.-K. Lee,
M.-J. Kang, S.-H. Ahn, D.-H. Gill, D.-G. Yoo,
B.-C. Park, J.-H. Koo, E.-H. Choi, J.-G. Kang,
G. S. Cho, “Impedance Matching in the Inverter Circuit of a 32-inch LCD TV with EEFL
BLUs”, SID’06 Digest, pp.506-509 (2006).
[29] K.-M. Cho, W.-S. Oh, G.-W. Moon, M.-S. Park,
S.-G. Lee, M.-G. Kim, “External Electrode
Fluorescent Lamp Model Based on the
Equivalent Resistance and Capacitance
Variation”, IMID’06 Digest, pp.1263-1266
(2006).
[30] G. S. Cho, D.-H. Gill, J.-H. Kim, S.-J Kim, S.-H.
Ahn, J.-M. Jeong, J.-G. Kang, B.-H. Hong,
J.-H. Koo, E.-H. Choi, J. P. Verboncoeur,
“Plasma Energy and Density in External Electrode Fluorescent Lamps”, Proceeding of the
Light Source Workshop 2006: Advance Light
Sources (24~26 Oct. 2006, Wonkwang Univ,
Korea), pp. 114-121 (2006).