Low Architecture for Multi-Lamp Systems with

Low Architecture for Multi-Lamp Systems with
Low Frequency Architecture for Multi-Lamp
CCFL Systems with Capacitive Ignition
Monm Doshi (I), Jianjian Bian ( I ) , Regan Zane ( I ) and Francisco J. Azcondo
[ I ) Colorado Power Electronic Center (CoPEC)
Department of Electrical and Computer Engineering
University o f Colorado at Boulder
Boulder, CO 80309-0425
[email protected] edu
(’) University of Cantabria
Electronics Technology, Systems and Automation
Engineering Department
Ave. de 10s Castros Sin 39005 Santander, Spain
[email protected]
Absfruci - This paper presents a low frequency architecture
for driving parallel cold cathode fluorescent lamps (CCFts) in
large screen LCD ’I’V backlighting applications. Key to the
architecture is a proposed capacitive coupling approach for ac
lamp ignition. The system consists of a single high voltage
converter, an ac lamp ignition circuit, current regulation devices
and a single primary controller. The topology is capable of
driving an arbitrary number of parallel lamps with independent
accurate lamp current regulation, while maintaining high
efficiency and achieving significant size, weight, and cost
reduction when compared to typical high frequency ac ballast
designs, Experimental results for a pair of parallel 800 V 40 cm
CCFLs demonstrate simultaneous ignition and dc current
The popularity of thin, light weight, wide screen televisions
has resulted in tremmdous interest and development of large
screen liquid crystal display (LCD) TVs. The increase in LCD
screen size has created a significant demand for longer CCFL
designs and parallel architectures suitable for efficient drive of
large CCFL arrays with high luminance uniformity and long
life [l-31. High frequency LCC resonant inverters, based on
push-pull (Royer oscillators) or bridge (full or half) topologies
are commonly used as electronic ballasts for powering single
and dual CCFL backlighting systems [4-81. The resonant
circuits, generating sinusoidal waveforms of 25-1 00 kHz,have
high losses associated with high frequency capacitive coupling,
resonant circulating currents and also cause luminance
uniformity degradation due to thermometer effect [9-121. As
the number of lamps per system grows (some estimates greater
than 40), it will not be feasible to provide an individual LCC
drive per lamp due to the significant increase in size, weight,
cost, complexity of enclosure design and losses [Ill. One
solution is to drive multiple lamps with a single LCC ballast.
However, it is generally not possible to simultaneously
maintain high efficiency, proper parallel lamp ignition, and
individual lamp current reguration in such designs.
In this paper, we present a low frequency architecture
suitable for high efficiency drive of large CCFL arrays. The
low frequency approach removes many of the challenges and
drawbacks associated with high frequency drive, including
capacitive coupling, thermometer effect luminance uniformity
degradation, and electromagnetic interference (EMI). The
architecture i s capable of driving a large parallel CCFL array
with only a single high voltage converter, resulting in reduced
size, weight, and cost over existing designs. Lamp ignition in
the low frequency system relies on a unique capacitive
coupling approach, resulting in near operating voltage lamp
ignition for smooth operating waveforms and long lamp life.
While capacitive coupling is based on similar principles to
normal operation of electrodeless and extemal electrode
fluorescent lamps (EEFLs) [13-181, displacement current is
only required in our proposed architecture as a short pulse
during cold lamp ignition. The architecture is based on a very
low frequency drive, a capacitively coupled lamp ignition
circuit and low voltage self-biased current limiting (CL)
devices as shown in Fig I . Low frequency square wave
electronic baIlasts, based on synchronous buck converter or
This work is co-sponsored by National Semiconductor Corporation (through
CoPEC), the National Science Foundation (under Grant No. 0348772), and the
Spanish Government (through project CICYT TEC 2004-02607iMIC).
0-7803-8975t/OSI$2C~.MI02005 IEEE.
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We experimentally tested our theory on lamp ignition by
gradually applying dc voltage to lamps of varying length up to
the rated lamp r m s voltage. As expected, no lamps achieved
ignition under these conditions. We then wrapped a small
copper foil around the center of the lamp, and applied a high
voltage, high frequency drive to the copper foil (just above
rated lamp operation) with the lamp electrodes grounded. This
resulted in lamp ignition and full operation in shorter lamps
(<IO cm) through capacitive coupling only. Operation in this
manner is essentially the same as used for electrodeless and
external electrode fluorescent lamps (EEFL) 113-181. However,
relatively high voltages (kV or more) and high frequencies
kHz or higher) to achieve ignition and maintain the arc
and high lumen output for longer Iamp lengths. Continuous
A key component in the system is a new approach to achieve operation in this mode results in the same coupling concems
lamp ignition at near operating voltage, allowing smooth and for efficiency and luminance uniformity as high frequency ac
simultaneous ignition of the paralie1 CCFL array with very drive of the electrodes.
little electrode degradation at startup, resulting in long lamp
We then tested operation with an applied dc voltage across
life and minimal component stress. We will first describe our the electrodes near rated rms value followed by short pulse
initial observations followed by the proposed ignition application of ac drive to the copper foiI wrapped around the
center of the lamp. Under these conditions, lamp ignition was
It is known that CCFLs (and most discharge lamps) are achieved immediately with sustained operation after removing
designed and specified for ac operation with typical operating the ac drive from the copper foil. Current limiting was added in
voltages ranging from 200 V rrns to 800 V rms and ignition series with the lamp to maintain a stable arc and regulate dc
voltages ranging from 800 V to over 2 kV. We have found that lamp current. With short lamps, the ac ignition drive was less
ignition voltages with gradually applied dc drive can be higher than 200V rms, 85 kHz, with lamp ignition in tens of
than the specified ac ignition with varying results under microseconds [one or two cycles). With longer lamps (up to
matched conditions. However, by applying the dc voltage 40 cm), use of a short copper foil did not result in ignition.
rapidly from zero, lamps occasionalIy ignite at significantly However, by moving the foil with ac drive applied closer to the
lower voltages. We also know from our experience with high electrodes, ignition can be seen over that portion of the lamp
frequency ac drives that reduced ignition voltages are attained and can be literally “pulled” down the lamp until complete
by sweeping the ac drive voltage more gradually, as seen in lamp operation is achieved and ac drive is removed. Note that
CCFLs as well as other discharge lamps [13], [Zl], [22]. We for each of these test, the polarity of the applied dc voltage was
believe that in each of these cases, the reduced ignition changed prior to each ignition to avoid asymmetrical
voltages are due to displacement current within portions of the degradation of the electrodes.
lamp chamber through parasitic capacitances back to the
Based on these results, we have found that by placing a
source prior to ignition. This current is associated with the copper plate beneath the lamp (-75% of lamp length),
induced movement of free eIectrons due to the changing sufficient capacitive coupling is created (- nF) for reliable
electric field and the relatively low impedance o f the parasitic ignition of single and parallel lamps from IO cm to 40 cm at
capacitances compared to the high impedance of the gas the respective operating dc voltage. Additional details of this
chamber prior to ignition.
behavior are given in the experiment results of Section IV.
While the capacitive coupling effect can benefit high
frequency ac ballasts by reducing the lamp ignition voltage
(cold ignition or re-ignition for PWM dimming), it also creates
A block diagram for the low frequency architecture is shown
a well known draw back: reduced efficiency and poor in Fig. 1. The system includes two high voltage converters that
luminance uniformity. This so called “thermometer effect” is generate a low frequency high voltage square wave, an ignition
discussed in most design guides and tips for CCFL ballasts and circuit that drives the lamp through capacitive coupling during
controllers, where significant measures are suggested to reduce the ignition sequence, a single primary controller that
parasitic capacitance in order to improve efficiency and light coordinates lamp ignition and steady state voltage output, and
quality [9-I2]. This is one of our primary motivations for current limiting (CL) devices that stabilize the lamp currents
considering IOW frequency drive for CCFLs. By driving the following ignition. The basic operation for each functional
lamps with essentially dc current, we can consider methods for block is described below.
taking advantage of capacitive coupling for smooth, soft lamp
A. High Vobage Converter
ignition while removing most concems associated with
The primary function of the converter is to generate a low
efficiency ioss due to ac coupling, EMI, and light quality.
frequency high voltage square wave to essentially drive the
buck-boost converter topologies, have been studied and are
frequently used to power HID lamps to avoid acoustic
resonance and conduction losses in the lamps [19], [20]. In this
paper, we discuss a suitable architecture for large CCFL arrays
and study new approaches for lamp ignition based on low
frequency drives.
A discussion is first presented in Section I1 on the proposed
approach for lamp ignition through capacitive coupling. The
complete low frequency architecture is then described in
Section 111, followed by experimental results in Section IV
based on a test system driving two parallel 40 cm, 5 mA, 800
V CCFLs. Our conclusions are summarized in Section V.
lamps with dc cumnt and equal duty cycles of positive and
negative polarity. Otie approach is to use two high voltage dcdc converters connected with their outputs tied in series with
the CCFL array, as shown in Pig. 1. The square wave is
generated by alternately activating each converter while
shorting the output of the inactive converter. A benefit of this
approach is that the converter specifications are quite common
with many well known high efficiency solutions, all control
drives and sensing are referenced to ground, and the short in
the inactive converter can provide ground referenced sensing
of the total current through all lamps, Current sensing is used
by the primary controller to detect lamp ignition or failure and
to fine tune the output voltage to minimize voltage stress on
the low voltage CL. devices. A critical aspect of achieving
these benefits is tho ability to perform lamp ignition at the
operating voltage of the lamp, which at less than 1 kV allows
use of readily available silicon components in the power
Square wave drive is applied to avoid migration effects in
the lamp and equalize electrode degradation and end
blackening. Our rewlts to date show that the square wave
frequency is not critical and can operate with periods on the
order of seconds, minutes or greater. One potential concern is
any lamp flicker or EM1 associated with the square wave
transitions. On this point, we have found that with the
operation of the CL devices, voltage ripple on the converter is
not critical. This allows use of low output capacitance
converters with square wave transition times on the order of ms
when driven by the lamp cument, which are below the
dominant lamp time constants and do not create visible lamp
B. Ignition Cimit
A long copper plate (or multiple metal strips) is placed
underneath the CCFL array to provide the capacitive coupling
needed for ignition, as described in Section 11. The ignition
circuit is used to drive the copper plate with a short pulse (or
pulses) for ignition of cold lamps. The applied voltage required
depends on many factors, including lamp characteristics,
temperature and distance between tamp side wall and copper
plate. As shown in Section IV, we have demonstrated
operation with pulses in the few hundred volt range with
periods in the ten microsecond range. We are continuing to
investigate the fundamental limits for reliable parallel lamp
ignition based on the driving electric or electromagnetic fields.
The key result for our low frequency architecture is that only a
single high frequency generator is needed, which is operated
for short pulses with a capacitive load. Thus a wide range of
resonant converter or pulse generator approaches are suitable
for the ignition circuit.
C. Cum" Limiting (CL) Devices
The CL device is required to stabilize individual lamp
currents following ignition and provide consistent lumen
output across the CCFL array. A hnctional block diagram of
one possible solution is shown in Fig. 2, where a MOSFET (or
Figure 2. Block diagram of the current limiting (CL)device for stable lamp
current regulation, The circuit provides self-biasing and current regulation with
positive voltage and acts as a short circuit for negative voltages (for oppasite
polarity current.
other active device) is used as a current source to provide high
impedance current limiting in series with the lamp. The device
must be capable of bIocking the maximum expected Iamp-folamp voltage variation, which was measured to be less than 50
V over a 5:l current range for CCFLs under dc drive. Two CL
devices are connected in series with each lamp; one made
active for each polarity of the dc voltage. Thus each device can
be self-biased from its drain voltage when blocking positive
voltages, and acts as a short circuit for negative voltages (when
the opposite device is regulating). Over-voltage protection can
be used to protect against high voltage during transients. A bidirectional current regulating circuit can aIso be designed to
replace MOSFETs as current source, allowing the use of one
CL circuit in series with the lamps.
D. Primary Controlfer
The primary system controller provides the drive signals for
all blocks, including the high voltage dc-dc converters, low
frequency switches, igniter circuit and may also provide
communication with the CL devices for dimming control. For
lamp ignition, high voltage dc is ramped up together with the
capacitively coupled ignition current. The controller then
monitors total lamp array current through the current sense
feedback from the inactive converter and continues ignition
current until all lamps are ignited (based on total current
sensed). For nominal operation, the dc voltage is adjusted until
the total sensed current is stabilized, which indicates that all
CL devices are regulating current. Lamp re-ignition for failed
lamps can be attempted by re-applying the ignition current.
The polarity switch of the dc voltage is achieved by tuming off
the active converter gate drive and switching to the opposite
converter at zero current in the sensing resistor.
An experimental test circuit was built to verify the key
concepts presented in the architecture of Fig 1. A Flyback
converter was designed to generate a single polarity of dc high
voltage output used to drive the lamp (realizing one half of the
architecture shown in Fig. 1). The copper plate is driven by a
high frequency high voltage damped sinusoidal waveform
generated by a series LC resonant tank igniter circuit. The CL
device was implemented using a series discrete MOSFET
connected in a source follower configuration. The circuit
Variable control was used to study the required
characteristics for paralIeI lamp ignition under different
conditions. Additionally, a delay circuit is used to introduce a
variable delay between the ignition pulse and application of the
dc lamp voltage to study the effect of ignition characteristic of
the lamp under various timing conditions
C. Curreat Limiting (CL) Device
The CL device was implemented using a series discrete MOSFET
(50 V d n g ) with 1.5 161 source resistance and a fixed gate voltage
for setting the CCFL m t .
D. Eprimental Resuks
Experiments were conducted to study the lamp ignition
characteristics for the pair of parallel 40 cm, 800 V CCFLs.
The input voltage to the Flyback converter was set to 40 V, and
the current Iimiting (CL) MOSFETs were biased to act as a 5
mA current source. The actual lamp current was calculated by
measuring the voltage drop across the RCLresistor connected to
Figure 3. (a) Experimental Flyback Converter and (b) basic operating
waveforms when operated with a fixed on-time (2.5 ps) and variable
frequency. ,V = 800 V, ,V = 40V,j; = 50 kHz,igpt = 2.8 A.
diagram for each block diagram along with the detail
experimental findings is presented in the section below:
Flyback C o ~ w o e S i g n
The Flyback converter design and basic operating
waveforms are shown in Fig. 4, with VDC= 40 V, tums ratio
n = 10, ton= 2.5 ps,fJ = 130 lcHz, fiOmp
= 800 V. The Flyback
converter represents haIf of the low frequency drive circuit
described in the architecture of Fig. 1 and is used to
demonstrate the key concepts of parallel lamp ignition and
B. Ijpiier Circuit
The igniter circuit consists of a MOSFET switch and series
resonant LC circuit, where the voltage across the capacitor is
used to drive the copper plate. The igniter circuit diagram is
shown in Fig. 4(a) and the voltage waveform across the copper
plate capacitor is shown in Fig. [email protected]). The circuit is designed to
generate a damped sinusoidal output voltage with a peak
voltage, V, = 450 V, frequency, fc = 85 kHz. The ignition
voltage can be controlled by varying the pulse width of the
gating signal to the MOSFET (pulse width of tp = 40 ps,
generates the peak output voltage of 450 V).
Figure 4. (a) Igniter circuit and (b) igniter circuit output voltage waveform
(760 V,, 85 kHz)
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. . . . . . . . . . . . . . . . . . . . . .-
. . . . . . . . . . . . . . . . . . .. _
Figure 5 . Experimental results demonstrating lamp ignition at near nominal
operating voltage for two pacallel 4 0 c m CCFLs following application of
ignition current through calracitive coupling after a delay of 2.4 ms from the
instance of driving the Flybick converter.
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Figure 7. Experimental results demonstrating lamp ignition at near nominal
operating voltage for two parallel 40cm CCFLs following application of
ignition current through capacitive coupling after a delay of 4.4 ms from the
instance of driving the Flyback converter.
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. . . . . . . .....+.....
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Figure 6. Experimental results demonstrating lamp ignition at near nominal
operating voltage for two parallel 40 cm CCFLs. In this case, the ac ignition
pulse is applied just prior to applying the high voltage dc drive.
Figure 8. Lamp ignition at different instances for two parallel 40 cm CCFLs.
Lamp 1 ignites much before kamp 2 due to high lamp voltage. Lamp 2 ignites
after driving the copper plate by ignition pulse.
the source of the MOSFET.
Simultaneous ignition and 5mA regulation of the two lamps
in parallel is shown in Fig. 5, where the ignition pulse was
applied approximately 2 ms after applying the gating signal to
the Flyback converter, The delay circuit was tuned to apply the
ignition pulse at the point where the nominal operating dc
voltage is achieved. This tuning circuit is an artifact of our
verification setup and would not be required in the full
architecture proposed in Fig. 1 since the converter would be
regulated to the desired operating voltage. We also adjusted the
ignition pulse timing to study the sensitivity of the ignition
behavior to the startup sequence. Figures 6 shows the result
when applying ac ignition prior to dc drive, which surprisingly
still results in successhl lamp ignition once the operating dc
voltage is applied. Figure 7 shows the result when applying the
ac ignition after allowing the applied dc voltage to exceed the
operating voltage. In this case, a significant lamp current spike
is created that could degrade lamp life. Similarly, Fig. 8 shows
a condition where one lamp ignites due to the applied high dc
voltage, followed by successful ignition of the second lamp by
the ignition pulse, demonstrating the ability to ignite lamps in
parallel even in sequence if desired. The time length of the
spikes in Figs. 7 & 8 is set by the response time of our CL
device, which could be significantly faster using feedback
circuitry. However, in general it is preferred to either apply the
ac ignition pulse prior to reaching rated operating dc voltage or
to regulate the output to avoid exceeding this voltage during
Table I summarizes the dc operating results for the two
40 cm lamps over a range of regulated dc current settings. A
number of important results are illustrated in Table I. First, the
lamp voltage shows relatively little variation over a wide
dimming range (up to 80 V), which simplifies the requirements
on the power converter. Second, the dc lamp voltage is actually
proportional to the lamp current over a range of dimming
levels and the incremental lamp impedance is relatively small
(tens of kR), which affects the design requirements on our CL
devices. Third, the lamp-to-lamp voltage difference at any
given current is less than 30 V. This is the worst-case voltage
that the CL devices must block to maintain current regulation.
Certainly additional testing is required, but initial results
indicate that relativcly low voltage ICs could be used to realize
the CL devices. Since the CL devices are the only components
in the proposed architecture that scale with the number of
lamps, a small low voltage IC for the CL devices would result
overall in a low cost, compact, light system solution for
backlighting in wide-screen LCD TVs.
It is also worth noting that expanded dimming range can be
achieved using traditional PWM techniques in this system by
creating a dead-time between alternate converter operation. A
PWM approach for dimming would also require higher square
wave drive operating frequency to avoid lamp flicker.
We have also tested multiple lamp lengths with operating
voltages from 200 V to XOOV to verify that ignition is
achieved at nominal operating voltage in each case. Figure 9
shows the ignition sequence for a single 6 cm, 250 V CCFL,
where the BC ignition pulse is applied during the ramp up of the
dc voltage. As expected, the lamp ignites at the rated voltage
with no voltage or current spikes. The dc voltage and current
dimming characteristics for the 6 cm CCFL are shown in Table
2, again demonstrating relatively small voltage variations over
the dimming range.
. . . . .
. . . . . . . . . . . . . . . . ..,." . . . . . . . . . . . . . . . . .
c h z m
Figure 9 Experimental results demonstrating lamp ignition at near nomnal
operating voltage for a single 6 cm CCFL with the ac ignition pulse applied
dunng the ramp up of the dc drive
We have presented a low frequency architecture for high
efficiency drive of parallel CCFL arrays used in large screen
backlighting applications. The proposed system provides the
benefits of a single power converter capable of driving parallel
CCFL arrays for significant size, weight, and cost reduction in
large screen LCD W S Low
. frequency drive also results in
improved eficiency, elimination of sensitivities to parasitic
capacitances associated with packaging and significantly
reduced EMI. A unique approach for lamp ignition was
proposed based on capacitive coupling that results in reliable,
soft ignition of parallel lamps for long lamp life and low
component stress. Experimental results were prcsentcd that
demonstrate simultaneous ignition and current regulation of a
pair of 40 cm CCFL:; based on the proposed architecture using
capacitively coupled ignition and dc current regulation. By
using low voltage ICs for current regulation, the architecture
results in a low cost, compact, light system solution for
backlighting in widescreen LCD TVs.
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