26.1: Low-Frequency Square-Wave Drive for Large Screen LCD-TV Backlighting Systems

26.1: Low-Frequency Square-Wave Drive for Large Screen LCD-TV Backlighting Systems
26.1 / M. Doshi
26.1: Low-Frequency Square-Wave Drive for Large Screen LCD-TV
Backlighting Systems
Montu Doshi and Regan Zane
Department of Electrical and Computer Engineering, University of Colorado at Boulder,
Boulder, CO-80309 0425
Francisco J. Azcondo
Electronics Technology, Systems and Automation Engineering Department, Universidad de Cantabria
Ave. de los Castros s/n Edif. ETSIIT, E. 39005 Santander, Spain
Abstract
This paper presents a low-frequency square-wave drive,
consisting of a single high voltage converter, an ac lamp ignition
circuit, current control devices and a single backlight controller,
capable of driving an arbitrary number of parallel cold cathode
fluorescent lamps (CCFLs) with independent accurate lamp
current regulation. Key to the architecture is a proposed
capacitive coupling approach for ac lamp ignition that results in
reliable, simultaneous ignition of parallel lamps with a maximum
ignition voltage near the normal lamp operating voltage. A brief
summary of the lamp model and behavior is presented to explain
the findings during the capacitive ignition and low-frequency
operation. Experimental results are presented demonstrating
parallel lamp ignition and current regulation for four, 250 mm
CCFLs.
1.
Introduction
Large screen LCD TV backlighting systems, generally consisting
of 16 or more cold cathode fluorescent lamps (CCFLs), require a
ballast capable of driving all or groups of lamps in parallel with
accurate current control and regulation. The most common
electronic ballasts, based on high frequency LCC resonant
inverters, are capable of driving at most four lamps in parallel and
require complex multi-winding transformers and specialized
control circuitry. This solution suffers from several disadvantages,
the major limitation being the inability to simultaneously maintain
high efficiency, proper lamp ignition, and individual lamp current
regulation [1, 2].
We present a suitable architecture, capable of driving a large
parallel CCFL array with high efficiency, accurate lamp current
control and near operating voltage lamp ignition. The system
block diagram, shown in Fig. 1, is based on low-frequency
square-wave (LFSW) drive and removes many of the drawbacks
associated with a high frequency drive, including energy loss
through capacitive coupling, the thermometer effect luminance
uniformity degradation and electromagnetic interference (EMI).
The proposed architecture is capable of driving a large parallel
CCFL array with only a single high voltage and high efficiency
converter, resulting in reduced size, weight and cost over existing
designs. Reduced ignition voltage (at near operating voltage) is
achieved by a unique capacitive coupling approach, based on the
similar principles to normal operation of electrodeless or external
electrode fluorescent lamps. However, the displacement current is
only required in our proposed architecture as a short high
frequency pulse during cold lamp ignition, which triggers
simultaneous lamp ignition of all parallel lamps. High impedance
current source circuit (based on MOSFET current mirror) is
connected in series with each lamp to ensure individual lamp
current regulation and control. In this paper, we discuss a suitable
architecture for large CCFL arrays and study new approaches for
lamp ignition to enable low frequency drive.
2.
2.1
Study of Lamp Behavior
DC Operation of Fluorescent lamps
A steady electric field across the plasma generated in the lamp
causes migration of active ions (light emitting specie), mercury in
the case of fluorescent lamps, from cathode to anode setting up a
gradient along the length of the lamp. The light luminance is
directly related to the mercury pressure and hence the axial
segregation of mercury under dc drive results in non-uniform
axial luminance distribution along the lamp. In order to avoid this
effect, sinusoidal ac voltage is conventionally used to drive linear
fluorescent lamps and most other discharge lamps.
In a recent investigation, addressing the problem of axial mercury
segregation (axial cataphoresis), the measured gradient in the
mercury pressure along the length (z) of the lamp (∂pn / ∂z) was
found to be proportional to the local mercury vapor pressure (pn):
(∂pn / ∂z) ∝ pn [3, 4]. Further, the study showed that axial
segregation under dc drive is predictable and can be quantified
based on lamp design parameters. If a limit is set on the allowed
luminance variation due to segregation, then bounds can be placed
on lamp parameters to meet the specifications. In our study of
low-frequency architecture, we observed that the non-uniformity
due to the thermometer effect was greater than that due to
cataphoresis. Axial luminance degradation is observed with near
dc drive, which from our experience and theoretical modeling is
within acceptable limits. The theoretical study show that it is also
possible to predict the phenomenon of cataphoresis, decrease its
effect and achieve a higher efficiency dc drive along with better
optical efficiency by redesigning the lamp and selecting optimal
parameters.
2.2
Lamp Ignition & Capacitive Coupling
From previous studies conducted to model the breakdown
phenomenon in long cylindrical tubes, it has become clear that the
tube wall and the electrical environment of the lamp play an
integral role in the initial phases of the breakdown process [5-7].
In [5], it is shown that starting at the active electrode (cathode),
subsequent sections of the wall are negatively charged and this
surface charge electrically shields the inside of the tube and
extends the cathode potential within the tube until the ionization
front reaches the anode. The process was modeled as a distributed
RC-line, with resistors representing the discharge gas and
capacitors representing the capacitance between the wall and the
metal tube at ground potential around the lamp. We now propose
our method of capacitive ignition based on our experiments.
ISSN0006-0966X/06/3701-0000-$1.00+.00 © 2006 SID
26.1 / M. Doshi
Igniter
Circuit
Floating Current
Control
Level
Shift
1: n
Ignition pulse command
Lamp current, PWM and
phase shift command
VDC
+
–
Level
Shift
Metal
Plate
Flyback
DC Power Supply
Backlight
Controller
Lamp
Terminal 1
(V1 )
Standard
DC Power
Supply
Standard
DC Power
Supply
Low-Frequency Square-Wave
Power Supply
Fig. 2: Typical high voltage Flyback converter circuit
Lamp
Terminal 2
(V2)
Supply,
biasing,
dimming ,
aux circuitry
Gate drive
(polarity control)
Rs
Fig. 1: Block diagram representation of low-frequency
square wave CCFL architecture.
We placed a metal plate beneath the lamp (~ 75% of lamp length)
and excited the plate using a high frequency ignition pulse drive.
The plate acts as an external electrode and energy is coupled into
the plasma through the metal plate (electrode) and conducting gas
capacitance (based on RC-line model). Lamp ignition is achieved
by applying a dc voltage across the electrodes followed by a short
pulse application of ac drive to the metal foil or plate. Our
experiments verified successful ignition and operation for long
lamps (> 25cm) with sustained operation after removing the ac
drive from the metal plate.
3.
Low Frequency Architecture
A block diagram for the low frequency architecture is shown in
Fig. 1. The system includes a high voltage LFSW source, an
ignition circuit that drives the lamp though capacitive coupling
during the capacitive ignition sequence, a single backlight
controller that coordinates lamp ignition and steady state voltage
output, and a current control (CC) circuit that stabilizes lamp
currents following ignition. The basic operation for each
functional block is described below.
3.1
High Voltage LFSW Power Supply
The primary function of the source is to generate a controlled low
frequency high voltage square wave to essentially drive the lamps
with dc current and equal duty cycles of positive and negative
polarity (to equalize electrode degradation). Depending upon the
lamp operating voltage, different approaches can be adopted to
generate a LFSW in order to achieve maximum drive efficiency.
For lamps with operating voltage less than 600 V, a full bridge
(H-bridge) employing high voltage switches powered from a
single high voltage dc-dc converter can be used. For higher
operating voltages, the square-wave can be generated by
controlling a cascade connection of two dc-dc converters, each
with their output connected in series with the lamp array. Figure 2
shows a typical Flyback converter topology that can used to
generate high voltage (with high turns ratio, n). The converter can
be controlled through the drive MOSFET and the control switch
(Senable) to power the lamp during one half the period and ground
the lamp terminal during the other half of the period of square
wave drive. Alternative designs, based on dc-dc or resonant
converters, can be used to generate the high output voltage [8, 9].
S enable
Current Source Circuit
Fig. 3: Block diagram representation of a unidirectional
current source
3.2
Igniter Circuit
The ignition circuit is used to drive the metal plate with a short
pulse (or pulses) for ignition of lamps. The applied voltage
required depends on many factors, including lamp characteristics,
temperature and distance between lamp sidewall and metal plate.
As shown in Section 4, we have demonstrated operation with
pulses in the few hundred volt range with periods in the ten
microseconds range. 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 wide ranges of resonant converter or pulse generator
approaches are suitable for the ignition circuit.
3.3
Current Control (CC)
The CC circuit is required in series with each lamp to stabilize
individual lamp currents following ignition and provide consistent
lumen output across the CCFL array. The CC devices must block
the maximum lamp-to-lamp voltage variation, which we have
measured to be less than 30 V [1], in our experimentation. The
low blocking voltage allows active devices to be used for current
control while still maintaining high efficiency. Figure 3 gives a
block diagram of a discrete unidirectional MOSFET current
source circuit, with current set by resistor Rs in series with
MOSFET. Note that each string of CC circuits (tied to the same
terminal) can be realized using a single integrated circuit (IC).
3.4
Backlight Controller
The backlight controller provides the drive signals for all blocks,
including the high voltage dc-dc converters, low frequency
switches, Senable (or bridge network), and the igniter circuit, and
may also provide communication with the CC circuits for
dimming control. For lamp ignition, high voltage dc is ramped up
together with the capacitive coupled ignition current. The
controller then monitors the total lamp array current through the
current sense feedback and continues ignition current until all
lamps are ignited (based on the total current sensed). For concept
evaluation purposes, the backlight controller is implemented using
a Xilinx Virtex IV FPGA board.
26.1 / M. Doshi
Differential
Voltage across
lamp
(V1 – V2)
Ignition Pulse
Terminal 1
Voltage
(V1)
Terminal 2
Voltage
(V2)
(a)
Differential
Voltage across
lamp
(V1 – V2)
Individual
Lamp
Current
Total Lamp
Array
Current
(a)
Differential
Voltage across
lamp
(V1 – V2)
Individual
Lamp
Current
Total Lamp
Array
Current
Ignition Pulse
Individual
Lamp
Current
Total Lamp
Array
Current
(b)
Fig. 4: (a) Four lamps operating in parallel [Length 250 mm,
dia 3.2 mm (top pair) and 2.6 mm (bottom pair)], (b)
operating waveforms of low frequency architecture:
Ch. 1: Differential lamp voltage (V1 – V2) (500 V/div),
Ch. 3: Individual lamp current - across Rs (5 mA/div),
Ch. 4: Total lamp current (scale: 20 mA/div)
Time Scale: 10 s/div
4.
Experimental Results
Experiments were conducted to study the lamp ignition
characteristics with 4 lamps in parallel as shown in Fig. 4(a). The
ac specifications are listed in Table 1.
Length Dia.
(mm) (mm)
White
Point
(K)
Ignition
Voltage
(Vrms)
Operating Lamp
Voltage Current
(Vrms)
(mArms)
250
3.2
5400
1300
520
5
250
2.6
8810
1300
525
5
Table. 1: AC lamp specifications [10]
500
Lamp
Ignition
Voltage
(VDC)
670
Typical
Operating
Voltage
(VDC)
610
500
670
600
Length
(mm)
Dia.
(mm)
Ignition
Pulse
(Vpk)
250
3.2
250
2.6
Lamp
Current
(mADC)
5
5
Table. 2: Experimental DC lamp operating conditions
(b)
Fig. 5: Experimental results demonstrating simultaneous
lamp ignition at near nominal operating voltage for (a)
negative going transition and (b) positive going transition.
Ch. 1 & 2: 500 V/div, Ch. 3: 5 mA/div, Ch. 4: 20mA/div
Time Scale: 200 μs
The experimental setup consisted of two CCM Flyback converters
(Fig. 2), a CC circuit built using discrete MOSFET current source
circuit (Fig. 3) and an ignition circuit with series resonant LC
circuit that generated damped sinusoidal ignition pulses. The
individual lamp current was measured across the sense resistor RS
in series with the MOSFET current source and the total current
was measured using a dc current probe at the output of LFSW
power supply (Terminal 1). The lamp ignition was achieved at a
dc drive of 670 V with a capacitive ignition pulse voltage of
500 Vpk (Table. 2). The steady state voltage drop across the lamp
was observed to be in range of 590 V to 620 V (lamp-to-lamp
variation of 30 V) and the current was regulated at a dc value of
5 mA by the current control circuit.
Figure 4(b) illustrates the operation of the low frequency
architecture. The polarity across the lamps was changed after 30 s
corresponding to LFSW of frequency = 0.016 Hz. The individual
lamp current and total lamp current is shown to be regulated at 5
mA (corresponding to 5 V across 1 kΩ resistor) and 20 mA,
respectively. The lamp current changes polarity from positive to
negative according to the lamp voltage transition. The LFSW
period is controlled thorough the FPGA and made to vary between
200 ms to 120 s (5 Hz to 0.008 Hz) to study the lamp behavior.
26.1 / M. Doshi
We varied the LFSW frequency through the complete range and
found no notable change in the output waveform characteristics.
Figure 5 illustrates the transient behavior of the low frequency
architecture during both positive and negative transitions of the
square wave. The ignition circuit is set to generate a pulse output
with a peak voltage of 500 Vpk at each LFSW transition. By
observing the total current, we see that simultaneous ignition of
the parallel lamps is achieved during both positive and negative
transitions. The control is provided such that the energy stored in
the output capacitor is recycled during the transition, leading to
higher efficiency. The effect is seen from the differential lamp
voltage waveform for e.g. in Fig 5(a), the voltage V1 drops
exponentially transferring energy to the inactive converter whose
voltage, V2, then rises from an initial value of few 100 V. It is
important to note that the peak lamp voltage during ignition is
near the normal operating voltage. In contrast, the rated lamp ac
ignition voltage is generally two to three times the rated (Table 1)
normal operating voltage. The current waveforms also show that
the electrode currents do not exceed normal operating levels. The
smooth transition waveforms and reduced component stress
during ignition lead to long lamp life. Since the total transition
time is approximately 500 μs, there is no visible flicker in the
lamp. Further, the converter transition can be controlled (lower
rise and fall times) to limit any generated EMI.
From the waveforms in Fig. 5 we can also see that the lamp
current drops to zero in less than 100 μs with a small decrease in
the applied voltage (e.g. < 50 V). This allows use of the CC
circuits to perform pulse-width-modulation (PWM) dimming of
the lamp since the required blocking voltage is low and the lamp
dynamics are sufficiently fast. Dimming is achieved through
on/off control of the CC circuit or by controlling the dead-time in
the LFSW transition. Continuous dimming can also be achieved
by controlling the command reference for the CC circuit. Again,
the fast lamp and circuit dynamics lead to smooth and continuous
dimming without giving rise to lamp flicker.
We have tested multiple lamp lengths with voltages from 200 V to
greater than 800 V to verify that ignition is achieved at nominal
operating voltage in each case. Further investigations on dimming
ranges and the lamp-to-lamp voltage variation were also carried
out, with results presented in [1].
5.
Conclusion
The proposed low frequency architecture results in a high
efficiency and low cost drive for parallel CCFL arrays used in
large screen backlighting applications. The system provides the
benefit of a single power converter capable of driving parallel
CCFL arrays for significant size, weight and cost reduction in
large screen LCD TVs. The low frequency drive results in
improved efficiency through the elimination of sensitivities to
parasitic capacitances associated with packaging and significantly
reduced EMI. A unique approach for lamp ignition is used based
on capacitive coupling and creates reliable, soft ignition of
parallel lamps for long lamp life and low component stress.
Experimental results demonstrate simultaneous ignition and
current regulation of an array of 4 lamps of 25 cm CCFLs based
on the proposed architecture using capacitively coupled ignition
and dc current regulation.
6.
Acknowledgements
The authors thank Dick McCartney of National Semiconductor
Corporation (NSC) for his conversations and input related to this
work. The work is co-sponsored by NSC through the Colorado
Power Electronics Center, the National Science Foundation (under
Grant No. 0348772) and Spanish Government through the project
CICYT TEC 2004-02607/MIC. Any opinions, finding and
conclusions or recommendations expressed in this material are
those of the authors and do not necessarily reflect the views of the
National Science Foundation.
7.
References
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Architecture for Multi-Lamp CCFL Systems with Capacitive
Ignition,” Applied Power Electronics Conference (APEC),
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