Andrew Smith from Power Integrationslooks at the
benefits and techniques for switching from CCFL to LED backlighting
on displays
The established technology for backlighting LCDs has been CCFL
(cold cathode fluorescent light), however, LED technology now
results in better displays than CCFL can provide.
The colour spectrum for CCFL lamps only spans about 80 per cent
of the permissible range due to the mercury arc spectrum and the
type of phosphors used.
CCFL lamps are based on mercury vapour arcs in glass tubes which
are not as robust as solid-state devices, which is why laptop users
can get a black screen if they drop their computer, even though the
rest of the electronics still function. Even without the computer
being dropped, the operational lifetime of LED technologies is much
longer than that of CCFL.
The mercury vapour is considered a toxic material in many areas
of the world, leading to government regulations requiring
appropriate disposal processes for fluorescent lamps. The RoHS
Directive in Europe and other national standards specifically
identify mercury compounds as hazardous. Although the CCFL tubes in
portable electronic systems are currently granted an exemption from
RoHS, industry analysts report that many display companies are
developing strategies to eliminate mercury, as well as the other
banned materials, from their products.
Added to that, CCFL lamps and their power circuitry use about a
third of the total power in laptop computers, making them the
single largest user of power within the laptop. Any technology that
can reduce this power drain will provide significant assistance to
the ongoing quest for longer system run time from a single battery
charge cycle.
Transition to LEDs
Given these limitations, equipment manufacturers are looking
into alternatives such as LEDs to replace CCFLs in backlighting
applications.
However, the design of a highly regulated, protected power
supply suitable for driving LED backlights is far from trivial.
In LCD TVs, one technique to replace CCFLs is to construct
strings of red, green and blue LEDs in a panel behind the LCD. A
string can be 20 or more LEDs in series. There are multiple strings
in a display, with the precise number of strings depending on
screen size. The LED strings are interleaved in a
red-green-green-blue sequence to produce zones of white light (see
Figure 1 below).
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Figure 1: RGB backlight power and control system
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This combination of LED strings produces a spectrum that covers
98 per cent of the NTSC colour space. The LED-backlit panel
produces more uniform illumination than a set of CCFL tubes and can
be adjusted for colour balance over a greater range of intensities.
This form of illumination is referred to as an RGB backlight.
To drive the LED string in the 60V – or 40V for red LED – range,
a constant current source is required.
A typical constant current/constant voltage-transitioning
characteristic for an actual LED string power supply is shown in
Figure 2. Around the operating voltage point, current is held
constant. The normal protection features can be applied to the
control scheme.
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Figure 2: Constant current characteristic for a 500W, 60V RGB
LED power supply
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The power supply for a string with 20 red LEDs would require a
40V output, since a single red LED has a Vf of about 2V at an If of
30mA max. The supply for each of the blue and green LED strings
would require a 60V output since the Vf is about 3.2V at an If of
30mA. A panel would use multiple parallel strings with
correspondingly increasing power requirements.
A moderately sized LCD TV system would require perhaps 15
parallel strings of LEDs with a 450-500mA current requirement. The
overall supply efficiency needs to be at least 90 per cent to
minimise heating issues and to achieve EnergyStar ratings for
operational efficiency. The LED supply would have over-temperature
and over-current protection to prevent thermal runaway and circuit
damage from short circuits.
LED brightness is adjusted by pulse width modulation of the
power supply unit output current in the downstream control
circuitry. The overall current distribution scheme needs
independent current adjustments for each string for colour balance
and to set the strings for uniform illumination.
Matching requirements and load balancing may not be too
critical, as the LEDs should be fairly closely matched as a set for
Vf, in colour and intensity. The control circuitry for colour and
brightness would also be the stages that compensate for low or high
Vf LED strings and overall illumination balance across the
screen.
RGB backlight power supply
The requirements for a 20 or 30W DC-DC constant current power
supply are straightforward.
First, the basic function of boosting the lower voltages in the
system to the higher ones needed by the LED string requires an
architecture that simultaneously meets the stringent requirements
of high efficiency, tight regulation, protection and low cost. Then
the designer has to develop the driver and control circuitry for
the specific supplies to satisfy the voltage and current
requirements.
The DPA switching regulator family has been designed to simplify
the task of designing power supplies for applications such as LED
backlighting. By combining the drive and control circuitry and
power Mosfet together in a single monolithic device, the complexity
of the switching and control functions to implement the DC-DC
converter has been removed.
The IC includes circuitry to accurately monitor temperature,
voltage and current. In addition, reference designs with full bills
of materials and PCB layouts for the specific applications are
available
The DPA-Switch family performs the conversion with minimal
external components.
By choosing a 400kHz switching frequency, the size of the boost
inductor and the filter capacitors are minimised. A monolithically
combined control circuit and power Mosfet simplifies the task of
the designer and minimises
board space.
The topology for the DC-DC converter is a non-isolated inductive
boost configuration (see Figure 3).
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Figure 3: A 60V supply for LED backlighting
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Circuit operation
L1, D1 and U1 are configured to provide a standard boost
converter topology. Boost voltage is regulated by U1 via Q2, which
provides a feedback current signal to Pin C of U1. C3, R1, C2 and
C6 provide loop stabilisation.
At turn on, input voltage rises (to 40Vdc), and the base of Q2
is driven by zener diodes VR1 and VR2 controlling the Q2 collector
feedback current signal. The load current causes a voltage drop
across R3 and R4. This voltage is filtered by R2 and C4 and drives
the base of Q2 taking over the control loop when the output voltage
drops below 40V (and VR1 and VR2 stop conducting). As such, the
output can be controlled for both constant output voltage (VR1,
VR2) and constant current (R3, R4).
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Figure 4: Efficiency versus power output
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The 40V supply converts a 24V input to a 40V, 500mA constant
current / constant voltage supply at an efficiency of greater than
90 per cent (see Figure 4 left).
The 30W 60V green and blue power supply is also shown. The
DPA-Switch family of IC’s includes all the necessary circuitry to
perform soft start, hysteric thermal protection, under-voltage,
over-voltage and current limit functions.
Andrew Smith is DC-DC product marketing manager at Power
Integrations
www.powerint.com