201303.pdf 36567KB Mar 22 2013 09:11:22 PM

Highly touted organic light-emittingdiode
(OLED) technology, as recently
reported, may not come anytime soon,
if ever. The investment required for this
“bleeding edge” large-panel technology
is prohibitive. In contrast, large display
panels with state-of-the-art thin-filmtransistor
LCD technology and “smart”
direct LED backlighting with local dimming
are far less expensive than OLEDs
but compare well for both power consumption
and picture quality.
Today’s LCD TVs—even those with
LED backlighting—are still some distance
from achieving the efficiency targets
they will face in the coming years.
New design techniques in LED-driver
circuits, however, promise to deliver
significant energy savings that will go
a long way toward helping TV manufacturers
meet tough requirements for
power consumption.
Changing requirements
Standards for TV power consumption
were raised in 2008, and each year the
specification reduces the amount of
power a TV can draw. The current maximum
for any size screen is 85W, making
the design challenge even tougher for
large-screen TVs.
Energy Star is an international
standard for energy-efficient consumer
products. Compliance is voluntary—
and highly influential—but it’s not the
only form of regulation. The state of
California’s Energy Commission, for
example, introduced its own standard,
which went into effect in 2011. This
regulation is a bit tougher than Energy
Star and also has real teeth: It prohibits
the sale of TVs in California that do not
meet the state’s efficiency specifications.
In Europe, regulations have for many
years allowed direct comparison of the
energy consumption of white goods (EU
Energy Label), and customers use it as
a basis for purchasing decisions. These
regulations are now mandatory for TVs,
cars, and household appliances.
at a gL anC e
↘ Standards regulating TV power
consumption continue to strengthen,
meaning designers must develop
new ways to meet efficiency
targets.
↘ LED backlighting accounts for
30% to 70% of overall system
power in LCD TVs, making this area
a good candidate for improving
energy efficiency.
↘ Manufacturers are investigating
sophisticated methods for reducing
power consumption, including
feedback regulation and smart
dimming.
efficiency can deliver a large combined
saving.
There are two ways to implement
LED backlighting (Figure 1). In indirect,
or edge-lit, backlighting, the LEDs
are arranged at the edges of the screen.
A light guide distributes the light uniformly
across the display. This arrangement
can be deployed with good optical
uniformity in screen sizes up to 60 in.
and enables backlighting units with a
thickness of just 5 to 10 mm.
In direct-backlit systems, the LEDs
are located directly behind the LCD,
enabling low power, good thermal
design, and excellent scalability with
practically no limit to the screen size.
These panels tend to be thicker than
edge-lit versions, but with the latest
technologies for light distribution, displays
as thin as 8 mm can now be found.
An important advantage of direct
backlighting is that it enables sophisticated
local dimming, which lowers
power consumption and increases the
dynamic contrast ratio, allowing the
latest TV designs to compare favorably
with OLED technology.
system arChiteCture
The choice of architecture for a backlit
LED-driver system is the decision with
the greatest potential to produce power
savings and significantly enhanced picture
quality. The designer looks for the
best balance between local control of
strings of LEDs and the lowest possible
BOM.
In a single-string, single-dc/dc-converter
backlit system, a switched-mode
power supply (SMPS) is used to provide
the voltage for backlit LEDs arranged
in strings. A current sink is included to
regulate the current through the LED
string. To minimize power dissipation,
the voltage at the I LED
sink needs to be
a fraction above the voltage necessary
to guarantee that the LEDs receive their
specified current (Figure 2).
A common design approach is to
use a feedback path from the I LED
sink
to the SMPS to regulate the SMPS’s
output voltage. This feedback path is
required to allow for variations in forward
voltage (V F
) from one LED to
another. The typical forward voltage of
a white LED is approximately 3.2V—
and may vary as much as ±200 mV per
LED—so, for example, in a string of
10 LEDs, the total for V LED
may range
from 30 to 34V.
The voltage that is required at the
dc/dc converter can be expressed as
V DC-DC
=V LED
+V SINK
; V LED
=n×V F(LED)
. V SINK
LeD baCkLighting
LED backlighting power ranges from
30% to 70% of overall system power
in LCD TVs, so improvements in the
efficiency of the backlighting power
circuit can make a considerable contribution
to system efficiency. As is
often the case in power system design,
a number of small improvements in
(a)
(b)
Figure 1 LCD TVs can adopt one of two arrangements for LED backlighting: indirect
(edge light) (a) or direct (b).
32 EDN | March 2013 [ www.edn.com ]