LED lighting teardowns: Five lighting designs that illuminate the future of lighting
Early SSL products are making their way onto store shelves and into inventory. These products can indicate what direction SSL design will take, at least in its early stages.
Margery Conner, Technical Editor -- EDN, October 7, 2010
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LED-based lighting is still far from a mainstream technology,
and its designs are in flux. Consumers have
not signaled the price-to-performance ratio for which
they will open their wallets and homes, and businesses
are reluctant to spend money in the current economic
climate. Nevertheless, early SSL (solid-state-lighting)
products are making their way onto store shelves and into inventory.
These initial designs can indicate what direction SSL design
will take, at least in its early stages.This article describes the tear-down of five LED-lighting products to see how they perform and what components and design topologies they use. It’s OK to design in the abstract or to speculate about the most effective ways to use a brand-new technology. The designers and manufacturers of these products, however, have made many assumptions about component pricing and availability, manufacturing and distribution pricing, features that prospective customers will want, and the prices that the market will bear. This level of uncertainty is common when manufacturers are introducing technologies. Five years from now, you’ll know what the market wants and is willing to pay for in efficient lighting, but no one now has a clue, partially because of the existence of so many as-yet-undetermined variables. What will the price of energy do? Will the government set an energy policy and stick with it? Will global-energy needs affect investment in energy-efficient hardware?
Considering all the unknowns that
face the introduction of SSL products,
it’s amazing that companies and investors
have the courage to invest. It falls
to the engineers’ lot to make the best
design they can with available components
for the price point set by the marketers.
It’s thus interesting and even
exciting to peek inside these products
and see the mind of the engineer and
the mind of the marketer.
This tour of tear-downs begins with
a 48-in. LED T8-sized tube light. You
can’t call it a “replacement” T8 light because
it doesn’t go into a fixture for fluorescent
tubes. Fluorescent tube lighting
requires a fixture with a ballast, the
lighting industry’s term for a fixture-enclosed
power supply for a light source.
This arrangement works for technologies
in which the light source, on average, wears out before the power-control
circuitry. Fluorescent-lighting fixtures
apply a voltage to a glass tube containing
vaporized mercury. The excited
mercury emits photons in the ultraviolet
wavelength; these photons strike
the phosphor coating on the inside of
the tube, in turn emitting light in the
visible wavelength. High-quality T8
fluorescent lights have efficacies of 100
lumens per watt and greater.It is impractical to directly replace
a fluorescent tube with an LED tube
because the two lights have different
power requirements. Most currently
available LED tube lights contain their
own ac/dc power supplies. In contrast,
fluorescent-light fixtures contain the
power-converting ballast.
Figure 1 shows an LED T8 tube light
from Alpine Electronics. Alpine also
provided a modified fluorescent-light
fixture with no ballast. Each 18W tube
emits 820 lumens, which works out to
almost 46 lumens per watt, or just about
half of what a high-quality fluorescent
tube light emits. Figure 2 shows that
each tube contains three rows of 96
LEDs. 
When the tube lights, the center
row of LEDs are a warmer, yellowish
white (Figure 3). The end cap routes
ac power down to the tubes’ internal
power supply (Figure 4). The aluminum
back is a thin, rounded cover that
touches the LED PCB (printed-circuit
board) only at the edges and doesn’t
provide much heat sinking. The PCB
has no metal core; it looks like a garden-variety fiberglass board. The board itself is thus not a heat sink. Figure 5 shows the power supply.
The part number on the three-terminal
power regulator is missing, so it yields
no part information. However, the part
includes a lot of electrolytic capacitors
(Figure 6). Two stapled-together PCBs
make up the 4-foot-long light. Figure 7 shows the staples that connect the
two boards, and Figure 8 shows the
top view of the staples and the jumpers
that route the power bus.
The specs for the light claim that
the tube has a 50,000-hour lifetime;
with all those electrolytic capacitors,
though, this figure seems dubious. It’s
possible to get a 50,000-hour lifetime
with electrolytic capacitors, but I think
the manufacturer may have just picked
the general-lifetime number for LED
components and used that figure for
the whole light. The tube’s innards exemplify
excellent manufacturing quality—much better than many other LED
lights and CFLs (compact fluorescent
lights).The LEDs are in a matrix of 288 diodes in 18 parallel strings with 16 diodes in each string. Each LED has a drop of approximately 3.2V, totaling approximately 50V across the array. The specifications state that the light is 18W, so each string consumes about 1W, meaning that each diode uses approximately 0.0625W. This figure is a far cry from the HB (high-brightness) LEDs that you usually encounter in designing for LED lighting, which use approximately 0.5 to 1W.
The power supply apparently outputs
51V dc—that is, although it measures
51V dc at the load, it could be a constant
current rather than a regulated-voltage power supply. Regardless,
all of the diode strings are paralleled
across the power-supply output—not
an ideal load for an LED matrix because,
as LEDs age, their current profiles
change. In an array like the one
in Figure 9, the string with the lowest
resistance pulls the most current, heating
the diodes and yielding differences
in LED output. One of the most important
characteristics of a light source is
an even, consistent intensity and color;
a matrix such as the one in this figure
is asking for hot spots. A better choice
would be a constant-current driver for
each string (Figure 10).Many power-management-IC vendors have developed their own LED-driver chips, such as Texas Instruments’ C2000 DSP-based IC driver, which lends itself well to applications with several strings. National Semiconductor, International Rectifier, Marvell, NXP, NEC, On Semiconductor, and several others also offer LED-driver chips, but the C2000 uses a DSP core with multiple PWM (pulse-width-modulated) outputs; one chip can provide a constant-current source for as many as seven LED strings.
You may be thinking that 18 strings would require 18 control loops. This requirement would be a problem for a cost-constrained tube light. Why not dispense with those wimpy 0.0625W LEDs and use some HB LEDs that will each put out 0.5W? Then you would need to use only 36 HB LEDs. This approach brings up a couple of other constraints, though. For example, 0.5W HB LEDs provide distinct, intense-point sources of light, and lighting designers and consumers alike don’t want that type of illumination. In addition, HB LEDs of this power have heat-dissipation issues: The 288 0.0625W LEDs more uniformly disperse heat and can use an inexpensive PCB. Using fewer high-power LEDs, however, requires a heat-dissipating substrate and may require the use of heat sinks, increasing the price of the tube light.
This design uses fewer expensive
LEDs, power-management
devices, and intense-point
sources of light, but it has uneven
current sourcing because
LEDs age unevenly, which affects
light quality and reliability.
The challenge for an LED-based
T8 LED-replacement light is to
cost-effectively replace today’s
$2 fluorescent light and maintain
the quality of light. Prices
for the Alpine T8 tube light
range from $65 (1000) to $95 (one)
per tube.Alpine can find customers, even
though its tube is competing against
$2 fluorescents, because LEDs’ longer
life can justify their higher costs in
some difficult-to-reach applications
when you consider replacement
costs, including labor,
downtime, and difficulty of
access. Early adopters who value
the color quality of LEDs and
who simply like having the latest
in technology also may be
willing to pay the premium.
You can’t consider the tube
light as a true replacement of a
fluorescent light because of the
modifications you must make to
a fluorescent-light fixture. An
example of a true replacement
light for a 40W incandescent
bulb is Home Depot’s recently
introduced EcoSmart dimmable
LED bulb (Reference 1).
The 8.6W light sells for $20 and
comes with a five-year warranty. The
light provides warm, diffuse light; dims
nicely; and produces no noticeable audio noise. It has a glass, dome-shaped
outer shell that covers the LEDs and
that doesn’t easily come apart, as you
can see in Figure 11. The LEDs are
not the usual intense light sources you
see in other LED lights, such as the
nondimmable, 7.5W bulb from TESS
(Topco Energy Saving System) Corp
that I disassembled in March (Reference
2 and Figure 12). That light uses
seven LEDs that output 560 lumens,
according to the specifications on the
packaging. These large-surface-area
LEDs provide a pleasant, diffuse light
source, and only two of them output
429 lumens at 8.6W.
Figure 13 shows a close-up of the
EcoSmart LEDs: I removed one to look
for a manufacturer’s label or mark because
officials at LSG (Lighting Science
Group), the bulb’s designer, don’t
want to divulge the company’s suppliers.
I couldn’t find a manufacturer’s label,
but there is an apparent part number,
AM6L1, and the part looks like
an LED array, meaning that the LED
packages several tiny LED chips in one
package and covers them with a single
phosphor. It’s a good choice to use such
a diffused light source because there is
no pixilation.To determine whose LEDs the light uses, I perused an LED catalog from Japanese LED manufacturer Citizen (Reference 3). It looks as though AM6L1 is similar to Citizen’s 6W CCL-L251 LED. In other words, the LSG derates the bulb’s two LEDs and runs them at less than 6W each—a smart, conservative design choice.
The electrolytic capacitor, which is
partially visible in the right side of the
figure, is a potential weak link, and
this design uses a high-quality part to
mitigate the risk of failure. The solder
joint is the Achilles’ heel of LED-lighting
reliability (Reference 5); using a
highly integrated LM3445 LED driver decreases the number of solder joints.
The metal baseplate of the LEDs
mounts directly on the finned metal
heat sink using a dab of thermal grease
(figures 16 and 17).
These two tear-downs have been
of lights that comply with a lighting
form factor. For my next project, I’ll
take a look inside a new lighting engine
from Cree, a manufacturer of LED
components. You can think of the
Cree LMR4 module as a light engine
that can serve as a building block for
a lighting luminaire (Figure 18 and
Reference 6). You can quickly disassemble
this light by removing several
screws. A large, white-metal hood encloses
the entire device (Figure 19).
The back of the LED unit lies to the
right of the penny, and the light cover
is above the unit. The cover has a simple
paper cone that serves as a reflector,
and a diffuser sits between it and a
clear-plastic light cover.Cree’s TrueWhite color-mixing technology
combines discrete white and red
LEDs. Other approaches to creating a
consistent warm white from LEDs rely
on combining multiple colored emitters
in one LED package or by tweaking
the phosphor. The LMR4’s TrueWhite implementation has five white
LEDs and three red LEDs. When you
turn on the light and gradually turn
up the power, the four primary white
LEDs and the two primary red LEDs
come on somewhat uniformly (Figure
20). As you continue to crank up the
power, the secondary white one and
then the secondary red one turn on. If
you crank it all the way up, the secondary
white LED comes fully on and rivals
the primary white LEDs in brightness,
whereas the secondary red LED
never appears to turn on much at all.
Plus, if you leave the light on at a power level in which the fifth white LED initially is off, it comes on after a couple of minutes, which perhaps means that the LED lights’ color changes with temperature and that the other two are balancing LED-color changes for both temperature and power. Cree’s marketing videos refer to “active color management” when describing TrueWhite, so the power and thermal response must be the active part.
An eight-pin TI 9C L2903 dual differential
comparator sits next to the
LEDs. This chip perhaps compares the
current through the main LEDs and
turns on the secondary color-balancing
LEDs when the current exceeds a maximum
threshold (Reference 5). Figure
21 shows the substrate after it has
popped off the baseplate, displaying the
metal core. The marking on the front,
“Berg MP A2,” looks like a Bergquist
Thermal Clad metal-core substrate, which comprises a circuit layer over a
dielectric layer over a base-metal layer. The substrate clamps onto an adhesive,
thermally and electrically conductive
layer to the baseplate of the power supply
(Figure 22). The National Semiconductor
LM3445 TRIAC-dimming
SULB is probably the power-management
IC, and the capacitors are 22-μF,
200V, 100°C Nichicon devices. The
power and ground wires loop through a
large ferrite bead to filter noise.
Cree specifies the power factor of the
LMR4 module at greater than 0.80 for
120V ac/60 Hz, or more than 0.90 for
230V ac/50 Hz. Measuring with my
trusty $20 Kill A Watt power meter
from P3 International yields a power
factor of 0.56. Granted, the Kill A Watt
is not the most sophisticated power meter
going, but 0.56 is a far cry from 0.8.
Removing the Lutron dimmer from the
circuit causes the module to operate directly
off ac-line voltage, increasing the
power factor to 0.91, so the TRIAC
dimmer is evidently not fully on even
when the switch indicates 100%.The previous tear-downs were all
production-LED lights currently for
sale. The next example is a Helieon
demo unit from Bridgelux (Figure 23).
Bridgelux and Molex teamed up to design a socket-and-module combination
for new installations (Figure 24).
The Helieon module includes a Bridgelux LED array mounted on an aluminum spreader, a lens, and a socket. The LED array can deliver 500 to 1500 lumens in 3000K warm white or 4100K neutral white, and the module’s optics shape the light path to deliver narrow, medium, or wide flood-beam angles. You can change the white-light unit’s color temperature and beam focus by swapping out the LED module. The socket attaches the LED module to the ceiling or the wall and delivers power to the fixture. The Helieon lacks the heat sinking necessary for a fully functional light; I suspect that this omission is the reason that it has a momentary on switch: to prevent evaluators from turning on and leaving on the evaluation kit, resulting in overheating.
The Helieon design also lacks power-management circuitry, but
it serves as another example of LED
emitters for LED lighting. Bridgelux
LEDs package a matrix of LED emitters
into one LED device, an approach
that’s similar to—but on a larger scale
than—the one that Citizen LED takes
in the EcoSmart bulb. The Bridgelux
device provides as many as 1500 lumens
in this package (Figure 25).Bridgelux intends the demo unit
for designers who want to evaluate
the Helieon LED module-and-socket
combination; the power-management
circuitry is there only to enable the
demonstration of the Helieon module.
Nevertheless, it illustrates that
the power management for LEDs is not trivial. The unit audibly ticks whenever
you plug in the brick and hums
when you hold down the momentary
power switch. The dimmer circuit
doesn’t use a TRIAC and dims the
light by only approximately 50%, rather
than virtually off. “Power management
is the bane of my existence,” says
Jason Posselt, vice president of sales for
Bridgelux, commenting on these undesirable
characteristics and likely voicing
the thoughts of many other LED
manufacturers.
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Talkback
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Nicely done, Margery Conner. Good analysis and insight. This article reminded me of the editor reviews and hands-on projects EDN used to do.
Millard Johnson - 2010-27-10 09:31:49 PDT -
Excellent article and objectivity in the analysis of LED products and their future. Thanks.
Deo V. Shenai, Dow Electronic Materials. - 2010-11-10 12:50:46 PDT
