25650-Five_lighting_designs_illuminate_the_future_of_lighting_PDF

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LED-Lightingtear-downs:

A speciAl EDNsection


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52 EDN spEcial sEctioN | october 7, 2010

LED-based lighting is still far from a mainstream tech-nology, and its designs are in flux. Consumers havenot signaled the price-to-performance ratio for whichthey will open their wallets and homes, and businessesare reluctant to spend money in the current economicclimate. Nevertheless, early SSL (solid-state-lighting)

products are making their way onto store shelves and into inven-tory. These initial designs can indicate what direction SSL designwill take, at least in its early stages.

Early ssl ProductsarE making thEirway onto storEshElvEs and intoinvEntory. thEsEProducts canindicatE whatdirEction ssldEsign will takE,at lEast in itsEarly stagEs.

This article describes the tear-downof five LED-lighting products to seehow they perform and what compo-nents and design topologies they use.Its OK to design in the abstract or tospeculate about the most effective waysto use a brand-new technology. Thedesigners and manufacturers of theseproducts, however, have made manyassumptions about component pricing

and availability, manufacturing anddistribution pricing, features that pro-

spective customers will want, and theprices that the market will bear. Thislevel of uncertainty is common whenmanufacturers are introducing technol-ogies. Five years from now, youll knowwhat the market wants and is willingto pay for in efficient lighting, but noone now has a clue, partially because ofthe existence of so many as-yet-unde-termined variables. What will the price

of energy do? Will the government setan energy policy and stick with it? Willglobal-energy needs affect investmentin energy-efficient hardware?

Considering all the unknowns thatface the introduction of SSL products,its amazing that companies and inves-tors have the courage to invest. It fallsto the engineers lot to make the bestdesign they can with available compo-nents for the price point set by the mar-keters. Its thus interesting and evenexciting to peek inside these products

and see the mind of the engineer andthe mind of the marketer.

This tour of tear-downs begins witha 48-in. LED T8-sized tube light. Youcant call it a replacement T8 light be-cause it doesnt go into a fixture for flu-orescent tubes. Fluorescent tube light-ing requires a fixture with a ballast, thelighting industrys term for a fixture-en-closed power supply for a light source.This arrangement works for technolo-gies in which the light source, on aver-

Figure 1 The LED T8 tube light from Alpine

Electronics uses a fixture with no ballast

and has its own ac/dc power supply.

By margEry connEr tEchnical Editor

lightiNg DEsigNsuma fuu f


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leD-lightingtEar-DoWNs


age, wears out before the power-controlcircuitry. Fluorescent-lighting fixturesapply a voltage to a glass tube contain-ing vaporized mercury. The excitedmercury emits photons in the ultra-violet wavelength; these photons strikethe phosphor coating on the inside ofthe tube, in turn emitting light in thevisible wavelength. High-quality T8fluorescent lights have efficacies of 100lumens per watt and greater.

It is impractical to directly replacea fluorescent tube with an LED tubebecause the two lights have different

power requirements. Most currentlyavailable LED tube lights contain theirown ac/dc power supplies. In contrast,fluorescent-light fixtures contain thepower-converting ballast.

Figure 1 shows an LED T8 tube lightfrom Alpine Electronics. Alpine alsoprovided a modified fluorescent-lightfixture with no ballast. Each 18W tubeemits 820 lumens, which works out toalmost 46 lumens per watt, or just abouthalf of what a high-quality fluorescenttube light emits. Figure 2 shows that

each tube contains three rows of 96LEDs. When the tube lights, the cen-ter row of LEDs are a warmer, yellowishwhite (Figure 3). The end cap routesac power down to the tubes internalpower supply (Figure 4). The alumi-num back is a thin, rounded cover thattouches the LED PCB (printed-circuitboard) only at the edges and doesntprovide much heat sinking. The PCBhas no metal core; it looks like a gar-den-variety fiberglass board. The board

itself is thus not a heat sink.Figure 5 shows the power supply.

The part number on the three-terminalpower regulator is missing, so it yieldsno part information. However, the partincludes a lot of electrolytic capacitors(Figure 6). Two stapled-together PCBsmake up the 4-foot-long light. Figure

7 shows the staples that connect thetwo boards, and Figure 8 shows thetop view of the staples and the jumpersthat route the power bus.

The specs for the light claim thatthe tube has a 50,000-hour lifetime;with all those electrolytic capacitors,though, this figure seems dubious. Itspossible to get a 50,000-hour lifetimewith electrolytic capacitors, but I thinkthe manufacturer may have just pickedthe general-lifetime number for LEDcomponents and used that figure forthe whole light. The tubes innards ex-emplify excellent manufacturing quali-tymuch better than many other LEDlights and CFLs (compact fluorescentlights).

The LEDs are in a matrix of 288 di-odes in 18 parallel strings with 16 di-odes in each string. Each LED has adrop of approximately 3.2V, totalingapproximately 50V across the array.The specifications state that the lightis 18W, so each string consumes about

Figure 3 The T8 balances cool-white LEDs in the outside rows and warmer-white LEDs

in the center row to determine color.

Figure 4 The T8s end cap routes ac power down to the tubes internal power supply.

Figure 2 The T8 comprises three 96-LED rows.


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1W, meaning that each diode uses ap-proximately 0.0625W. This figure is afar cry from the HB (high-brightness)LEDs that you usually encounter in de-

signing for LED lighting, which use ap-proximately 0.5 to 1W.The power supply apparently outputs

51V dcthat is, although it measures51V dc at the load, it could be a con-stant current rather than a regulat-ed-voltage power supply. Regardless,all of the diode strings are paralleledacross the power-supply outputnotan ideal load for an LED matrix be-cause, as LEDs age, their current pro-files change. In an array like the onein Figure 9, the string with the lowestresistance pulls the most current, heat-ing the diodes and yielding differencesin LED output. One of the most impor-tant characteristics of a light source isan even, consistent intensity and color;a matrix such as the one in this figureis asking for hot spots. A better choicewould be a constant-current driver foreach string (Figure 10).

Many power-management-IC ven-dors have developed their own LED-driver chips, such as Texas Instru-ments C2000 DSP-based IC driver,which lends itself well to applications

with several strings. National Semi-conductor, International Rectifier,Marvell, NXP, NEC, On Semiconduc-tor, and several others also offer LED-driver chips, but the C2000 uses a DSPcore with multiple PWM (pulse-width-modulated) outputs; one chip can pro-vide a constant-current source for asmany as seven LED strings.

You may be thinking that 18 stringswould require 18 control loops. Thisrequirement would be a problem for acost-constrained tube light. Why not

dispense with those wimpy 0.0625WLEDs and use some HB LEDs that willeach put out 0.5W? Then you wouldneed to use only 36 HB LEDs. This ap-proach brings up a couple of other con-straints, though. For example, 0.5WHB LEDs provide distinct, intense-point sources of light, and lighting de-signers and consumers alike dont wantthat type of illumination. In addition,HB LEDs of this power have heat-dissi-pation issues: The 288 0.0625W LEDs

Figure 6 The five electrolytic capacitors (right) represent potential failure points in the

design; you must derate them for use at higher temperatures.

Figure 7 Two stapled short boards comprise the longer PCB.

Figure 8 A top view of the boards shows the staples and jumpers that connect the boards.

Figure 5 The lights internal power supply extends halfway along the back of the LED PCB.


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more uniformly disperse heatand can use an inexpensive PCB.Using fewer high-power LEDs,however, requires a heat-dissi-

pating substrate and may requirethe use of heat sinks, increasingthe price of the tube light.

This design uses fewer ex-pensive LEDs, power-manage-ment devices, and intense-pointsources of light, but it has un-even current sourcing becauseLEDs age unevenly, which af-fects light quality and reliability.The challenge for an LED-basedT8 LED-replacement light is tocost-effectively replace todays$2 fluorescent light and main-tain the quality of light. Pric-es for the Alpine T8 tube lightrange from $65 (1000) to $95 (one)per tube.

Alpine can find customers, even

though its tube is competing against$2 fluorescents, because LEDs long-er life can justify their higher costs in

some difficult-to-reach appli-cations when you consider re-placement costs, including la-bor, downtime, and difficulty of

access. Early adopters who valuethe color quality of LEDs andwho simply like having the lat-est in technology also may bewilling to pay the premium.

You cant consider the tubelight as a true replacement of afluorescent light because of themodifications you must make toa fluorescent-light fixture. Anexample of a true replacementlight for a 40W incandescentbulb is Home Depots recentlyintroduced EcoSmart dimma-ble LED bulb (Reference 1).The 8.6W light sells for $20 and

comes with a five-year warranty. Thelight provides warm, diffuse light; dimsnicely; and produces no noticeable au-

CAN=CONTROLLER-AREA NETWORK

DALI=DIGITAL ADDRESSABLE-LIGHTING INTERFACE

I2C=INTER-INTEGRATED CIRCUIT

SPI=SERIAL-PERIPHERAL INTERFACE

UART=UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER

CPU

32-BIT,

60-MHz

DSP CORE

I2C

SPI

UART

CAN

32-BIT,

60-MHz

FPU

16-CHANNEL,

12-BIT,

5M-SAMPLE/SEC

ADC

VREF

COMMUNICATIONS

PWM7

PWM6

PWM3

PWM4

PWM5

PWM2

PWM1

DALI

POWER-LINE

COMMUNICATIONS

C2000 PICCOLO F2803X

BUCK/BOOST SINGLE-ENDED

PRIMARY-INDUCTANCE CONVERTERLED CURRENT

CONTROL AND DIMMING

PFC

OPTIONAL

CYCLE-

BY-

CYCLE

CURRENT

LIMIT

VACFILTER

AND

DIODE

++

+

+

3.3V

Figure 10 A constant-current driver for each LED string is a better power topology for LED matrixes.

LED2

LED16

LED1

LED2

LED16

LED1

LED2

LED16

LED1

51V DCSTRING1 STRING2 STRING18

Figure 9 In a multistring LED array with one power source,

the string with the lowest resistance pulls the most current.


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dio noise. It has a glass, dome-shapedouter shell that covers the LEDs andthat doesnt easily come apart, as youcan see in Figure 11. The LEDs arenot the usual intense light sources yousee in other LED lights, such as thenondimmable, 7.5W bulb from TESS(Topco Energy Saving System) Corpthat I disassembled in March (Refer-ence 2 and Figure 12). That light usesseven LEDs that output 560 lumens,according to the specifications on thepackaging. These large-surface-area

LEDs provide a pleasant, diffuse lightsource, and only two of them output429 lumens at 8.6W.

Figure 13 shows a close-up of theEcoSmart LEDs: I removed one to lookfor a manufacturers label or mark be-cause officials at LSG (Lighting Sci-ence Group), the bulbs designer, dontwant to divulge the companys suppli-ers. I couldnt find a manufacturers la-bel, but there is an apparent part num-ber, AM6L1, and the part looks likean LED array, meaning that the LED

packages several tiny LED chips in onepackage and covers them with a singlephosphor. Its a good choice to use sucha diffused light source because there isno pixilation.

To determine whose LEDs the lightuses, I perused an LED catalog from Jap-anese LED manufacturer Citizen (Ref-erence 3). It looks as though AM6L1is similar to Citizens 6W CCL-L251LED. In other words, the LSG deratesthe bulbs two LEDs and runs them at

Figure 13 The EcoSmart LEDs have a large surface area that provides a pleasant, dif-

fuse light source.

Figure 14 Removing the encapsulant that surrounded the EcoSmarts drive electronics

was messy, but surprisingly easy.

Figure 11 The EcoSmart bulb has a

glass outer shell that doesnt easily

disintegrate.

Figure 12 An LED bulb from TESS Corp has multiple intense-LED sources; two of them

output 429 lumens at 8.6W.


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october 7, 2010 | EDN spEcial sEctioN 61

less than 6W eacha smart, conserva-tive design choice.

A rubbery compound encapsulatesthe electronicsa good choice for

lighting technology because encapsu-lation cushions the electronics fromall the vibrations inherent in a small,easily accessed light bulb (Figure 14).Its not so nice for a tear-down, how-ever. Nevertheless, the rubbery encap-sulation material comes off fairly easily,exposing all of the drive electronics.The most promising ICthat is, theone with the most leadsis a 10-pinMSOP with a cryptic SULB mark-ing on the top (Figure 15). A quickGoogle search reveals SULB as thetop mark for National Semiconduc-tors LM3445 TRIAC (triode-alter-nating-current)-switch-dimmable LEDdriver (Reference 4). I could see onlyone 50-F Nichicon electrolytic ca-pacitor, which operates at 105C. The

black capacitor-like components in thefigure are inductors.

The electrolytic capacitor, which ispartially visible in the right side of the

figure, is a potential weak link, and

this design uses a high-quality part tomitigate the risk of failure. The solderjoint is the Achilles heel of LED-light-ing reliability (Reference 5); using a

highly integrated LM3445 LED driver

Figure 15 SULB indicates that the LED

driver for the EcoSmart bulb is National

Semiconductors LM3445 TRIAC-switch-

dimmable LED driver.

Figure 16 The bottom of the LED base-

plate shows the LEDs with a dab of ther-

mal grease on their substrates.


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decreases the number of solder joints.The metal baseplate of the LEDsmounts directly on the finned metalheat sink using a dab of thermal grease

(figures 16 and 17).Compare this approach with theseven-LED light from TESS, in whichthe LEDs sit on a metal-core substrateand then on a flexible thermal inter-face before mounting on the heat sink.EcoSmart uses a simple approach thatquickly removes the heat from theLEDs. Its overall design philosophy in-creases reliability by reducing the partscount and, thus, the associated solderpoints.

These two tear-downs have beenof lights that comply with a lightingform factor. For my next project, Illtake a look inside a new lighting en-gine from Cree, a manufacturer of LEDcomponents. You can think of theCree LMR4 module as a light enginethat can serve as a building block fora lighting luminaire (Figure 18 andReference 6). You can quickly disas-semble this light by removing severalscrews. A large, white-metal hood en-closes the entire device (Figure 19).The back of the LED unit lies to theright of the penny, and the light cover

is above the unit. The cover has a sim-ple paper cone that serves as a reflec-tor, and a diffuser sits between it and aclear-plastic light cover.

Crees TrueWhite color-mixing tech-nology combines discrete white and redLEDs. Other approaches to creating aconsistent warm white from LEDs relyon combining multiple colored emit-ters in one LED package or by tweak-ing the phosphor. The LMR4s True-White implementation has five whiteLEDs and three red LEDs. When you

turn on the light and gradually turnup the power, the four primary whiteLEDs and the two primary red LEDscome on somewhat uniformly (Figure20). As you continue to crank up thepower, the secondary white one andthen the secondary red one turn on. Ifyou crank it all the way up, the second-ary white LED comes fully on and ri-vals the primary white LEDs in bright-ness, whereas the secondary red LEDnever appears to turn on much at all.

Plus, if you leave the light on at a pow-er level in which the fifth white LEDinitially is off, it comes on after a cou-ple of minutes, which perhaps means

that the LED lights color changes withtemperature and that the other two arebalancing LED-color changes for bothtemperature and power. Crees market-ing videos refer to active color man-agement when describing TrueWhite,so the power and thermal responsemust be the active part.

An eight-pin TI 9C L2903 dual dif-ferential comparator sits next to theLEDs. This chip perhaps compares thecurrent through the main LEDs and

turns on the secondary color-balancingLEDs when the current exceeds a maxi-mum threshold (Reference 5). Fig-ure 21 shows the substrate after it haspopped off the baseplate, displaying themetal core. The marking on the front,Berg MP A2, looks like a BergquistThermal Clad metal-core substrate,

Figure 17 The baseplate of Citizens LED mounts directly on the large metal heat sink.

Figure 18 You can quickly disassemble Crees LM4 light engine by removing several

screws.


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which comprises a circuit layer over adielectric layer over a base-metal layer.The substrate clamps onto an adhesive,thermally and electrically conductive

layer to the baseplate of the power sup-ply (Figure 22). The National Semi-

conductor LM3445 TRIAC-dimmingSULB is probably the power-manage-ment IC, and the capacitors are 22-F,200V, 100C Nichicon devices. The

power and ground wires loop through alarge ferrite bead to filter noise.

Figure 19 A large, white-metal hood encloses Crees LM4. An LED unit lies to the right

of the penny, and the light cover sits above it. The cover has a simple paper cone that

serves as a reflector; a diffuser sits between the cone and the clear-plastic light cover.

Figure 20 Cree uses five white LEDs, which appear yellow when off, and three red

LEDs to perform active color management.


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Cree specifies the power factor of theLMR4 module at greater than 0.80 for120V ac/60 Hz, or more than 0.90 for230V ac/50 Hz. Measuring with my

trusty $20 Kill A Watt power meterfrom P3 International yields a powerfactor of 0.56. Granted, the Kill A Wattis not the most sophisticated power me-ter going, but 0.56 is a far cry from 0.8.Removing the Lutron dimmer from thecircuit causes the module to operate di-rectly off ac-line voltage, increasing thepower factor to 0.91, so the TRIACdimmer is evidently not fully on evenwhen the switch indicates 100%.

The previous tear-downs were allproduction-LED lights currently forsale. The next example is a Helieondemo unit from Bridgelux (Figure 23).Bridgelux and Molex teamed up to de-

sign a socket-and-module combinationfor new installations (Figure 24).

The Helieon module includes aBridgelux LED array mounted on analuminum spreader, a lens, and a sock-et. The LED array can deliver 500 to1500 lumens in 3000K warm white or4100K neutral white, and the modulesoptics shape the light path to delivernarrow, medium, or wide flood-beamangles. You can change the white-light

units color temperature and beam fo-cus by swapping out the LED module.The socket attaches the LED moduleto the ceiling or the wall and deliv-ers power to the fixture. The Helieonlacks the heat sinking necessary for afully functional light; I suspect thatthis omission is the reason that it hasa momentary on switch: to preventevaluators from turning on and leav-ing on the evaluation kit, resulting inoverheating. The Helieon design also

Figure 21 The LEDs sit on a substrate with a metal core and clamp onto an adhesive,

thermally and electrically conductive layer to the baseplate of the power supply.

Figure 22 This power-management IC is most likely National Semiconductors TRIAC-

dimming LM3445, and the capacitors are 22-F, 200V, 100C Nichicon devices. Note

that the power and ground wires loop through a large ferrite bead to filter noise.

the helieon mod-

ule includes a

bridgelux led

array mounted

on an aluminum

spreader, a lens,

and a socket.


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lacks power-management circuitry, butit serves as another example of LEDemitters for LED lighting. BridgeluxLEDs package a matrix of LED emit-ters into one LED device, an approachthats similar tobut on a larger scalethanthe one that Citizen LED takesin the EcoSmart bulb. The Bridgeluxdevice provides as many as 1500 lu-

mens in this package (Figure 25).Bridgelux intends the demo unit

for designers who want to evaluatethe Helieon LED module-and-socketcombination; the power-managementcircuitry is there only to enable thedemonstration of the Helieon mod-ule. Nevertheless, it illustrates thatthe power management for LEDs is not

trivial. The unit audibly ticks when-ever you plug in the brick and humswhen you hold down the momentarypower switch. The dimmer circuitdoesnt use a TRIAC and dims thelight by only approximately 50%, rath-er than virtually off. Power manage-ment is the bane of my existence, saysJason Posselt, vice president of sales forBridgelux, commenting on these unde-sirable characteristics and likely voic-ing the thoughts of many other LEDmanufacturers.EDN

RefeRences

1 EcoSmart LED A19 40 Watt Equiva-

lent Light Bulb, Home Depot, http://bit.

ly/a0FoqS.2 Conner, Margery, Tear-down: inside

a 7W LED light bulb, EDN, March 18,

2010, http://bit.ly/c4CoY4.3 CITILED: The Light Engine, Citizen

Electronics, http://bit.ly/csdQhi.4 Triac Dimmable Offline LED Driver

from the PowerWise Family, National

Semiconductor, http://bit.ly/aHfq8e.

5 Conner, Margery, Burnout: Weaklinks affect HB-LED lifetime, EDN, Feb

18, 2010, http://bit.ly/dcPoId.6 LM193, LM293, LM293A, LM393,

LM393A, LM2903, LM2903V dual dif-

ferential comparators, Texas Instru-

ments, 2010, http://bit.ly/9MsSYY.

Figure 23 Bridgelux and Molex teamed up to design a socket-and-module combina-

tion for new installations.

Figure 24 The Helieon module includes a Bridgelux LED array, a lens, and an aluminumspreader.

Figure 25 When you look through green

welders goggles, the arrays of emitters

in an LED show faintly.

+ Go to www.edn.com/101007led for

links to the Web sites of companies this

article mentions.

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