LED固态照明入门介绍资料

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Solidstatelighting(SSL)referstosolidstatelightemittingdeviceswhichdonotutilizefilimentsorelectronarcstreamstoproducelight.ThemostreadilyrecognizableSSLproductstodayutilizelight-emittingdiodes(LEDs),whichproducephotons

(light)directlyfromthepassageofelectronsfromonematerialwithintheconductivedevicetoanother.OthersformsofLEDsincludeorganiclight-emittingdiodes(OLEDs)whichuseorganicmaterialsbutareotherwisesimilartoLEDsandpolymerlight-emittingdiodes(PLEDs),whicharemadefromflexiblematerials.OtherformsofSSLproductsincludeCathodluminescence-wherelightisgivenoffbyamaterialbombardedbyelectronsgeneratingheatwhichcausesthematerialtoemitphotons(light),orElectrolumenescencewherevisiblelightisproducedbyasubstanceexposedtoanelectricfieldwithoutanythermalenergygeneration.Theterm"solidstate"referstothefactthatlightisemittedfromasolidobject-asemiconductororotherphoto-reactivematerial.

Unliketraditionallighting,SSLsourcescreatevisiblelightwithoutheatasacomponentoftheluminousemission.Thisdoesnotmeanthatheatisnotgenerated.

LEDsproduceasmuchheatasanysourceofasimilarefficiency.However,theheatfromanLEDiscontainedinthesemiconductormaterial,tobesinkedorextractedawayfromthevisiblelightemission.

Atthistime,thelargestissueinadaptingLEDstogeneralilluminationisthermalmanagement.Heathasaneffectonlightoutput,andcolor,andisthefocusofagreatdealofresearchinthismarket.Further,asefficienciesincrease,andmethodsaredevelopedtoextracttheheatfromthesemi-conductormaterials,theheatgeneratedbyLEDswillbelowerthanconventionalsourcesofsimilarlumenproduction.Theultimategoalistoreachefficiencieshighenoughtosubstantiallyreducetheissueofthermalmanagementinlightingproductdesigns.

Asageneralrule,solid-stateproductsexhibitgreaterresistancetoshock,vibration,andtemperatureextremes,therebyincreasingtheirpotentiallifespansignificantly.However,directcontactwiththeLEDsourcesthemselves(pressingontheirface,ordirectlystrikingthem)destroystheinternalstructureofthedevices,requiringtheybeprotectedinsomemanner.

SSLhasbeendescribedbytheUnitedStatesDepartmentofEnergyasapivotalemergingtechnologythatpromisestoalterlightinginthefuture.Itisthefirstnewlightingtechnologytoemergeinover40years,withthepotentialforenergyefficienciesandcostsavings,ispotentiallyaverydisruptivetechnologyinthemarketplace.

1.LEDHistory

For150yearslightingtechnologywasmainlylimitedtoincandescence,fluorescence,orarcdischarge.However,energyefficienciesoftheseconventionalsourceshaveessentiallyreachedapeakofroughly25%(75%ofenergyconsumedisheat),withincandescentlightingachievinganefficiencyoflessthan2%.WiththeadventofcommercialLEDsinthe1960s,however,adoortoanewandexcitingformoflightingtechnologywasopened.Fromthebeginning,researchersrealizedthatLEDshaveatheoreticalpotentialtoexceedtheefficiencyofconventionalsources.

Justastransistorsreplacedtubesinelectronics,solidstatecircuitsconsumelessenergythroughareductioninparasiticloss(heat)thatcannotberesolvedwithconventionalglasstubetechnologiescurrentlyprevalentinlighting.

InitiallyLEDswereredincolor,withgreen,yellow,andorangevariantsfollowingsoonthereafter.In1993,ShujiNakamuraofNichiaChemicalIndustriescameupwithablueLEDusinggalliumnitride(GaN).Withthisinvention,itwaspossibletocreatewhitelightwithanLED,openingtheopportunitytoputthesedevicestoworkingeneralillumination..

2.TechnologyOverview

(1)WhiteLightGeneration

AsingleLEDcanonlyproduceasinglecolor,specificallyred,green,blue,amber,oryellow.ToproducethewhitelightitisnecessarytoconvertlightfromanLEDtolightspanningthevisiblespectruminsomefashion.Toattainthis,threeapproachesareusedforgeneratingwhitelight:Wavelengthconversion;Colormixing;andmostrecentlyHomoepitaxialZnSe.

WavelengthconversioninvolvesconvertingsomeoralloftheLED'soutputintovisiblewavelengths.Methodsusedtoaccomplishthisfeatinclude:

(a)BlueLED&yellowphosphor-Consideredtheleastexpensivemethodforproducingwhitelight.BluelightfromanLEDisusedtoexciteaphosphorwhichthenre-emitsyellowlight.Thisbalancedmixingofyellowandbluelightsresultsintheappearanceofwhitelight.

(b)BlueLED&severalphosphor-Similartotheprocessinvolvedwithyellowphosphors,exceptthateachexcitedphosphorre-emitsadiferentcolor.Similarly,theresultinglightiscombinedwiththeoriginatingbluelighttocreatewhitelight.Theresultinglight,however,hasaricherandbroaderwavelengthspectrumandproducesahighercolorqualitylight,albeitatanincreasedcost.

(c)Ultraviolet(Uv)LED&red,green,&bluephosphors-TheUVlightisusedtoexcitethedifferentphosphors,whicharedopedatmeasuredamounts.Thecolorsaremixedresultinginawhitelightwiththerichestandbroadestwavelengthspectrum.However,LEDmaterialsarehighlysusceptibletoUVlightdegradation,andinherentlyiltershortwavelengthemission,soarelimitedto450nmwavelength,whichislessefficientthanthe280nmwavelengthreactivephosphorsusedinfluorescentsources.

(d)BlueLED&quantumdots-Aprocessbywhichathinlayerofnanocrystalparticlescontaining33or34pairsofatoms,primarilycadmiumandselenium,arecoatedontopoftheLED.Thebluelightexcitesthequantumdots,resultinginawhitelightwithawavelengthspectrumsimilartoUVLEDs.

Thisisanemergingtechnology,thatpromisestoexceedanyofthephosphortechnologiesinefficiencyandcolorcontrol.

(e)ColormixinginvolvesutilizingmultipleLEDsinadeviceandvaryingtheintensityofeachLEDtoproducewhitelight,orbyconfiguringthedevicewithamixofLEDcolorstocreateadesiredwhitebalancewithoutcontrol.Asnophosphorsareused,thereisnoenergylostintheconversionprocess,therebyexhibitingthepotentialforhigherefficacyatsomepointinthefuture.However,inthenearfuture,whiteLEDsusingblueorvioletLEDsandphosphorcoating,arethemostpopoularwaytogeneratewhitelight.Incolormixing,colorrenderingnearthatofincandescentandday-lightingcanbeachievedbyahybridsystemusingRed,Green,Blue,Amber,andaWhiteLEDs.Asmanyas7colorscanbeusedtoreproduceexceptionallygoodwhitecolorbalance,usingRed,Red-orange,Amber,Green,Cyan,Blue,andIndigo.However,themoreLEDsused,thegreaterthecomplexityoftheproduct,thusahighertheexpense.Theseblendedcolorsystemsallofferthecapacitytoadjustwhitebalancemanuallyorautomatically,usingmicroprocessors,creatingasystemthannoconventionalsourcecanequal.

WhitephosphorLEDsgenerateexcellentcoloratlowcost.

(f)HomoepitaxialZnSeBlueLEDisanotherrecentLEDdevelopedbySumomitoElectricwhereahomoepitaxialZnSeblueLEDiscreatedthatproducesbluelightfromtheactiveregionandyellowemissionfromthesubstrate.TheresultingwhitelighthasawavelengthspectrumonsimilartoUVLEDscoatedinyellowphosphor.However,sincenophosphorsareused,theresultisahigherefficiency"white"LED,althoughcolorperformanceisstillinneedofimprovement.ThepromiseofthistechnologyisinitscombinationwithQuantumDots,thecombinationofthesetechnologiesishighlycomplimentaryandmoreefficientthatphosphorbasedwhitelightLEDs.

(2)LEDComponents

LEDsthemselvesarecalLEDdie,andtheyaremadejustlikeanyothermicro-chipdevice,inwafers.FromthesewafersindividualLEDsarecutawayusinglasersandplacedindeviceswithhighlyspecializedroboticdevices.EachLEDdieisaboutthediameterofathinmechanicalpencillead,andthinnerthanahumanhair,andproducesalimitedaboutoflightbyitself.InSSLapplications,aseveralLEDsareplacedclosetogetherinapackagetocombinetheiroutputtocreateilluminatingeffects.Thesesmallclustersareintegratedcircuits,likememorychipsinacomputer,andpackagedinsomeway-eitherintoachip,orinsideanenclosurewithanopticalcovering-withelectricalcontactpoints.Thesepackagesarethensolderedtocircuitboards,orstrips,whichareinstalLEDinlightingproducts.Unlikestandardlamps,circuitboardscarryingLEDscanbeconfiguredeasilytosuitvirtuallyanyconfiguration,shapeorenclosure.

AllSSLproductsrequireanelectronicdriverdevice.ThisisusuallyseparateoftheLEDcircuitboard,andconvertsthebuildingenergytoelectricalcharacteristicstheLEDdevicesrequiretoproducelight.Twocoremethodsofcontrolareused:Currentcontrol,andVoltageControl.

Onecontrolshowmuchenergypassesthroughthedevice,allowingvoltagetovaryupordowntomaintainoperation.Theothercontrolsthevoltagebyregulatinghowmuchenergyispassingthroughthedevice.Thesearenotinterchangeable,andarematchedtotheLEDorSSLpackagebeingpowered.LEDdevicesaregenerallyDCcurrent,whichmeansanadditionalconversionfromthebuilding’sACcurrentisincludedinthedriverdevices.

DimmingofanLEDsourceisnotassimpleasdimmingincandescentsources,butismoreeffectivethanfluorescentdimminginbothcolorandintensitycontrol.

AcommonmethodusedisPulseWidthModulation(PWM).

ThisisaccomplishedbyoperatingtheLEDsatahighfrequency,withmanypulsesasecondthatareconstantwhenthedeviceisatfullbrightness.

AdjustingthewidthofthepulseswillreducethenumberofelectronspassingthroughtheLED,reducingitslightoutput.Mixingcolorsisaccomplishedby“dimming”eachindividualcolorindependentlytoachieveadesiredbalance-whichcanrangefromonecolor(allothersdimmedoff)towhite(allcolorsbalancedandfullbrightness.)Otherdimmingschemes,includingcurrentlimiting,voltageregulation,etc.areeffectiveincontrolofintensitylevels.

3.AdvantagesofSSL

(1)TechnologicalComparison

SSLwilleventuallybeappliedasacost-effective,energyefficient,highqualityreplacementforincandescentandfluorescentlamps.TobetterunderstandthetechnicalmeritsofSSL,itisimportanttounderstandthetechnologybehindthelampsitintendstoreplace.However,justasthecostoffluorescentproductsexceedthatofincandescenttoattaintheenergygains,SSLproductswilllikelyalwaysbeslightlymoreexpensivethanfluorescentproducts.Withthepotentialofhighefficiency,improvedthermalperformance,controllability,highcolorperformance,andverylonglife,LEDshavethepotentialofproducingveryeffectivelightingsystemswithanadvantageinoperationalcoststhatwillmorethanjustifytheinitialcostsinvolved.

Incandescentlamps(lightbulbs)createlightbypassingelectricitythroughathinfilament,therebyheatingthefilamenttoaveryhightemperaturethateventuallyproducessomevisiblelight.Theincandescingprocess,however,ishighlyinefficient,since98%ofitsenergyisheat.Incandescentlamps,however,areinexpensivetoproduce.Halogenlampsaresimilartoincandescentlamps,bututilizehalogenfillgasandaquartzenvelopetoproducealittlemorelightandlongerlife.Thetypicallifespanofanincandescentlampisbetween750and2,000hours.

Fluorescentlamps(lightbulbs)workbypassingelectricitythroughmercuryvapor,whichinturnproducesultravioletlight.Theultravioletlightisthenabsorbedbyaphosphoruscoatinginsidethelamp,causingittoglow,orfluoresce.Whiletheheatgeneratedbyfluorescentlampsismuchlessthanitsincandescentcounterpart,eficienciesarestilllostingeneratingtheultravioletlightandconvertingthislightintovisiblelight.Inaddition,olderlampsusemercury,whichisdetrimentaltohealth,andshouldthelampbreak,exposuretothesubstancecanbehazardous.Theuseofmercuryisbeingreduceddramatically,andmayeventuallybeeliminatedasthefluorescentlampmanufacturersrespondtoenvironmentalconcerns.Fluorescentlampsaretypicallyfivetosixtimesthecostofincandescentlamps,buthavelifespansbetween10,000and20,000hours,withmanyextendedlifesourcesnowreaching40,000hours.

HighIntensityDischargesourcesworkbycreatinganelectronarcstreamthroughagasthatcontainsmetalsalts,forexamplesodiumcreatesayellowlight,whilescandiumandsodiummixedmakewhitelight.Thesesourcescreateanintenselightwithinasmallarea,whichisopticallyveryefficient.Theselampsproduceaboutthesameoverallefficiencyasfluorescentlamps,withalifeofbetween7,500and35,000hoursdependingonthedesignandapplicationofthelamp.

SSLgeneratesilluminationbygroupingsmallerLEDsinanorderlyfashion,therebycreatingapackagedsource.ASSLproductcanbeutilizesinglecolorLEDs,multiplewhiteLEDs,orcollectionsofdifferentcoloredLEDsconfiguredtomixandproducewhitelight.TheinherentadvantagesanddisadvantagesofSSLarethesameasthoseoftheLEDsthemselves.Advantagesinclude:

(a)Durable-nofilamentortubetobreak

(b)Longlife-LEDslastbetween15,000and70,000hourswhenproperlyapplied

(c)Lowpowerconsumption-reducesoverallelectricitybillcomparedtosomesources

(d)Flexibleapplication-smallsizeofLEDscanleadtouniquelightingdevices

(e)Uniquecolorpotential-abletocreatelightinwaysconventionalsourcescannotequal

(f)Reducedheatgeneration-heatandlightareindependent-reducingimpactonoccupants

(g)Nomercurytobedisposedofattheendofalamp’slife(unlikefluorescentproducts).

Currently,thenumberofSSLproductsonthemarketthatfunctionastruereplacementsforincandescentorfluorescentlampsissmall,butgrowingrapidly.WhiteLEDsproducedtodayaretooexpensivetobeconsideredaffordableagainstIncandescent,eventhoughtheyproducemorelight.However,overtheirlife,thesavingsinenergywillpaybackthecostdifferencemanytimesover.Comparedtofluorescent,lumenproductionbyLEDsisnowequaltothemostcommonlyappliedproducts.

FuturedevelopmentsinLEDtechnologywillcontinuetoimproveefficiency,colorperformance,andextendedthelife.BasedonresearchconductedbytheDepartmentofEnergy(DOE)andtheOptoelectronicsIndustryDevelopmentAssociation(OIDA),itisexpectedthatbytheyear2025,SSLwillbethepreferredmethodofilluminationinhomesandoffices.

ThefollowingchartcomparesSSLdevices(projectedto2025)withincandescentandfluorescentsources:

(2)Benefits

In2001,theUnitedStatesconsumedover7.2quads(7.2×1015BTU=7.6EJ)ofenergyonlightingforcommercial,residential,andindustrialbuildings.(USDOE).WithAmerica'ssteadygrowthandlimitedresources,thiscontinuedrateofconsumptionisnotsustainable.Recognizingtheneedforchange,theDOEhassetagoaltoreduceelectriclightingconsumption50%by2025.SSLtechnologiesareuniquelypositionedtoparticipateinreachingthisgoalandattainthefollowing:

(a)educeCO2emissions,therebypositivelyaffectingthegreenhouseeffect

(b)Decreaseby50%theglobalamountofelectricityusedforlighting

(c)Providehigherqualitylighting

(d)Decreaseby10%thetotalglobalconsumptionofelectricity(projectedtobeabout1.8TW·h/yr,or$120billionperyear,bytheyear2025)

(e)Reduceprojected2025globalcarbonemissionsbyabout300millionmetrictonsperyear

(f)Createnewindustriesandjobs

TheU.S.Government,bywayoftheDOEandotheragencies,hasfundedmillionsofdollarsinresearchgrantsandprojectsrelatingtothedevelopmentofahighqualityyetaffordableSSL.Amajormotiveoffundingsuchresearch,inadditiontoitsenvironmentalimpactandenergysavingspotential,istodecreasedependenceonforeignfossilfuels.

4.ResearchandDevelopment

Inordertofurtherthedevelopment