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Research Article Experimental Study on Active Cooling Systems Used for Thermal Management of High-Power Multichip Light-Emitting Diodes Mehmet Kaya Department of Mechanical Engineering, Erzincan University, 24100 Erzincan, Turkey Correspondence should be addressed to Mehmet Kaya; Received 26 March 2014; Revised 10 July 2014; Accepted 16 July 2014; Published 5 August 2014 Academic Editor: Kamal Aly Copyright © 2014 Mehmet Kaya. Tis is an open access article distributed unde
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  Research Article Experimental Study on Active Cooling Systems Used for ThermalManagement of High-Power Multichip Light-Emitting Diodes Mehmet Kaya  Department of Mechanical Engineering, Erzincan University, 󰀲󰀴󰀱󰀰󰀰 Erzincan, urkey  Correspondence should be addressed to Mehmet Kaya; 󰀲󰀶 March 󰀲󰀰󰀱󰀴; Revised 󰀱󰀰 July 󰀲󰀰󰀱󰀴; Accepted 󰀱󰀶 July 󰀲󰀰󰀱󰀴; Published 󰀵 August 󰀲󰀰󰀱󰀴Academic Editor: Kamal Aly Copyright © 󰀲󰀰󰀱󰀴 Mehmet Kaya.TisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,whichpermits unrestricted use, distribution, and reproduction in any medium, provided the srcinal work is properly cited.Teobjectiveothisstudywastodevelopsuitablecoolingsystemsorhigh-powermultichipLEDs.othisend,threedifferentactivecooling systems were investigated to control the heat generated by the powering o high-power multichip LEDs in two differentcon󿬁gurations (󰀳󰀰 and 󰀲  ×  󰀱󰀵W). Te ollowing cooling systems were used in the study: an integrated multi-󿬁n heat sink designwith a an, a cooling system with a thermoelectric cooler (EC), and a heat pipe cooling device. According to the results, all threesystemswereobservedtobesufficientorcoolinghigh-powerLEDs.Furthermore,itwasobservedthattheintegratedmulti󿬁nheatsink design with a an was the most efficient cooling system or a 󰀳󰀰W high-power multichip LED. Te cooling system with a ECand 󰀴󰀶W input power was the most efficient cooling system or 󰀲  ×  󰀱󰀵W high-power multichip LEDs. 1. Introduction A light-emitting diode (LED) is actually a semiconductordiode. However, it differs rom other normal diodes becauseit is manuactured or lighting applications. LEDs convertelectrical energy to direct light [󰀱, 󰀲]. As the composition o  different substances used to manuacture LEDs determinesthe colour o light emitted, LEDs can be manuactured toprovide a wide spectrum o wavelengths, rom inrared toultraviolet.Lightingsystemssuchas󿬁lamentglowlampsand󿬂uorescent tubes emit light by a heating or chemical process,whereasLEDsproducelightthroughphotonsthatareemittedrom semiconductor junction areas [󰀲]. Hence, LEDs haveadvantages such as longevity, higher efficiency, and smallerdimensions over other lighting systems.In recent years, LEDs have been regarded as the most valuable light source because they have many advantages,such as energy consumption, durability, easy installation,󿬂exible design, environmental compliance, higher light out-put with the same power consumption, rapid response, andlongevity. LEDs have higher efficiency to reduce energy con-sumption. Because o reduction in the required energy,higher efficiency LEDs will reduce the using o ossil uelthat will provide a decrease in the environmental pollutionand global warming caused by carbon dioxide and suchgases [󰀳–󰀸]. oday, LEDs are replacing other light sources in many󿬁elds,suchasindoor-outdoorandautomotivelighting,screen backlighting, guideboards, signal systems, and panelscreen lighting [󰀴, 󰀵]. Te nature o LEDs is such that they  convert approximately 󰀲󰀰–󰀳󰀰% o unused energy to light andconverttherestotheenergytoheat[󰀵,󰀹].Teheatadversely  affects the light quality, efficiency, and longevity o LEDs.For example, a chip junction temperature above 󰀱󰀵󰀰 ∘ C or a󰀱󰀰 ∘ C increase in an LED device increases the power usedby the LED and also reduces light quality and LED lie by hal. Moreover, based on the physical interior structure o anLED,theLED’schipjunctiontemperaturemustbekeptunder󰀱󰀲󰀰 ∘ C to prevent damage [󰀴, 󰀷, 󰀱󰀰, 󰀱󰀱]. For this reason, the heat generated by LEDs should be controlled via an efficientcooling system and extracted rom LED device. Te need orhigher-levellightsourcesrequiresthemanuacturingoLEDswith higher energy levels. Compared to the heat generatedby LEDs with lower energy levels, those with higher energy levels generate larger amounts o heat. Tus, such LEDsrequire a more efficient cooling system [󰀴].Te literature suggests that there is no agreed-upon solu-tion regarding the development o systems that are relatedto the efficient extraction o the heat generated by high-energy LEDs rom LED devices themselves. Accordingly,studies are still being conducted on the development o  Hindawi Publishing Corporatione Scientific World JournalVolume 2014, Article ID 563805, 7 pages  󰀲 Te Scienti󿬁c World Journaleffective and efficient cooling systems or high-energy LEDs.Tereore, the development o more efficient cooling systemsor high-power LEDs remains a useul area o study [󰀱, 󰀴, 󰀵, 󰀷, 󰀱󰀰, 󰀱󰀲, 󰀱󰀳]. Among these reerences, one o the well- known studies was perormed by Lu et al. [󰀱] to improve thethermal characteristics o high-power LED (light-emittingdiode) package using a 󿬂at heat pipe. In that study obtainedresults indicated that the junction temperature o LED isabout 󰀵󰀲 ∘ C. o improve the heat dissipation o high-powerlight-emitting diodes having  6 × 3 W LEDs in two rowscooling system with thermoelectric cooler were used by Liet al. [󰀴]. It was ound that temperature o the substrateo LEDs reached 󰀲󰀶 ∘ C without EC, while it was only 󰀹 ∘ Cwhen the best rerigeration condition appears by using EC.Another well-known study was done by Cheng et al. [󰀵]to predict heat dissipation o high-power LED system andprediction o LED chip junction temperature by using 󿬁niteelement method. In that study it was ound that, using aan at the side wall o the heat sink channel to increase theconvective heat transer coefficient is an effective method toreduce the LED chip junction temperature. Moreover, Wanget al. [󰀷] perormed an experimental study to investigatethe thermal perormance o the vapor chamber which hasbeen used or cooling system o 󰀳󰀰 Watt high-power LEDs.Tey have ound that plate works out hot-spot problem o 󰀳󰀰Watt high-power LEDs was successully. Particularly, Li et al.[󰀱󰀲] perormed an experimental study on cooling o LEDillumination package ranging rom 󰀳󰀰W to 󰀳󰀰󰀰W using theloop heat pipe heat sink. Tey have indicated that measuredthermalperormanceoloopheatpipeheatsinkwassuperiortoanyconventionalpassivethermalmanagementsolutionsinterms o heat sink weight.ActiveandpassivecoolingsystemsareusedtocoolLEDs.Passivecoolingsystemsaresufficienttocoollow-powerLEDsbut not high-power LEDs. Tus, different cooling systems,such as the integrated multi󿬁n heat sink design with a an,coolingsystemseaturingathermoelectriccooler(EC),andheat pipe cooling devices, are used to cool high-power LEDs[󰀱, 󰀴, 󰀱󰀲]. High-power LEDs and LED devices are manu- actured using different materials and operated at differentpower levels and in different con󿬁gurations. Te success o the thermal management o such LED devices varies withthe cooling systems employed. In other words, an effectivecooling system in a certain LED con󿬁guration may not beeffective i there is a change in the LED con󿬁guration. oconclude,eachparameteroLEDdevicesandcoolingsystemsshould be evaluated simultaneously to choose a coolingsystem that is appropriate or the thermal management o LEDs [󰀴, 󰀱󰀴]. o the knowledge o the author although considerableresearches have been perormed regarding LEDs coolingsystems in the available literature, different cooling systemsused or high-power LEDs and comparisons in this study were not studied previously. Tereore, this study aimed todeterminesuitablecoolingsystemsorhigh-powermultichipLEDs.othisend,theeffectothedimidiationoLEDpoweron temperature values, two different LED con󿬁gurations,󰀳󰀰W, and series-connected  2 × 15 W were used. 2. Theoretical Analysis Approximately 󰀸󰀰% o the electrical energy used in high-powerLEDdevicesisconvertedtoheat[󰀴].Apartothisheatis dissipated to the environment rom the lighting and lateralsuraces o an LED by natural convection and radiation as aresultothetemperaturedifferencesbetweentheLEDandtheenvironment. However, the amount o heat transerred romthese suraces is small compared to the total amount o heatthat should be extracted rom the LED device. Consequently,the LED chip, where the maximum temperature occurs,transersheattothesubstrateotheLED,thentothethermalpaste that is used to reduce thermal contact resistance and󿬁nally to the cooling system by heat conduction. As anadditional an is used in cooling systems, the next heattransmission process occurs rom the cooling 󿬁ns to ambientair through orced convection. An equivalent thermal circuito this heat transer mechanism and a schematic 󿬁gure o an LED device are shown in Figure 󰀱. Te heat transerredbetween the junction o the LED chip and the substrate o LED is equal to that transerred between the junction o theLED and ambient air and can be de󿬁ned as ollows:  = 󰀨󽠵   − 󽠵 󽠵 󰀩􍠵 −󽠵 = 󰀨󽠵   − 󽠵 􍠵 󰀩􍠵 󝠵 ,  (󰀱)where  , 󽠵  , 󽠵 󽠵 , 􍠵 −󽠵 , 󽠵 􍠵 ,and 􍠵 󝠵  reertoheattransmission,thetemperatureotheLEDjunction,thetemperatureothesub-strate o LED, the thermal resistance rom the LED junctionto the substrate o LED, the environmental temperature, andthe total thermal resistance, respectively. Termal resistanceis de󿬁ned as ollows: 􍠵 󝠵  = 􍠵 −󽠵  + 􍠵 CS  + 􍠵 􍠵 ,  (󰀲)where  􍠵 −󽠵 ,  􍠵 CS , and  􍠵 􍠵  reer to the thermal resistancerom the LED junction to the substrate o LED, the thermalresistance o the cooling system, and the thermal resistanceo ambient air, respectively [󰀴, 󰀱󰀵]. As the junction o the LEDchipisaninternalcomponentothedevice,thejunctiontemperature ( 󽠵  ) cannot be measured directly. In (󰀱),  󽠵   canbe calculated by using (󰀳): 󽠵   = 󽠵 󽠵  + (󝠵 × 80 % ) × 􍠵 −󽠵 .  (󰀳)In (󰀳), the heat transmission is equal to 󰀸󰀰% o the LEDinput power. Te thermal resistance rom the LED junctionto the aluminum substrate o the LED is de󿬁ned as 󰀰.󰀲–󰀰.󰀵 ∘ C/W by the manuacturer o the device [󰀱󰀲]. Tus, giventhe temperature o the substrate o the LED, the temperatureo the LED junction can be determined because the otherparameters in (󰀱) are known [󰀴, 󰀱󰀲]. According to (󰀱), it is clear that heat transmission changes in direct proportion tothe difference between the temperature o the LED junctionandtheambienttemperatureandininverseproportiontothetotal heat resistance. It is not possible to control the temper-ature o the LED junction speci󿬁c to LED manuacturing orthe change in the ambient temperature o the environmentwhere the LED is used by determining the amount o heatdissipated rom the LED device efficiently by means o   Te Scienti󿬁c World Journal 󰀳 Termocouple  2 Chip of LEDTermal greaseTermocouple  1 Cooling system:(1) Cooling 󿬁ns( 2 ) EC and cooling 󿬁ns( 3 ) Heat pipe with cooling 󿬁nsSubstrate of LEDFan QT J R J – S T S R CS R a T a F󰁩󰁧󰁵󰁲󰁥 󰀱: Schematic diagram o the LED system and thermal equivalent circuit o the heat transer mechanism. High-power LED( 30 W)High-power LEDs( 2 × 15 W) F󰁩󰁧󰁵󰁲󰁥 󰀲: High-power multichip LEDs. the temperature difference. Because o this restriction, theefficient extraction o heat rom the LED device is only possible by developing high-perormance cooling systemsand materials or reducing the total heat resistance. Te hightemperature o the LED junction adversely affects the LED’slie span, light quality, and power consumption, and thus thetemperature should be maintained within a certain limit. I heat control or an LED device is provided by an effectivecooling system, the temperature o the LED junction shouldremain within this limit [󰀴, 󰀱󰀲]. 3. Experimental 󰀳.󰀱. Experimental System.  In the experimental systems usedor this study, a Bridgelux brand multichip high-power LEDwas used in two different con󿬁gurations, shown in Figure 󰀲.TedimensionsothesubstrateotheLEDwere  22×25 mm.Te LED models and power used were the BXRA-󰀵󰀶C󰀲󰀶󰀰󰀰-H-󰀰󰀰 model and the 󰀳󰀰W and BXRA-C󰀱󰀲󰀰󰀲 model serially connected to operate at 󰀱󰀵W [󰀱󰀱]. o cool the LEDs, thecooling systems presented in Figures 󰀳 and 󰀴 were used: an integrated multi󿬁n heat sink design with  80 × 45 × 85 mm o aluminium󿬁n aCoolerMaster model  80×80×25 mmoansupplying 󰀳󰀰CFM 󿬂ow rate at 󰀲󰀲󰀰󰀰RPM; a thermoelectriccooling unit obtained by integrating a EC󰀱-󰀱󰀲󰀷󰀰󰀸 model Cooling finsECHeat pipeHigh-power LED( 30 W) F󰁩󰁧󰁵󰁲󰁥 󰀳: Cooling system with thermoelectric cooler (EC) andheat pipe cooling device. thermoelectric element, operating at a maximum tempera-ture o 󰀶󰀸 ∘ C with dimensions o   40 × 40 mm, into the sameheat sink design [󰀱󰀶]; and a heat pipe cooling device withCooler Master RR-󰀲MN-󰀲󰀲p-R󰀱 model  74×45×88 mm o aluminium 󿬁n and  80×80×25 mm o an supplying 󰀳󰀰CFM󿬂ow rate at 󰀲󰀲󰀰󰀰RPM and with two copper pipes [󰀱󰀷].Te temperature was measured with a 󰀴-channel, Lutronbrand M-󰀹󰀴󰀶 model thermometer. o these channels, K-type thermocouples with a measurement range o   − 󰀱󰀹󰀹.󰀹 ∘ Cto 󰀱󰀳󰀷󰀰 ∘ C and a sensitivity o 󰀰.󰀱 ∘ C were connected [󰀱󰀸].Termocouple 󰀱, connected to the thermometer as shownin Figure 󰀴, was used to measure the temperature o thesubstrate o the LED and EC hot surace. Termocouple󰀲 was used to measure the top surace temperature o thecooling system, and thermocouple 󰀳 was used to measure theambient temperature. As shown in Figure 󰀵, channels wereopened to surace cooling 󿬁ns used in experimental study.Tenullcontactwasprovidedorthesuraceothesubstrateo the LED and EC hot by passing the thermocouple 󰀱rom these channels to sensitively measure the temperatureo the substrate o the LED and EC hot surace. Moreover,thermalpastewasappliedbetweentheLEDsubstrateandthetop surace o the cooling unit to prevent contact thermalresistance. Te experiment system was prepared with the junction o the system elements.  󰀴 Te Scienti󿬁c World Journal DC power supply of ECFanhermocouple 3hermocouple 2hermocouple 1High-power LEDhermometerCooling finsDC power supply of fanDC power supply of LEDS F󰁩󰁧󰁵󰁲󰁥 󰀴: Experimental setup and cooling system (an integratedmulti󿬁n heat sink design with a an). hermocouple 1(for substrate of LED)hermocouple 1(for substrate of LED) F󰁩󰁧󰁵󰁲󰁥 󰀵: Termocouple 󰀱 or used temperature substrate o LED. 󰀳.󰀲. Application of the Experiment.  LEDs systems that haveactive cooling systems reach heat balance in 󰀵–󰀱󰀰 minutes[󰀴, 󰀵, 󰀱󰀲]. Te LED or LEDs and the cooling an used in each system were powered at the same time. Te experimentinvolvingthethermoelectricelementwasbegunbypoweringthe thermoelectric element. Te duration o the experimentwas monitored using a chronometer, and temperature valueswere recorded at certain time intervals. Te experiment wasconducted until the temperature reached a stable value. Inthe experimental study it was observed that measured tem-peratureosamplesbecomesconstantafer󰀲–󰀵minutesromthe beginning o experimental study. Measurements wereperormed or 󰀱󰀲 minutes. In addition, the experiment wasnotbegununtilthetemperaturereachedtheambienttemper-ature. During the experiment, the ambient temperature wasapproximately 󰀲󰀲.󰀵 ∘ C. 4. Experimental Results and Discussion Te temperature o the LED substrate was determined tobe 󰀱󰀰󰀸 ∘ C by substituting the numerical values used in theexperimental LED systems into (󰀳), where the maximumtemperature o the LED junction was 󰀱󰀲󰀰 ∘ C, the LED inputpower was 󰀳󰀰W power, and the thermal resistance rom theLED junction to the substrate o the LED was 󰀰.󰀵 ∘ C/W. ocreateadurable,effective,andreliableLEDcon󿬁guration,thecooling systems used in the LED system must be maintainedat a temperature below that calculated or the LED. Te 20253035404550556065700 1 2 3 4 5 6 7 8 9 10 11 12 ime (min)    T  e  m   p  e  r  a   t  u  r  e    (         ∘    C    ) Substrate of LEDs temperature 30 W LED (cooled by only cooling 󿬁ns) 30 W LED (cooled by cooling 󿬁ns and fan) 30 W LED (cooled by heat pipe with cooling 󿬁ns) 30 W LED (cooled by heat pipe with cooling 󿬁ns and fan) 2 × 15 W LEDs (cooled by only cooling 󿬁ns) 2 × 15 W LEDs (cooled by cooling 󿬁ns and fan) 2 × 15 W LEDs (cooled by heat pipe with cooling 󿬁ns) 2 × 15 W LEDs (cooled by heat pipe with cooling 󿬁ns and fan) F󰁩󰁧󰁵󰁲󰁥󰀶:TechangeothesubstrateoLEDtemperatureovertimeduring the cooling process o LEDs with 󰀳󰀰W and  2 × 15 W LEDscon󿬁guration as a means o integrated 󿬁n heat sink design with orwithout an and heat pipe cooling device. change in the LED substrate temperature over time duringthe cooling o the LEDs with the 󰀳󰀰W and  2 × 15 W LEDscon󿬁guration using the integrated 󿬁n heat sink design withor without a an and a heat pipe cooling device is presentedin Figure 󰀶. According to Figure 󰀶, when the an was not usedinbothsystems,thetemperatureincreasedataconstantrate afer the ambient temperature was reached. As shown inFigure 󰀶, during the 󰀱󰀲th minute o the experiment, the anwas not run and the temperature o the substrate o LED o the 󰀳󰀰W LED decreased or both cooling devices reachinga temperature o approximately 󰀶󰀰 ∘ C. When the an wasrunning, the temperature o the substrate o LED was 󰀳󰀵 ∘ Cor the 󰀳󰀰W LED. o prevent an increase in temperature,the cooling systems capacity should be enhanced or a anshould be integrated into the same system. In Figure 󰀶, thetemperature o the substrate o LED in three cooling systemsusedintheexperimentsisseentobelowcomparedtopassivecooling systems. Low temperature o the substrate o LED iskeptwithdecreasingototalthermalresistanceduetotheuseo active cooling system in cooling LEDs system [󰀱, 󰀴]. Te change in the LED substrate temperature over timeduring the cooling process o the LEDs adopting the 󰀳󰀰Wand  2 × 15 W LED con󿬁gurations using an integrated 󿬁nheat sink design with a an and a heat pipe cooling deviceis presented in Figure 󰀷. Whereas the substrate temperatureo the 󰀳󰀰W LED was approximately 󰀳󰀵 ∘ C in both coolingsystems, this value was 󰀲󰀶 ∘ C when the heat pipe coolingdevice was used or the  2 × 15 W LEDs. Tis result indicates
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