Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyA Novel Procedure and Device to Allow ComprehensiveCharacterization of Power LEDs over a Wide Range of TemperatureGábor Molnár 2, , Gergely Nagy 1 , Zoltán Szőcs 1molnarg@micred.com, nagyg@eet.bme.hu, szucs@eet.bme.hu1 Budapest University of Technology and Economics (BUTE), Dept. of Electron Devices2 Microelectronics Research and Development LtdBudapest, HungaryABSTRACTLEDs are key elements in modern, energy efiicient lightingsolutions as well as impose some issues from thermalpoint of view, since light output and reliability both dependon LEDs' junction temperature. A comprehensiveand accurate measurement method is required and demandedby several leader LED manufacturers. Failing aproper combined thermal and radiometric/photometriccharacterization of LED light sources it is impossible tofulfill the reliability prescriptions for LEDs and to trustthe lifetime estimation given in LED datasheets. Lightoutput of LEDs is typically measured in integratingspheres. A key element in such a total flux measurementsetup is the appropriate set of standard LEDs which areboth current and temperature stabilized and are accompaniedwith certificate values of their own total flux tracableto primary etalons of national measurement laboratorise.So far there are hardly any such standard LEDsavailable for the high power range. In this paper we describethe design of such a device (having 5 colors) anddescribe a modification of the substitution type total fluxmeasurement method which is suitable for an automated,comprehensive measurement of LEDs over a wide rangeof operating conditions.In order to allow safe and good design of solid-state lightingsolutions, precise thermal characterization of powerLEDs is a must. LED vendors and users are starting to sharethe same point of view, that for design purposes, the thermalmetrics of power LEDs should be calculated by consideringthe emitted optical power – in other words, the radiometricflux of the LED. Farkas et al. have observed quite a largevariation in the total junction-to-ambient thermal resistanceof a 1W red LED as the case temperature and the operatingcurrent of the LED was varied [2]. This variation - representedin a so called cumulative structure function – isshown in Fig. 2.I. INTRODUCTIONThermal characteristics of power LEDs are vital from thepoint of view of their expected lifetime and reliability.Higher junction temperatures cause shorter lifetime andthrough induced thermo-mechanical stresses, result in differentfailure mechanisms like die attach delamination. This isone reason, why precise thermal characterization of LEDs isimportant. Another aspect of thermal resistance of LEDs –which is directly related to the junction temperature – is thatmost important parameters of LEDs depend on temperature.Again, increasing junction temperature results in degradationof these parameters – such as the wall-plug efficiency (emittedoptical power related to the supplied electrical power) asshown in Fig.1.Fig. 1. Wall-plug efficiency of a power LED at different temperatureIn their paper they gave different explanation for this as wellas they suggested (as early as 2003 [5]) to combine thermaltesting of power LEDs with radiometric and possiblyphotometric measurements, since part of this variation canbe attributed to the temperature and current dependence ofthe efficiency of converting the supplied electrical energy toemitted light energy. If this efficiency variation is not consideredin the calculation of the thermal resistance of anLED package then the thermal resistances seems to vary asthe operating conditions change.Even today, in many cases the efficiency of LEDs is not consideredwhen thermal resistance values are given in LEDs’©EDA Publishing/THERMINIC 2008 89ISBN: 978-2-35500-008-9