Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyOn the standardisation of thermalcharacterisation of LEDsPart II: Problem definition and potential solutionsAndrás PoppeMicReD Ltd,MTA-MMSZ Building, Etele út 59-61, Budapest XI, H-1119 Hungarypoppe@micred.comandClemens J.M. LasancePhilips Research LaboratoriesHigh Tech Campus 4, 5656 AE Eindhoven, the Netherlandsclemens.lasance@philips.comAbstract- Nowadays the demand for thermal standards forpower LEDs is increasing. On the one hand metrics for faircomparison of competing products are needed, on the otherhand, designers of power LED based applications demandreliable and meaningful data for their daily work. Today's datasheet information does hardly meet any of these requirements.Part I compares the current situation in the LED world withthe situation in the IC world over twenty years ago, observesthat much can be learned from the progress achieved, andconcludes with a proposal for action. Part II addresses thermalissues that are specific to light emitting diodes (in fact, alsosemiconductor devices), the drawbacks of the current situationwith respect to the information in the data sheets, andemphasises the need for electro-thermal models.I. INTRODUCTIONUnlike two decades ago in the IC world, T junction of an LEDis not just a performance indicator of the thermal design butalso plays a major role in lighting design since manyproperties of the light output of an LED depend on theabsolute junction temperature. This means that thermalmanagement should be an integral part of the overall designof an LED based lighting solution, resulting in changingroles of different engineering disciplines in the overalldesign process – as will be discussed later in more detail.Consequently, since T junction of LEDs is more widely usedin the overall design process of LED based lightingsolutions, well-established definitions of standardisedthermal metrics and models will be even more importantthan before, both for the LED manufacturers and the lightingsystem designers.In Section 2 we highlight the reliability and overallperformance aspects of thermal characterisation of LEDs,while in Section 3 we describe some major issues regardingtoday's LED data sheets and suggest some hints for possiblesolutions.II. GOALS OF LED THERMAL CHARACTERISATION:RELIABILITY AND OVERALL PERFORMANCE PREDICTIONThe most important reason for an LED designer to usethermal data is to get an idea about the reliability of the finalproduct (apart from adhering to official regulations such asUL, CE etc.). To get an idea of the failure and degradationmechanisms that influence lifetime it is useful to make adistinction between the device (the LED itself) and thepackage or system.• Device reliability: intrinsic light output reductionunder operating conditions• Package reliability: failures caused by thermalstresses and ageingA. Device and package reliabilityThe main temperature-related problem at the device levelis reduction in light output as a function of time. Because ofthis phenomenon, the light output may decrease to anunacceptable level before a ‘real’ irreversible failure occurs.This condition may be coined a ‘lumen maintenance’ failure.The level at which this is called a ‘failure’ should be subjectto standardisation. Fig. 1 shows a typical plot of the situationabout five years ago.Regarding the required accuracy, we may conclude fromthe graph that a 10% error in lifetime corresponds to about 2°C difference in die temperature. It is clear that a designerneeds accurate information about the absolute temperatureover time in order to reach a conclusion about the lifetime.In practice the question remains how to measure thetemperature with the required accuracy.©EDA Publishing/THERMINIC 2008 213ISBN: 978-2-35500-008-9