Package-Related Thermal Resistance of LEDs Application Note

Transcription

1 ackage-ated Thermal esistance of LEDs Application Note Abstract This application note offers an introduction to e definition and specification of e ermal resistance value for LEDs and IEDs (I emitting diodes). In addition, an overview is provided as to how OSAM Opto Semiconductors specifies e ermal resistance for various types of LEDs, how e reference points are defined for e various product types and which values are used for e data sheets of e LEDs. Introduction In order to achieve e expected riability and optimal performance for today's standard light emitting diodes (LEDs), e use of appropriate ermal management is advantageous in most cases. At modern high-power LEDs wi a power dissipation of several Watts, is is essential. Basically, e operation of LEDs is limited by various factors, depending on e material and technology deployed. An important parameter of practical interest for users is e temperature of e active semiconductor layer (junction temperature), since is influences e function and lifetime of e LED. The manufacturer's recommended maximum junction temperature should erefore not be exceeded during operation in order to prevent damage to e component. The goal in e devopment of ermal management is to keep e junction temperature as low as required for e given application. An important aid in e devopment of ermal management is e ermal resistance ( ) of e LED. Ideally, e user requires a ermal resistance at describes e ermal characteristics of e component independent of e environmental conditions and allows e user to determine e junction temperature of e LED wi sufficient precision. In practice, however, an environment-rated ermal resistance JA is specified, which is determined under defined measurement conditions (see JEDEC 51). Under technical conditions of use in an ectronic device, however, deviations from ese conditions arise. Those influence e accuracy of determining e junction temperature. An exact determination of e junction temperature is erefore only possible if sfdetermined, device-rated correction factors are available to e user. For e majority of users, however, is is probably not e case. Thermal resistance The ermal resistance ( ) or absolute ermal resistance is defined as e rate of temperature increase for e supplied power and in e end, is a measure of e capability to dissipate heat. T heat The temperature increase of LEDs is caused by at portion of e power dissipation at is not transformed into light. The temperature difference ΔT or upper temperature is erefore defined as e January, 2014 age 1 of 8

2 difference of e inner temperature of e heat source (semiconductor chip) and an external constant reference temperature. Since e temperature increase is proportional to e power dissipation, e ermal resistance can also be seen as e proportionality factor at contains e physical heat dissipation characteristics of e LED in question. Knowledge of e ermal resistance of an LED serves to: determine e junction temperature at arises in e LED under operating conditions determine e maximum allowable external reference temperature for a given internal temperature and power dissipation evaluate measures for dissipation of heat (= ermal management) Definition of e reference temperatures "solder point" T S or for an LED (such as e OSAM OSTA-Lighting), e "board" temperature T B. Here, e "solder point" is e transition from e active heat pa of e housing to e solder surfaces of e circuit board and is dependent on e packaging technology. For components like OSAM OSTA e "board" temperature refers to e underside of e components. TOLED product family The primary flow of heat for TOLED packages occurs by e transfer of heat from e chip and leadframe to e ends of e ermally active leads. It should be noted here at depending on e design of e semiconductor housing, a differentiation can be made between ermally active and inactive leads. Thermally inactive leads are ose which are only connected to e chip by bond wires. For e definition of e ermal resistance of an LED, two temperatures are required - e inner temperature of e heat source and an external reference temperature (temperature of an external point on e component). For an LED, e "temperature of e inner heat source" refers to e temperature of e active semiconductor zone in which e photons are generated. This is designated as e junction temperature T J. The appropriate section of e external temperature reference point depends on e type of housing and e respective rate of heat dissipation. For an OSAM Opto Semiconductors SMT LED (such as e TOLED or DAGON LED, for example) is reference point is e January, 2014 age 2 of 8 Figure 1: rimary heat pa of e OSAM TOLED family For ese components, e solder point temperature can be directly determined in practice by gluing a ermo ement at or on e solder pads of e ermally active leads/pins (see also application note Temperature Measurement wi Thermocouples).

3 DAGON LED product family In e case of e DAGON product family, e internal dissipation of heat from e chip occurs primarily via e integrated heat sink to its underside. Figure 2: rimary heat pa of DAGON products Here, e practical measurement of e solder point temperature is a bit more complicated, since e temperature cannot be directly measured at e heat sink. Instead, it is recommended to record e temperature directly adjacent to e DAGON housing on e long side by means of a ermo ement on e circuit board. Steady state e temperature gradient to e heat sink is negligible in is case. OSAM OSTA product family For e OSAM OSTA product family, e internal dissipation of heat occurs from e chip via e ceramic submount to e underside of e carrier board (Figure 3). Definition of power loss and heat flow Wi classical silicon-based semiconductor components such as integrated circuits and transistors, e entire ectrical energy is converted into heat. The heat flow can erefore be equated to e ectrical power loss and leads to a clear interpretation of e ermal resistance. January, 2014 age 3 of 8 Figure 3: rimary heat pa of OSAM OSTA products Wi light emitting diodes (LEDs) however, e supplied ectrical energy is only partially converted to heat. Depending on e optical efficiency, a portion of e ectrical energy is emitted as light/radiation (optical energy). This leads to two different definitions of ermal resistance for LEDs. Electrical ermal resistance The ermal resistance is defined in e same manner as classic semiconductor components. The energy coupled out (light/radiation) is not taken into consideration (see JEDEC 51). heat U ectr. F I F T eal ermal resistance This definition of ermal resistance considers e actual flow of heat at is dissipated rough e housing. The optical power of e LED at is coupled out is erefore taken into consideration.

4 heat opt T opt T (1 LED ) Normally, for e devopment of an application and especially in e evaluation of appropriate ermal management, one starts wi a predefined ermal resistance and e expected increase in temperature ΔT (= T j,max T ambient), at determines e maximum permissible power or current. where: η LED = optical efficiency of e LED max T T j T a This value is independent of e working point of e LED and can erefore be used to characterize e LED housing. Through e use of e ermal resistance when considering e ermal layout, it can be guaranteed at e maximum permissible junction temperature is not exceeded for all modes of operation which arise. Depending on which ermal resistance is drawn upon for e calculation ( ectr or ), different maximum permissible powers or currents result. ectr,max,max ectr,max T T ectr,max (1 LED ) It should generally be noted here, at in principle, ese powers or currents only describe e initial state (= 0h) for maximum LED efficiency and minimum power consumption. For e calculation at uses e ermal resistance ectr., is means at already at e beginning (t = 0h), e maximum junction temperature T j,max has been reached. Figure 4: Comparison of power and ermal resistance Consequence of e two ermal resistances For a better understanding of e two different ermal resistances, e issue and eir effect will once again be addressed and clarified by means of a comparative example. T T (wi t = 0h) j, ectr j,max rogressive aging and sustained temperatures lead to a decrease in optical efficiency, however, and erefore to an increase in power consumption. This in turn leads to an increase in e junction temperature. For e calculation at uses e ermal resistance ectr., is means at: T T (wi t > 0h) j, ectr j,max January, 2014 age 4 of 8

5 Determination of e ermal resistance JS at OSAM OS OSAM Opto Semiconductors uses e ermal transient measurement meod which is called e static ermal resistance test meod according to JEDEC 51 in order to determine e ermal resistance of OSAM Opto Semiconductors LEDs. Figure 5: Aftereffect for e use of e ermal resistance ect The same increase in junction temperature also applies for e calculation of e maximum power wi e ermal resistance. In is case, however, since e optical efficiency is also taken into consideration, e maximum achievable temperature is ly limited by e maximum permissible junction temperature. T T j, j,max Figure 6: Aftereffect for e use of e ermal resistance The temperature is measured wi a temperature-sensitive parameter of e LED - e forward voltage drop of e diode. However, e accurate and reproducible measurement of junction to solder point ermal resistance JS is far from trivial and up to now, not defined by international standards. In order to determine e ermal resistance JS, e device under test has to be measured twice. For e first transient ermal measurement, e device is applied to e heat sink (isoermal plate) wiout ermal grease; e second measurement is done wi e same setup but wi ermal grease. For low power LEDs, different CB materials are used. Figure 7 shows two Z -curves of an OSAM OSTA module which has been measured wi and wiout ermal grease between board and e water-cooled cold plate. The transient ermal behavior of e device in is two-measurement setup is identical as long as e heat propagates from e junction rough e device. When arriving at e solder point or e end of e board to heat sink interface, e curves are different depending wheer or not ermal grease is present at e interface. This change in e ambient condition indicates e end of e device. Therefore e JS value can be determined as e value Z (ts) at e point of time ts when e two curves start to separate. January, 2014 age 5 of 8

6 2.5 No74_1 - Ch. 1 No74_2mK - Ch. 1 T3Ster Master: Z 2 Normalized temperature rise [ C] e-6 1e-5 1e Time [s] Figure 7: Z -Curve of e OSAM OSTA-Lighting LED module Thermal resistance JS in e LED datasheets from OSAM Opto Semiconductors In e data sheets from OSAM Opto Semiconductors, e value JS is specified by: JS T J T S opt The data refers to e ermal resistance. For comparison e effective ermal resistance ( ectrical = effective) is also listed by e high power LED devices. These are determined by means of measurements of a random characterization test and entered into e data sheets in e following manner: Standard LED (e.g. TOLED, owertoled) In e data sheets for ese components, e maximum value of e ermal effective resistance is specified. JS, TJ T S opt The maximum ermal resistance is statically determined from e distribution of e random characterization test. max. typ. 6 January, 2014 age 6 of 8

7 I devices In e data sheets for e I components, e data sheet contains e typical ermal resistance of e sample characterization test and e resulting maximum ermal resistance. typ. max. typ. 6 High power LEDs (e.g. Advanced owertoled, DAGON LED, OSAM OSTA) In e case of e high-power LED, e data sheet contains e typical ermal resistance of e sample characterization test, e resulting maximum ermal resistance, and for comparison e effective ermal resistance wi e optical efficiency of e LED. typ. max. typ. typ. and LED 6 By stating e typical ermal resistance, e comparability of different housing packages is guaranteed. Appendix Don't forget: LED Light for you is your place to be whenever you are looking for information or worldwide partners for your LED Lighting project. Auors: ainer Huber, Thomas Zahner, Andreas Stich ABOUT OSAM OTO SEMICONDUCTOS OSAM, Munich, Germany is one of e two leading light manufacturers in e world. Its subsidiary, OSAM Opto Semiconductors GmbH in egensburg (Germany), offers its customers solutions based on semiconductor technology for lighting, sensor and visualization applications. Osram Opto Semiconductors has production sites in egensburg (Germany), enang (Malaysia) and Wuxi (China). Its headquarters for Nor America is in Sunnyvale (USA), and for Asia in Hong Kong. Osram Opto Semiconductors also has sales offices roughout e world. January, 2014 age 7 of 8

8 For more information go to DISCLAIME LEASE CAEFULLY EAD THE BELOW TEMS AND CONDITIONS BEFOE USING THE INFOMATION SHOWN HEEIN. IF YOU DO NOT AGEE WITH ANY OF THESE TEMS AND CONDITIONS, DO NOT USE THE INFOMATION. The information shown in is document is provided by OSAM Opto Semiconductors GmbH on an as is basis and wiout OSAM Opto Semiconductors GmbH assuming, express or implied, any warranty or liability whatsoever, including, but not limited to e warranties of correctness, completeness, merchantability, fitness for a particular purpose, title or non-infringement of rights. In no event shall OSAM Opto Semiconductors GmbH be liable - regardless of e legal eory - for any direct, indirect, special, incidental, exemplary, consequential, or punitive damages rated to e use of e information. This limitation shall apply even if OSAM Opto Semiconductors GmbH has been advised of possible damages. As some jurisdictions do not allow e exclusion of certain warranties or limitations of liability, e above limitations or exclusions might not apply. The liability of OSAM Opto Semiconductors GmbH would in such case be limited to e greatest extent permitted by law. OSAM Opto Semiconductors GmbH may change e information shown herein at anytime wiout notice to users and is not obligated to provide any maintenance (including updates or notifications upon changes) or support rated to e information. Any rights not expressly granted herein are reserved. Except for e right to use e information shown herein, no oer rights are granted nor shall any obligation be implied requiring e grant of furer rights. Any and all rights or licenses for or regarding patents or patent applications are expressly excluded. January, 2014 age 8 of 8