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# Internal Thermal Resistance of LEDs: LED Fundamentals

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In this presentation on Internal Thermal Resistance of LEDs we will look at the definition and significance of internal thermal resistance in LED packages, the path of heat transfer in some OSRAM LED packages, and the calculation of the junction temperature in LEDs.

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### Internal Thermal Resistance of LEDs: LED Fundamentals

1. 1. LED FundamentalsInternal ThermalResistance of LEDs09-07-2011
2. 2. Thermal Resistance (Rth ) (T1  T2 ) The thermal resistance (Rth) is defined as Rth   Q where: Difference between the inner temperature of the heat T1  T2   source (semiconductor chip) and an external constant reference temperature. f t t  Q Dissipated power For an LED  The temperature of the inner heat source refers to the temperature of the active semiconductor zone in which the photons are generated. This is designated as the junction temperature (TJ).  The appropriate selection of the external temperature reference point depends on the type of housing and the respective rate of heat dissipation. This reference point is the "solder point temperature" (TS).LED Fundamentals | Internal Thermal Resistance | Page 2
3. 3. Two Definitions of Thermal Resistance (Rth JS)For LEDs t e t e a resistance from ju ct o to so de po t ( thJS) can be de ed in t o ways o s the thermal es sta ce o junction solder point (R ca defined two ays Electrical thermal resistance Real thermal resistance Electrical Rth JS Real Rth JS T T RJX el  RJX  real  Pel Pel  Popt  Q  Pel  Popt  Q  Pel  VF  I F l Q  Pel  1   LED   where : where : Pel  Supplied electrical power VF  Forward voltage Pel  Supplied electrical power I F  Forward current Popt  Supplied optical power  LED  Optical efficiency of the LED The OSRAM datasheets lists value of the real thermal resistance (Rth JS) in K/WLED Fundamentals | Internal Thermal Resistance | Page 3
4. 4. Internal Thermal Resistance Rth JS Heat Conduction and Heat Path of LED Packages g TOPLED Family DRAGON Family LED  Conductive heat transfer through leads to  Heat spreading within the internal heat PCB spreader  Thermal active and thermal inactive leads  Conductive heat transfer through heat and solder pads p spreader to PCB  The chip is mounted on the thermal active  The heat spreader has electrical potential lead. The bond wire goes to the thermal inactive leadLED Fundamentals | Internal Thermal Resistance | Page 4
5. 5. Internal Thermal Resistance Rth JS (contd…) Heat Conduction and Heat Path of LED Packages g OSLON SSL Family  The chip is mounted on a thermal pad  The thermal pad is electrically isolated by the g ceramic housing  Conductive heat transfer takes place though the ceramic housing and all the three pads.LED Fundamentals | Internal Thermal Resistance | Page 5
6. 6. Thermal Resistances (Rth JS ) of OSRAM LEDs 150 sistance Rth [KW-1] 125 hJS 120 K/W 100 75 Real Thermal Res 50 40 K/W 36 K/W 25 7 K/W 6.5 K/W 4.2 K/W Advanced Power OSRAM OSTAR- OSTAR DURIS E 3 DURIS E 5 OSLON SSL DRAGON TOPLED Plus Lighting PlusLED Fundamentals | Internal Thermal Resistance | Page 6
7. 7. Significance of Thermal Resistance Rth JSKnowledge of the thermal resistance (Rth JS ) of an LED serves to: Determine the junction temperature that arises in the LED under operating conditions. Determine the maximum allowable external reference temperature for a given internal temperature and power dissipation. Evaluate E l t measures f dissipation of h t i th LED system (th for di i ti f heat in the t (thermal management). l t)LED Fundamentals | Internal Thermal Resistance | Page 7
8. 8. Thermal Measurement Estimation of Junction Temperature (TJ) p ( LCW CP7P 80.00 If = 350 mA 70.00 Te m pe ra ture (C ) 60.00 60 00 Vf = 3 2 V 3.2 50.00 Solder Point Temp (C) Ts (measured) = 75°C RthJS = 7K/W 40.00 Ambient temp (C) 30.00 ФV(25°C) = 100 lm Ta (measured) = 20°C 20.00 10.00 Approximation (White LED): 100 lm ~ 320 mW = 0.00 Popt 0.00 50.00 100.00 150.00 200.00 Pel = 3 2 x 0 3 0 = 1 12 W 3.2 0.350 1.12 Time (min) PD = Pel – Popt = 1.12 W - 0.320 W = 0.8 WCalculation : T  T  Rth  P J S JS D = 75°C + 7 x 0.8 W = 81°C Junction Temperature Estimation at Tamb(20°C) = 81°CLED Fundamentals | Internal Thermal Resistance | Page 8
9. 9. Thermal Management System Thermal Resistance of System RthJA Internal External Thermal Resistance Thermal Resistance RthJS RthSB RthBA  Materials  Substrate Technology  Housing  Package Geometry  Solder Pads  Heat Sink  Thermal Spreader  Thermal Vias  Air Velocit VelocityLED Fundamentals | Internal Thermal Resistance | Page 9
10. 10. Disclaimer All information contained in this document has been checked with the greatest care. OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with the use of these contents. OSRAM Opto Semiconductor GmbH makes no representations and warranties as to a possible interference with third parties intellectual property rights in view of products originating from one of OSRAM Opto Semiconductor GmbHs partners, or in view of products being a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners. Furthermore OSRAM GmbH s partners Furthermore, Opto Semiconductors GmbH cannot be made liable for any damage that occurs in connection with the use of a product of one of OSRAM Opto Semiconductor GmbHs partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners.LED Fundamentals | Internal Thermal Resistance | Page 10
11. 11. Disclaimer This document constitutes neither an offer to sell nor a solicitation to buy or subscribe for securities. Any such offer will be made solely on the basis of the Securities Prospectus yet to be approved by the German Financial Supervisory Authority (BaFin) and published thereafter. The information legally required to be provided to investors will be contained only in the S Securities Prospectus. The information contained herein is not for distribution, directly or indirectly, in or Into the United States of America (including its territories and possessions of any State of the United States of America or the District of Columbia) and must not be distributed to U.S. persons (as defined in Regulation S under the U.S. Securities Act of 1933, as amended ("Securities Act")) or publications with a general circulation in the United States of America. This document is not an offer of securities for sale in the America United States of America. The securities have not been and will not be registered under the Securities Act and may not be offered or sold in the United States of America absent registration or an exemption from registration under the Securities Act. The g p g Issuer does not intend to register any portion of the offering in the United States of America or to conduct a public offering of the securities in the United States of America. This document is not an offer of securities for sale in the United Kingdom, Canada, Japan or Australia.LED Fundamentals | Internal Thermal Resistance | Page 11
12. 12. Thank you for your attention.LED Fundamentals | Internal Thermal Resistance | Page 12