Conception et réalisation thermique

1. Conception et réalisation thermique Troyes, 23 février 2012 Thierry Suzanne Ingénieur d’application

2. Designer’s Complaint… • LEDs are specified @ single test current @ 25°C Tj • My application is different! • What is the real light output for my application?

3. A new trend in the data- sheet characterization of the LEDs The LEDs are tested and binned at real world operating conditions LED Datasheet Specifications Hot Binning @ 85oC

4. What is LED Junction Temperature • Temperature directly on the LED chip/die LED Junction Temperature (Tj) • What does tested and binned at 25oC or 85oC at a specific drive current of for example 700mA mean? • The LED was driven at 700mA and light output measurements were made while the junction temperature at the LED was maintained at 85oC

5. Measurement Point • Application Brief AB33 http://www.philipslumileds.com/uploads/10/AB33-pdf

6. Heat Generation • LEDs are not 100% efficient  power consumed is not completely converted to light • Approximately, 30% to 50 % (depending on the technology) is converted to light and the rest is converted to heat Heat Radiometric Power (power converted to light)

7. Thermal Pad Heat Flow • LED thermal pad does not provide enough surface to dissipate the heat • No heat in LED’s main light beam • Heat generated by the LED is dissipated via the thermal pad underneath the LED • We add board, thermally conductive material and heat • sink to transfer the heat from the LED junction to the • air surrounding the LED No heat in the light beam

8. Effects of Heat on LEDs • Heat affects the LEDs in 5 different ways: • Light output • Color shift • Forward voltage shift • LED lifetime • Permanent damage

9. Reduces Light Output 200% Royal Blue Green Cyan Amber Red Blue 150% White 100% Relative Light Output (LOP) 90% 100% light output at 25oC 50% 0% 70C -40 -20 0 20 40 60 80 100 120 Junction Temperature TJ [°C] More sensitive to heat Effects of Heat on LEDs • AlInGaP: Red, Red-Orange, Amber • InGaN: Royal-Blue, Blue, Green, Cyan, White

10. Effects of Heat on LEDs Shifts dominant wavelength *

11. 400 350 Royal Blue, Blue, Cyan, Green, White (InGaN) 300 250 Red, Reddish Orange, Amber(AlInGaP) Forward Current (mA) 200 150 Vf -2.0 to -4.0mV/°C 100 50 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Forward Voltage (V) Effects of Heat on LEDs Tj Vf LED Driver: Vout= 43-48V @ 25oC  Vf=3.0V 15 LEDs: → 15 x 3.0 = 45V OK!! @ 87oC  Vf=2.85V 15 LEDs: → 15 x 2.85 = 42.75V Not OK!!

12. Effects of Heat on LEDs (B50, L70) What is (B50, L70)?

13. Lumen Maintenance - (Bxx, Lyy) • Notation used to describe the average lumen maintenance characteristic of the LEDs. • Lumen maintenance for SSL devices is typically defined in terms of the percentage of initial light output remaining after a specific period of time. • (Bxx, Lyy) • Bxx: percentage of LEDs, on average • Lyy: percentage of light output remaining • Example – (B50, L70) at 50000hours: • On average, the light output of 50% (B50) of the LEDs within the system will drop to lower than 70% (L70) of their initial light output after 50000hours.

14. Effects of Heat on LEDs Reduces operating life ~50.0% (B50, L70) ~155k ~175k ~165k

15. Effects of Heat on LEDs May cause severe damage Thermal management is critical

16. Convection Conduction Radiation Basic cooling considerations • Conduction: • The transfer of heat energy through a substance or from one substance to another due to temperature difference • Convection: • The process in which hot air rises and cool air delves down. Hot air will cool down as it flows through the cooler air mass until it reaches equilibrium. • Radiation: • The transfer of heat via electromagnetic waves through space

17. Thermal Management • It is critical to extract the heat away from the LED module and transfer it to ambient • This can be done using the principles of conduction, convection and radiation

18. Heat Sinks Efficiency of heat sinks depends mainly on: • Surface area • The larger the surface area, the more heat dissipated • Structure or shape • Proper structure increases turbulent airflow which creates a more efficient heat sink

19. Heat Sinks • Material • Use of materials with better thermal conductivity gives a more efficient heat sink • Ex. cooper 401 W/m-K vs. aluminum 235 W/m-K Turbulent Flow Laminar Flow

20. Thermal Resistance RTH • Thermal resistance describes how much that • material resists the flow of heat through it • Units: oC/W or oK/W • It changes with the material type, thickness, • surface area, and power (number of LEDs) • We want this number to be as low as possible • to make sure heat flows easily from one point • to another

21. LED thermal resistance: RTH junction to slug Board thermal resistance: RTH board Thermal interface material thermal resistance: RTH thermal interface Heat sink thermal resistance: RTH heatsink Thermal Resistance RTH + + + RTH R TH junction to slug + R TH board + R TH thermal interface + R TH heat sink =

22. Thermal Conductivity (k) • The measure of a material’s ability to conduct heat (W/mK) Thermal Conductivity Units are in W/mK.

23. Case Study

24. Scenarios: • Scenario A: Passive Cooling • Open Frame • Closed Fixture • Scenario B: Active Cooling • Open Frame • Closed Fixture

25. FLS has jointly developed with Qfinsoft, QLED, a thermal design and simulation software In parallel, FLS has launched a thermal design and simulation service to assist customers 4 FLS Engineers are assigned to carry out this service QLED – Thermal Simulation

26. What is QLED? • FLS jointly developed QLED with Qfinsoft • QLED is a thermal design and simulation software developed for modeling LUXEON LED lighting systems • The accuracy of the LED models and their behavior were endorsed by Philips Lumileds

27. What is QLED? QLED is a virtual environment which allows customers to create different models. For example, models can range from: A single LED on a heat sink to Multiple LEDs on a custom made board within an enclosed space or casing with active cooling

28. Benefits of Using QLED • It minimizes the number of design cycles, reduces development costs, and decreases time to market Concept Prototype Testing Product QLED Concept Prototype Testing Product

29. Benefits of Using QLED 2. Simple user interface 3D Model View Main Toolbar 3D Toolbar Simulation Manager Message Window Component Toolbar

30. Key Features • Provides very fast simulation results, with most simulations taking only minutes • Offers an easy to use library system for material selection • Includes a powerful, yet easy to use design optimizer

31. QLED Capabilities • Simulation modes include: • Steady state: DC current (constant ON) • Transient: Pulse or strobe LEDs • Parameterized Trials • Optimization

32. Scenario A Passive cooling

33. Fortimo DLM 1100lm Thermal path basic solution • Temperatures: • 1 = test point Tc • 2 = heat sink @ module side • 3 = ambient • Resistances: • R1 = LED DLM path 1-2 • R2 = heat sink path 2-3 3 3 1 2 2

34. Fortimo DLM 1100lm Thermal Resistances Tc Side view 0.2 K/W Rth c-hs Ths Rth hs-amb Rth hs-amb 3 3 3 1 1 1 Tamb Top view 2 2 2

35. Fortimo DLM 1100lm Thermal resistance of heat sink • Example of standard heat sink: • Needed 4.214 K/W (max) • Heat sink: Aavid Thermalloy • Length @ 4.01 K/W = 35 mm • Width= 76.2 mm, height= 38.1 mm, #fins= 8

36. Thermal Simulation – Open Frame • Ambient = 35oC • Tc ≈ 62oC • Matches the theoretical calculations LEDs junction temp. Tc

37. Thermal Simulation – Closed Fixture • Tc = 90oC • Exceed the max. Tc • Thermal design must be modified • Fully enclosed can (air tight) • No vents for air to go in and out • Steel Fixture Tc

38. Solutions? – larger heat sink • Larger heat sinks: • Tripled the heat sink height • Tc ≈ 73oC • We still need to lower Tc to 65oC Tc

39. Solutions? – larger heat sink • Fins extended to touch the fixture • Tc ≈ 59oC Tc

40. Solutions? – vented fixture • Vents on upper and lower sections of the fixture • Tc ≈ 82oC • Even with larger heat sinks, it may be difficult to reduce Tc Tc

41. Scenario B Active Cooling

42. Nuventix – Open Frame • Each setting has a thermal resistance depending on the performance setting

43. Nuventix – Open Frame • At the standard setting and ambient temperature = 35oC, Tc ≈ 44.7oC • Tc = P x Rth(hs-ambient) + Tambient • Tc = 13 x 0.75 + 35 = 44.75oC

44. Nuventix – Closed Fixture • Experimental testing • SynJet to be modeled in QLED