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LED Thermal Management
Light emitting diodes (LEDs) like other solid state electronic devices produce heat that must be managed in order to ensure optimum device performance. The benefits of LEDs include long useable life, brilliant light, and continuously increasing efficacy values. An important caveat: if the heat produced by the LEDs is not mitigated properly, these benefits will not be fully realized. Lighting Matrix® has decades of outdoor lighting fixture design experience. More recently, Lighting Matrix has also gained very valuable LED design experience with proven LED fixtures enjoying many years of optimum performance.
This Information Brief provides specifiers and customers with an understanding of Lighting Matrix’s depth of knowledge in LED thermal management, giving them piece of mind that Lighting Matrix designs LED fixtures for optimum long term performance. Being part of Philips means Lighting Matrix has access to the latest technologies and products – the LEDs themselves, LED drivers, LED light engines, and our own foundry for manufacturing heat sinks – enabling Lighting Matrix to design and manufacture the best LED fixture solutions available.
Heat Transfer
To optimize performance, the impact of heat on LEDs must be understood. Heat or heat transfer occurs when a temperature difference results in energy in motion.Heat transfer occurs from the higher to the lower temperature within a medium or between media (e.g. solids, liquids, and gases) via three processes. Conduction is heat transfer
across or through a medium, such as heat conduction through metal. Convection occurs between a surface and a moving fluid. Evaporation is an example of natural convection; forced convection occurs when a fan blows air over a hot surface. Radiation is heat transfer between surfaces4, such as the heat from the sun or the heat rising off a road or sidewalk on a hot summer day.
LEDs
LEDs are solid state devices that directly convert electricity to light. They are called “solid state” because the solid chip or die is the active component that produces light. As a diode, an LED has a positive (p) anode and a negative (n) cathode that meet at a junction. It is at this p-n junction where the work of converting electricity to light occurs6.
See Figure 1.
Illustration courtesy of IESNA TM-16-05.
What is commonly thought of as an “LED” is a packaged device that includes the LED chip itself, a primary lens for protection, and a thermal pad for heat transfer7. In this Information Brief, “LED” refers to an LED package. While traditional lamps radiate light and heat in virtually all directions, LED light is emitted forward but the heat is retained. See Figure 2.
Illustration courtesy of Philips Lumileds Lighting Company.
LED efficiencies are improving, but nothing is 100% efficient; the by-product of light production is heat. The junction temperature (TJ) is the heat that must be conducted through the thermal pad and away from the LED. For more technical information on LEDs, see Illuminating Engineering Society of North America (IESNA) TM-16-05 – IESNA Technical Memorandum on Light Emitting Diode (LED) Sources and Systems.
Lower Tj = long useable life and brighter light
LED data sheets and other technical information provided by device manufacturers provide indications of how thermal management affects performance. In particular, maintaining a lower TJ results in long life, but how is “long” defined – and how is “life” defined? For lighting, it must go beyond measuring the functional lifespan of a component. The LED may still have electrical “life” (i.e. current is still flowing through it), but how much useable light is it producing? LEDs are similar to Mercury Vapor lamps in that they do not exhibit significant, noticeable light degradation; rather, their light output gradually dims over time.
It is commonly cited that LEDs have up to a 100,000 hour “lifetime”. In 2005 the Lighting Research Center (LRC) proposed 70% lumen maintenance or L70% due to it being“…close to the threshold for detecting gradual reduction in light output. Research shows also that reductions to 70% of initial light output are considered acceptable by the majority of occupants within a space.” This proposal leads to a more meaningful definition of “life”.
Both Philips and HADCO specify life with respect to 70% lumen maintenance which means at least 70% of the LED’s initial light output will be maintained. Philips characterizes this as L70 for two lifetimes – B10 and B50. B10 designates that only 10% of the population of LEDs are expected to fail, meaning 90% are expected to survive. Likewise, B50 designates that 50% are expected to survive. As a comparison, traditional lamp sources typically rate
lamp performance based upon 50% lifetimes. Philips publishes graphs for (B10, L70) and (B50, L70) lifetimes for their LUXEON Rebel LEDs. See Figure 3 and Figure 4.
Illustration courtesy of Philips Lumileds Lighting Company.
Illustration courtesy of Philips Lumileds Lighting Company.
As an electronic device, LED lifetime and other specifications are based upon electrical characteristics, such as forward current through the device, because changes in electrical characteristics change performance. That is why there are different graphs for different forward currents (e.g. 350mA, 700mA, or 1A – see graphs) depending upon desired LED drive and performance characteristics, including lifetime and lumen maintenance.
The unique nature of LEDs means their performance characteristics are also based upon the management of the critical heat generation factor TJ as illustrated in these graphs. LED performance data is provided with respect to TJ directly and also with respect to corresponding points in the thermal path, such as thermal pad temperature. See Figure 5.
Illustration courtesy of Philips Lumileds Lighting Company.
Conclusion
As an LED fixture manufacturer, Lighting Matrix understands heat transfer, LEDs, how heat is produced by and affects LEDs, and how to mitigate that heat. We also understand the critical role fixtures play in the thermal management of the whole LED system. Using LED manufacturer supplied specifications including performance graphs, Lighting Matrix designs all of our LED fixtures for years of optimum performance based upon measurable, meaningful metrics. To learn more about Lighting Matrix’s growing line of LED fixtures and our entire commercial / Residential/Industrial product line, Please Contact us: Sales@lightingmatrix.com
(From Philips Inc)