LEDs are often described in marketing materials as "cool" lighting, and in fact LEDs are cool to the touch because they generally don't produce heat in the form of infrared (IR) radiation. On the other hand, LEDs generate heat in the diode semiconductor structure (in addition to photons) and this heat must exit the system through conduction and convection. Consequently, luminaries’ designers must be conscious of potential heat dissipation challenges and how those challenges may affect LED performance, longevity, and even lamp safety.

  Elevated junction temperatures have been shown to cause an LED to produce less light (lumen output) and less forward voltage. Over time, higher junction temperatures may also significantly accelerate chip degeneration, perhaps by as much as 75 percent with an increase from about 100°C to 135°C during regular use.

  Engineers and material scientists have been and are developing new LED-related thermal management solutions including improved drivers, diaphragm-driven forced convection methods, better heat sinks, and even the introduction of graphite foam as a cooling medium. This article will first describe three junction temperature considerations--basic thermal resistance, power dissipation, and junction temperature measurement--then briefly look at advances in each of the aforementioned approaches to improved LED thermal management.

  The following are the various important parameters in selecting Thermal Management:

1. Thermal resistance
2. Airflow
3. Volumetric resistance
4. Fin density
5. Fin spacing
6. Width
7. Length

  Calculate the required LED heat sink for thermal management. The basics to do that is to understand the scheme at the right Each part of the design adds up some heat due to individual thermal resistances of each material – the adding up can be calculated as T = Pd x Rth. In this case we have the thermal resistance of the LED module (Rj-c), the thermal resistance of a gap filler (thermal pad or grease) we want to place between the led module and the heat sink (Rb), and the thermal resistance from our heat sink (Rh) which has to make that the total design stays below the maximum required junc_on temperature Tj

  If our led light is in a recessed environment I want to calculate with an ambient temperature Ta of 45°C Means the maximum temperature added in the total design is Tj – Ta = 135°C – 45°C = 90°C

  The total power to dissipate is of course lower than the total power the LED consumes. Some part of the power becomes light - the more efficient your LED module, the bigger part of the total power will be transferred in to light, easy to verify if you compare the luminous flux to the power. As a fist rule we use 80% of the total power to be dissipated (Pd.

  Now we just define mathemacally what would be the maximum thermal resistance our heat sink should have, or define the maximum raise in temperature our heat sink will create when dissipating Pd 11.76W.

  Suppose we will use a phase change gap filler thickness 0.18mm (thermal pad which becomes fluid on first heating cycle) with a thermal resistance of 0.4°C/W.

  Let's see what we know already and what is missing.

  Only thing missing now is the needed thermal resistance of the heat sink Rh.

  Choose a heat sink for thermal management with an Rth value of < 4.65°C