Do LEDs produce heat?
A frequently listed advantage of LEDs is that they do not produce heat. In a way, this is true, LEDs are cool to the touch because they usually do not produce heat in the form of infrared (IR) radiation. This obviously doesn’t go for IR specific LEDs.
IR radiation is what actually heats incandescent bulbs and other light sources, making them hot to the touch. Without IR radiation, LEDs are able to be placed in spots where the heat from other sources would cause a problem (grow lights, reef tank lights, illuminating food, etc.).
Although LEDs are cool to the touch, within the devices themselves, there is plenty of unwanted heat. This heat comes from the inefficiency of the semiconductors that generate the light. The radiant efficiency (total optical output power divided by total electrical input power) of LEDs is typically between 5 and 40%, meaning that 60-95% of the input power is lost as heat. So what do you do with all this excess internal heat?
In one sense this is true: LEDs are cool to the touch because they generally don't produce heat in the form of infrared (IR) radiation (unless of course they are IR LEDs).
IR radiation heats the enclosures and surroundings of incandescent bulbs and other sources, making them hot to the touch. The absence of IR radiation allows LED fixtures to be positioned in locations where heating from conventional sources would cause a particular problem e.g. illuminating food or textiles.
However, crucially, heat is produced within the LED device itself, due to the inefficiency of the semiconductor processes that generate light. The wall-plug efficiency (optical power out divided by electrical power in) of LED packages is typically in the region of 5-40%, meaning that somewhere between 60 and 95% of the input power is lost as heat.
Why should LED junction temperature be managed?
With high-power LEDs, it is crucial that you remove heat through efficient thermal management. Without good heat sinking, the junction (internal) temperature of the LED rises, causing the LED characteristics to change for the bad.
As the junction temperature of an LED increases, both the forward voltage and the lumen output decreases (see Picture 1). Not only is this decreasing the brightness and efficiency of your LED but this junction
temperature affects the overall lifetime of the LED as well. LEDs don’t usually fail catastrophically (although some may, especially if you over heat them); instead, the lumen output of the LED will decrease over time. Higher junction temperatures lead to faster LED deterioration. This is why it is crucial to keep your LED junction temperature low. Also take note that if you are over driving your LED (putting more current to it than what it is rated) this will drive temperatures up so high that permanent damage can occur.
What affects junction temperature?
The ambient temperature and the drive current both affect the junction temperature of LEDs. Other influences are the nature of the light, whether it is steady state or pulsed, and then the one we are really interested in, LED wattage per unit area of heat sink (surface that dissipates heat).
The most important part of LED cooling is the thermal path from the LED junction to the outside of the light fixture. Heat needs to be conducted away from the LED in an efficient manner, and then removed from the area by some sort of cooling or dissipation.
So there you have it, now you should know why you need an LED heat sink. When choosing your heat sink and finding which heat sink to use, remember to take into account all the factors that contribute to heat and the cooling process:
- LED wattage
- The number of LEDs you are powering
- The temperature of the environment where the LEDs will be running
- Whether the LEDs are mounted in an enclosed or open space.
Stay tuned for more posts on LED heat sinks and choosing the right one, but this should get you started in setting up your LED system!
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