Wednesday, January 11, 2012

Why do low-wattage LED lamps need such big heatsinks?

When you think about it doesn't it seem strange that a 10W LED light bulb needs a heavy heatsink with fancy fins when a 100W incandescent doesn't?

Here's what I have found.

A 100W incandescent light is too hot to touch - about 90-130C (200-270F). But that is using the surface of that glass bulb as the heatsink. The filament that actually gives the light is 2000-2500C (3600-4500F)!!! The gasses inside of the bulb actually don't carry heat to the surface of the glass at all. That's an advantage in this bulb since the heat is key for getting the light.

LEDs are semiconductors. Like most semiconductors (like all the ones that drive your cell phone, PC, iPod, etc.) in order to survive at all they need to keep their temperature (Tj) below 150C (300F). In order to have a decent lifetime and keep other parameters within control it needs to be closer to 85C (185F - a good temperature to brew coffee at). That's thousands of degrees cooler than the filament!!

The the actual light producing part, the LED junction, is tiny (less than 10 square mm). Since it is small it doesn't have enough surface area to get rid of the 10W* and still maintain this temperature. So it needs to have more surface area - which is the heatsink. The heatsink is made out of Aluminum since the metal carries the heat to its own surface pretty efficiently. Without the heatsink the LED would get too hot for a semiconductor (not too hot if it was a filament).

*10W is actually not used only by the LED. There are some other electronics inside an LED bulb that convert AC down to the low DC voltage that the LED needs. These parts are not perfectly efficient so they use some of the 10W themselves - which is as much as to say they generate heat too.

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