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Do high power LEDs emit IR heat in forward manner?

Posted: 28 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:high power LED? PN junction? power semiconductor? IR meter? thermographic measurements?

If you could review the all the lighting industry's literature of the last 10 years about high power LED lamps and luminaires, would you question this headline? That is, if you read that a high power LED emits no UV or IR and that any heat is created only in the PN junction and then transferred to a heat sink. Okay on the UV part; but the IR part? Read on.

What you will further seem to know for sure is that the PN junction is the hottest point in an LED (or any other power semiconductor for that matter) with the LED mounting substrate and or heat sink being somewhat cooler. How much cooling is needed can be rather accurately calculated if you know a) the heat sink temperature (easy to determine) and b) the junction-to-case thermal resistance (easy to determine for the data sheet).

You likely agree with what you have just read. Having been involved over several decades with silicon power semiconductor and power LEDs in one way or another I also would have agreed... until I did not.

It all started to unravel during some simple experiments relating to a high-power LED array used to illuminate a phosphor-coated sheet (remote phosphor application) to create white of certain properties. The application related to a need for high CRI (over 90) /high CCT (5600K) lighting for TV and motion picture studio lighting.

Operating the array at only 25 watts (about 30% of maximum, and a fan-cooled heat sink and LED substrate both under 35§C (meaning the junction was, with 100% certainty, under 40-45§C), I observed, as I positioned the small remote-phosphor sheet over it, that my hand immediately got too hot. In the past I had never paid much attention to this kind of thing. I put my hand virtually on top of LEDs and almost got burned. How could this be, I asked, if the LED substrate is less than 40§C.

I positioned a thermocouple wire across the LEDs and measured over 125§C!! Again, how could this be? I measured again with non-contact IR meter and got same temperature. I wondered if this involved some peculiarity associated with these specific royal blue LED arrays. Over a period of time I proceeded to operate a whole range of LED arrays rated from 10 to 100 watts- white, royal blue, greenCOB arrays, multi-SMD arrays and large-single-chip typesÿ(figure 1: A- Bridgelux white RS multi die; BPhilips royal blue LXK multi-SMD ; CEpistar royal blue multi-die; DLuminus CBT90 green (single large die) ECitizen CLL330 white multi-die FCree CXA2520 white multi-die).

Figure 1: A variety of LEDs tested with consistent results.

Virtually identical observations. Operating at less than full power, with substrate below 50C, the top surfaces of the array were measured at well over 100C and as high as 150C. Instrumentation error? Really only 35-40C? My imagination? A drop of water placed on top of any one of the devices boiled off in seconds. The last I knew, water boiled at 100C.

A sliver of paraffin, specified for a 70C melting point, placed on any one of those surfaces, melts "immediately". During a break in that comparative process, I placed a small square of phosphor coated PET (high temp plastic) over one of the royal blue arrays, while monitoring colour temperature (i.e. CCT). Immediately at LED turn-on, the CCT was about 5600K but in less than 30 seconds it rose to 7,000, 10,000 and finally above 12,000K and the light became very "bluish.

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