Design Con 2015

True or false? High-power LEDs don’t generate IR heat in the forward direction like a filament lamp

-October 08, 2013

If one 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 35ºC (meaning the junction was, with 100% certainty, under  40-45ºC), 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 40ºC.

I positioned a thermocouple wire across the LEDs and measured over 125ºC!! 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, green-- COB arrays, multi-SMD arrays and large-single-chip types  (See Figure 1A- Bridgelux  white RS multi die;  B - Philips royal blue LXK multi-SMD ; C - Epistar royal blue multi-die;  D - Luminus CBT90 green (single large die)  E - Citizen CLL330 white multi-die F - Cree 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 50°C,  the top surfaces of the array were measured at well over 100°C and as high as 150°C. Instrumentation error? Really only 35-40°C?  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 100°C.                                               

A sliver of paraffin, specified for a 70°C 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 color 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.

I then examined the tiny phosphor sheet and found that the phosphor coating, with an organic, somewhat temperature-sensitive binder, in a couple of places, had actually melted away, allowing blue light passage.

To further determine if this perceived high temperature was a “phantom”,  I proceeded to place over all devices at one time or another a small (1” by 1” by  .032) piece of aluminum.   Without exception, each became a hot plate, elevating the aluminum temperature (as measured by a thermocouple on the top illumination-immune side away from the light). Without exception, the temperature got to between 125-150ºC.

To make it more interesting  I put a 3.5-inch tall  “Lena” model collimating reflector from Ledil  over the blue LED array, and the thin aluminum plate on top  of it  with a tiny water-filled aluminum cup on top of that plate (Figure 2).                                                                    

 

Figure 2 The experiment—A 3.5-inch tall  “Lena” model collimating reflector from Ledil  over the blue LED array, with a thin aluminum plate on top  of it  and a tiny water-filled aluminum cup on top of the plate. 

Next: The results

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