Norman99
Well-known member
A led's 50K hour lifespan is an approximation based on the die's thermal junction temperature of well below 85 degrees C. A led's usual specs when placed in their proper "binning" code is taken at 25 degrees junction, but that is like when the led is really "idling" at a low input current of say 350 milliamps. When that led gets up to full rated output, that junction temperature may get to 85 degrees C or possibly higher depending on the input current. Designers know all too well that heat above the junction limit will eventually destroy the led die and when it reaches that point is called thermal runaway. The led has only a few minutes of life left or perhaps seconds if the heat is not dissipitated fast. Thermal management is the prime consideration of leds, especially when dealing with a 100 watt emitter. The heat sink by itself and the given size constraints of the enclosure is not enough to allow passive ( no fan ) cooling, and therefore a fan of sort had to be used. If the heat sink, by very rough rule of thumb, is hot enough to be bearable, then that would look good. When it gets so hot as you can touch it for only no more than 2 seconds, then the led junction temperature may well be over 85 deg C. It is almost impossible to measure the led's junction temp as it is the area immediately below the die surface and over the supporting substrate. The design specs of that particular led will tell what that temp can be as the max. but trying to find the specs are slim at best. If your newly installed fan is doing the job at a marginal level, then perhaps the led will last more than 10K hours. You will definitely know for sure if within 50 hours of use at full power that the fan is adequate. At lower dim levels I don't think you will encounter any problems as the heat is logrythmically reduced by just throttling down the brightness just a bit. As an example, reducing the brightness by 25% may emit less than 50% worth of heat. Leds work this way and some led lighting MFRS know this and will only let the led go as far as 80% of their rated full potential. That extra 20% makes very, very little difference in perceived brightness but will require more heat to achieve, not worth it. You can do exactly the same by dimming just a wee bit. I would keep touching that heat sink at the warmest side and see if it reaches the point of getting too hot to touch. If you have to change the fan for whatever reason, the Sunon MagLev fans are tops in my book. I think DgiKey may stock them. They are known to be well made and silent.
