How hot is too hot?

[LLCoolBeans]

I realize that every model of high-power LED will have a different normal operating temperature, but is there some general rule of thumb in regards to operating temperature? What temperature would likely cause damage or significantly reduce the lifespan of your average high-power LED?

 

[Oddjob]

I always figured if I can't hold a light it probably should not be left on too long but I'm only guessing.

 

[LukeA]

Keep it as cold as possible.

 

[LLCoolBeans]

Keep it as cold as possible.

Yes, understood, but as cold as possible may not be cold enough or maybe too cold (i.e. heat sink overkill). I need to know what an acceptable operating temp range is.

 

[LLCoolBeans]

Really? No one has an answer?

Ok, let's put the question another way. I built a floor lamp using a single MC-E. After letting the lamp warm up for more than an hour, I can put a thermometer up against the heat sink and it reads 110 degrees F.

Is that too hot? 110 doesn't sound too bad, but what if it were higher? At what point would I need to redesign my heat sink?

 

[LukeA]

Cree specifies max junction temperature as 100˚C. How hot the heatsink is when the junction is that temp is determined by the thermal resistance of the heatsink.

 

[Illum]

when eggs solidify....around there +/- 80C would be what I'll be reaching for.

In the words of mcgizmo

Proper thermal relief results in warm to the touch and not "hot".

 

[Axkiker]

im right with you buddy.... Im fooling around with crees and soon k2's and im trying to figure out how hot as well.

I have been toying with the idea of getting one of those laser IR thermometers.

I wont be able to get a true junction temp but i could get a reading at the hottest point possible by just pointing the laser.

 

[jtr1962]

Junction temperature is more important than heat sink temperature. The heat sink can be at room temperature but if you're running an MC-E or P7 at 2 amps per die then the junction temperature will be in excess of 130°C.

See this post for the effect of junction temperature on lifetime. Basically for longest life we want junction temperatures no higher than 55°C. For an XR-E running at 700 mA this means the heat sink must be around 35°C or less.

 

[LLCoolBeans]

Junction temperature is more important than heat sink temperature. The heat sink can be at room temperature but if you're running an MC-E or P7 at 1 amp per die then the junction temperature will be in excess of 130°C.

See this post for the effect of junction temperature on lifetime. Basically for longest life we want junction temperatures no higher than 55°C. For an XR-E running at 700 mA this means the heat sink must be around 45°C or less.

Difficult to measure junction temp.

Well, I'm running the MC-E at 700ma with dies wired in series. I did another reading and it was 115 degrees F, so that's just slightly over 45 degrees C. I guess that means I'm probably safe, but I might not get ideal longevity.

It's a fairly large heat sink BTW, MC-E can really throw some heat.

 

[jtr1962]

Difficult to measure junction temp.

You can't measure it directly but you can calculate it based on typical junction to thermal pad resistances and heat sink temperatures. For the XR-E this resistance is about 8°C/W. For the MC-E and P7 it's around 3°C/W. In the example you gave I'm calculating a junction temperature of around 75°C. This isn't bad at all. According to the chart the LED should last about 70,000 hours to 70% brightness.

 

[SemiMan]

Depends on the LED and how long you want it to last.

Realistically for a flashlight, as long as you keep it below the maximum junction temperature, life is likely going to be as long as you are going to ever want (or till a better LED comes along). If you are building a light for your house that is on 12 hours a day, then you may want it a lot colder.

Also depends on the LED vendor. With Cree, they may spec a maximum die temp of 145C, but the temperature at which they rate the life may be at say 90C. Lumileds tends to rate their life at much higher die temperatures, say 125C or higher. That may allow you to run the leds a lot hotter and still have them last.

Semiman