help with K2 Junction Temp

[Axkiker]

Hey someone on here explained how I could calculate my junction temp based on voltage drop for one of the cree leds I was experimenting with.

Can someone tell me how to do this for one of the k2 leds? I am experimenting with some of the k2's (not the new ones with ttfe or whatever they have now) Im trying to figure out the same thing but im unsure what figures to use. Im pushing them to 1000ma.

I looked on the phillips website but am unsure if im reading this stuff right.

Under the section which said electrical characteristics at 1000ma it says mv/C and C being 2 degrees.

So my voltage drop was .480 I assume this means 480 / 2 = 240 degrees C. However this doesnt make any sence. I am using a larger heat sink than my crees and the actual led doesnt seem as hot


so am I doing something wrong?? I just dont think it is running that hot

 

[Oznog]

Determining die temp through dv/dtemp is not that easy. For one, you need to know exactly what the forward voltage is at that current (1A, and it must be accurate) with the die at a known temp. One way to do this is deliver short pulses of known current, too short to heat up the die, and measure voltage. However, it's too short for a voltmeter and many scopes won't deliver an extremely accurate reading so there's some testbed considerations there.

The dv/dtemp is not all that tightly controlled, so there may be significant variation from the spec sheet which will really mess with your result. To actually measure dv/dtemp, you could place the mounted device on a hotplate or whatever, accurately measure the sink temp, and perform the same test as above to see what the dv was for the new temp.

Like I say, it's a bit difficult.

Most people only want to know how well their sink is working, and sink temp is easy to measure. dv/dtemp is important for measuring the performance of thermal board or thermal epoxy where the temp on the other side of the interface cannot be measured directly. Most people don't need to do this honestly, you just need to use AS epoxy, or solder a solderable pad. I mean if you've applied AS or soldered properly, you can't do much better so why bother with a complex test.

 

[Axkiker]

Determining die temp through dv/dtemp is not that easy. For one, you need to know exactly what the forward voltage is at that current (1A, and it must be accurate) with the die at a known temp. One way to do this is deliver short pulses of known current, too short to heat up the die, and measure voltage. However, it's too short for a voltmeter and many scopes won't deliver an extremely accurate reading so there's some testbed considerations there.

The dv/dtemp is not all that tightly controlled, so there may be significant variation from the spec sheet which will really mess with your result. To actually measure dv/dtemp, you could place the mounted device on a hotplate or whatever, accurately measure the sink temp, and perform the same test as above to see what the dv was for the new temp.

Like I say, it's a bit difficult.

Most people only want to know how well their sink is working, and sink temp is easy to measure. dv/dtemp is important for measuring the performance of thermal board or thermal epoxy where the temp on the other side of the interface cannot be measured directly. Most people don't need to do this honestly, you just need to use AS epoxy, or solder a solderable pad. I mean if you've applied AS or soldered properly, you can't do much better so why bother with a complex test.

Well my problem is that im not dealing with a traditional heat sink material (aluminum copper) Im experimenting with a substance which I can vary the aluminum content. So im trying to figure out a way i can figure out or test for the juction temp to see how well im dissipating the heat.

if you have any ideas let me know.

 

[ledstein]

Maybe you should give us more details about your project (if its not a secret, ofcourse:caution:).

 

[HarryN]

I hope you don't mind, I posted my answer to your question in the K2 technical information thread.

 

[Axkiker]

Maybe you should give us more details about your project (if its not a secret, ofcourse:caution:).

No its nothing secret. Im trying to work out a lighting project using a epoxy type material filled with aluminum. This stuff has good heat dissipation properties but im not sure if its enough to dissipate the heat these new leds generate.

Im experimenting with different mixtures etc but cant really grasp how well or not well its workign cause I dont have a way to measure junction temp.

its nothing flashlight related