Thermal Resistance Calculation

That should be a lower case k, so as to NOT confuse it with degrees Kelvin (K).

k is the "coefficient of heat transfer". You need it to calculate the junction resistance. It's a complex issue though, you should get an ME involved who has experience in this. But if you insist on pursuing it yourself, do a search on that term in quotes, it will pull up academic websites where you might be able to delve into the complexities.

Or deal with a supplier NOT in China who knows what they are doing instead of just spewing out cheaper product designed by others. The reason they have not responded is likely because they they have no clue, but are frantically posting on ME forums trying to get some poor unsuspecting ME to give them the answer.

Hi JRaef, I don't think that's correct. In this case, I believe that it is indeed just degrees Kelvin. Thus, K/W = Kelvin per watt, or deg C per watt.

Double check with a google of Rthjc

Or look at this:

http://www.signalpro.biz/heatsink.pdf

And this:

http://www.vishay.com/docs/84062/84062.pdf

and this:

http://www.newark.com/pdfs/techarticles/vishay/InstructionsAssemblyInstructions-SolderingandHeatRemoval.pdf

FYI Ithought I was dealing with US company Vishay but the product packaging states made in China.

K/W is the same as oC/C.

1 K (one kelvin) increment in temperature is identical to 1 oC (1 degree Celsius).

brgds

Snel

You would have got the answer straight away if you would have checked Wikipedia instead of waiting for answer from manufacturer or CR4.

Still looking? Follow this link: http://en.wikipedia.org/wiki/Kelvin

good luck

Got answer from manufactures. Units are degrees kelvin/watt but their original data sheet gave a figure of 15 K/W , they have now amended this 0.15K/W and issued new data sheet.

Calcs now work fine.

Thanks for all replies

Since the units he has given are K/W, he probably refers to the thermal resisivity.

The units for thermal resisivity (usually represented by lower-case k) should be Kelvin-meters per Watt, or K·m/W.

This property may be used to obtain the thermal conductivity by taking its reciprocal.

It's not thermal resistivity, it's thermal resistance - from the hottest spot in a junction to some external reference. You apply some power (watts) to a semiconductor and you want to be sure that that the maximal allowable temperature is not exceeded on the junction(s). Hence K/W is standard (kelvins per watt - not degrees Kelvin or Kelvins).

brgds

Snel