I've read datasheet of some NPN transistors. In some cases, datasheet barely gives me the value of thermal resistance from junction to case, but without the one from junction to ambient. So how I can determine the thermal resistance from junction to ambient.
Here let's take SA1943 as an example.

 

They are assuming you are going to attach it to a heatsink so they don't give it. But the TO264 package is 40C/Watt.

 

Here's a paper showing the thermal resistance to air of many common semiconductor packages.

 

We can use data sheet to calculate Rhtj-c
Rthj-c = (Tj_max - Tc)/Ptot_max = (150°C - 25°C)/150W = 0.84°C/W

Or we can find it here
https://www.fairchildsemi.com/datasheets/FJ/FJL4315.pdf
http://www.st.com/web/en/resource/technical/document/datasheet/CD00249630.pdf

 

I think these two following articles might be helpful.
1.Thermal resistance
2.List of thermal conductivities
In some case, datasheets only mention thermal resistance from junction to case, and I want to determine thermal resistance from junction to ambient, so how can I do ?
Please take a look at the equation at the end of the first article. And my question is how we determine x, the length of the material (measured on a path parallel to the heat flow) (m), in case we don't use heatsink?

 

It will be a science project to get it very close, but to get pretty close you can use:

Junction to case plus 50 / square root of the exposed surface area in cm

For some reason it is never quite this good but you can try it with a few packages to get a rule of thumb for yourself.

PS.
Works for heat sinks too.

 

Hello,

You might want to have a look at this page of ESP on heatsinks:
http://sound.westhost.com/heatsinks.htm

It also contains a lot of calculations and tips.

Bertus

 

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In some case, datasheets only mention thermal resistance from junction to case, and I want to determine thermal resistance from junction to ambient, so how can I do ?
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Thermal resistances simply add.
So you take the junction-case thermal resistance and, if it's free air, you add the case-ambient value given in my reference in post#3.
If it's case to heat-sink then you add junction-case resistance to the thermal resistance from case to heat-sink (quite small if it's properly mounted with a thin layer of thermal grease) and then add the heat-sink to air thermal resistance.

 

Crutschow, Am I reading something wrong in your chart? It shows TO220 as 35C/W. I thought they were almost always 62C/W.

 

Crutschow, Am I reading something wrong in your chart? It shows TO220 as 35C/W. I thought they were almost always 62C/W.

So do I! It seems Mr.Crutchow's reference belongs to Linear Tech.Coporation, so it might be not true for other ICs from other manufacturer!
Please take a look at my attachments ! Both are the TO 220 type !