# An Example of Misusing Thermal Resistances

Quote of the Day

Being defeated is often a temporary condition. Giving up is what makes it permanent.

— Marilyn vos Savant

In my previous post, I provided some definitions of thermal resistances and thermal characteristics. In this post, I want to show the problem that got me interested in this subject. An engineer misused the numbers and got answers that did not make sense. I require the folks in my group to estimate the junction temperatures of the parts they use so that the know if there design may have a temperature problem. In this case, the calculation required the use of the ψJT thermal characteristic. The basic problem is straightforward:

• Estimate the junction temperature of a part that dissipates 1.25 W.
• We have a large number of thermal parameters available. Which one do we use?

Figure 1 shows how the calculation went.

I thought this was a good example to illustrate the importance of understanding the thermal parameters and using them properly.

As an aside, I also looked at estimating the ψJT using the estimating formula from the previous post (Figure 2).

The result is not too bad considering the limited data I have.

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