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.

Figure 1: Three Wacks at Estimating Junction Temperature.

Figure 1: Three Wacks at Estimating Junction Temperature.

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).

Figure 2: Psi-JT Esimate with Comparison to the Measured Value.

Figure 2: Psi-JT Esimate with Comparison to the Measured Value.

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

 
This entry was posted in Electronics. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *