Tool/software:
I refer to the TO-252 package for LM317AMDTX.
I am trying to understand what may be the reason(s) behind the large thermal resistance between the Theta_JC (Bottom) and Theta_JB values?
Thank you
Thanks for the document.
I have a better understanding of the test methodology now but still could not understand how the Theta_JC (bottom) and theta_JB values could have such a big difference.
1. My strongest suspicion would be that the PCB did indeed play a substantial role in the measurement for the Theta_JB value. Would this understanding be correct?
2. If so, for the purpose of conducting thermal simulations using the block-and-surface resistance method as defined in JESD15-3, I am undecided on which of the following approaches would be more representative.
i) Theta_JC (top) + Theta_JB
ii) Theta_JC (top) + Theta_JC (bottom)?
Would you be able to advise? My application has heat sink on the top of the component and heat sink under the component through the thermal vias.
Hi Benedict,
1. Yes, the theta_JB value does rely on the PCB. As we can see with the junction temperature, this parameter requires the PCB's temperature:
This paper also helps explain the theta_JB parameter: https://www.ti.com/lit/an/slua844b/slua844b.pdf
2. I believe the paper above also highlights the formulas to use to calculate the junction temperature of each use case, theta_JB, theta_JC, etc...
This table listed on page 10 shows how to calculate junction to board thermal resistance:
When using a heat sink, typically we would use the junction to case thermal resistance not the junction to board thermal resistance.
Please see page 7 of Semiconductor and IC Package Thermal Metrics (Rev. D) (ti.com).
Using equation 5 and equation 6, you are able to calculate the junction temperature with a heat sink.
Please let me know if you have any additional questions or concerns.
Best,
Hannah
