You can calculate the junction temp based on the Theta_JA parameter and the expected PLoss.
As an example For EN6347QA: Theta_JA=30C/W (Please check the datasheet page 5)
- Assuming your Vin=5, Vout=3.3 and Iload=4A
- Eff with that input output setting will be 92% (Please check the datasheet)
- Pout= Vout*Iload= 13.2W
- So PLoss= Pin-Pout=1.148W
- Delta_T= Theta_JA* PLoss = 35C
- So if we assume the ambient temp (TA)= 25
- Then the T_Junction = TA+ Delta_T = 60C
I hope that example is clear on how to calculate the expected junction temp.
Yes I know how to calculate the junction from Theta_JA. However, this is not very helpful as I need to measurethe junction when the chip is in vacuum and here the Theta_JA is not any good. I need to know the junction to top case so I can measure the top case temperature and the calculate the junction