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TPS7A84A: Natural Convection Heat Transfer Coefficient for PCB

Will McKinney
Prodigy 30 points
Part Number: TPS7A84A

I am trying to calculate the natural convection heat transfer coefficient for a PCB with several components on it, including TPS7A8400ARGRR. I have been referencing document SNVA419C. Looking at Table 2, the calculations are gone through to estimate h. However, the h value calculated (14.39) is different than what is described in text above Table 2. Additionally, I was wondering why the Nusselt Equation was calculated for 2 surfaces of a horizontal plate? Is this to account for heat transfer on both sides of the PCB? It seems like the Grashof number was miscalculated because the volume expansivity was left out of the formula. 

  • 0
    TI__Mastermind 37895 points

    Hi Will,

    What is the purpose of these calculations? We do not normally do calculations manually; the thermal information in the datasheet is obtained using computational fluid dynamics simulation tools. Are you attempting to obtain some figures that are not included in the datasheet?

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 30 points

    The purpose of the calculation is for use of the convection coefficient in a thermal simulation. Yes, I am trying to estimate the convective heat transfer coefficient for a 70C ambient temperature. 

  • 0 in reply to Will McKinney
    TI__Genius 13085 points

    Hey Will,

    Natural convection coefficients are a bit outside of our LDO product line's wheel house. We've consulted with our usual expert for thermal simulations but he wasn't the one who wrote the App Note and doesn't do these calculations by hand so didn't know how to help. We're going to have to look further afield to see if we can find someone who can answer your questions. 

  • 0 in reply to Will McKinney
    TI__Genius 13085 points

     Hi Will,

    I reached out to the apps engineer who wrote this and here are his comments, I hope they are helpful.  

    1) Table 2 calculates the Heat transfer equation using Grashof number which is more accurate for natural convection.  The calculations in the text above are for the calculation of the heat transfer equation using the Reynolds number which is more accurate for forced convection conditions.

    2) Although the text in the table says the nusselt number for two surfaces, the calculation was for one surface, but two surfaces are assumed in the board area calculation because of the two sided PCB.

    3) On the Grashov question for volume, I believe this is covered by the length^3, but I’ll be honest it’s been 10 years since I wrote this and since I am primarily a power guy, I would need to go back to my text books to see if I made a calculation error.

     

     

  • 0 in reply to Kyle Van Renterghem
    Prodigy 30 points

    Hi Kyle, 

    Thanks for the reply and the answer to my questions. I think that clears up some things. I appreciate your help.