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ADC32RF80: Junction Temp (Tj)

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Replies: 6

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Part Number: ADC32RF80

hi, TI

If developers measure the case temperature and the board temperature,
do they obtain the junction temperature from the equation below?

Thanks and Best Regards,

OHTA

  • OHTA:

    The equations did not come through the post. Regardless, you can calculate the junction temp by:

    Tj [degC] = Tc [degC] + theta-jc [degC/W] * Pdiss [W].

    Alternatively you can substitute board temp and theta-jb.

    --RJH
  • In reply to RJ Hopper:

    Hi, RJH

    Thank you for your reply.

    I confirmed SPRA953C”Semiconductor and IC Package Thermal Metrics”.

    It says that the equation(Equation 4) is invalid.

    Also, SPRA953C says that Ψjt should not be used when application of a heat sink is intended. Instead, Equation 5 and Equation 6 should be used.

    but, I cannot measure θsa. so, I cannot calculate the junction temp by Equation 5 or Equation 6.

    For that reason, I deduced that equation( I thought that equation with the idea of a two-resistance model)

    which equation should I use for calculating correct junction temperature? 

  • In reply to RJ Hopper:

    hi, RJ Hopper

    sorry, I did not notice that the equations did not come through the post.

    I have attached files(the equation and Equation4~6 of spra953c)

    Could you please see files.

    Thank you and Best regards

    OHTA

  • In reply to OHTA:

    spra953c says that Equation 4 is invaild.
    Because, a very small percentage of heat energy in a typical plastic package is convected and radiated off the top surface of the package.
    so, I deduced the equation with the idea of two-resistance model.

    OHTA
  • In reply to RJ Hopper:

    Spra953c says that Equation 4 is invaild.

    Because, a very small percentage of heat energy in a typical plastic package is convected and radiated off the top surface of the package.

    Tj [degC] = Tc [degC] + theta-jc [degC/W] * Pdiss [W] _______Equation 4

    So, I deduced a equation below with the idea of two-resistance model.

    Is the equation below correct?

  • In reply to OHTA:

    I am unsure if the equation is correct.

    The ADC32RF80 is a QFN package with a thermal ground pad. Most of the heat is intended to be dissipated through the ground pad to the board. A heat sink on the bottom of the board improves the performance; this is implemented on the EVM and on the TSW40RF80 reference design.

    The picture above depicts adding a heat sink on the top of the device. Whereas adding a heat sink on top can't hurt thermally, it would be much more effective on the bottom. The app note references that dissipation to the top (i.e. case) is not accurate since most of the heat is not dissipated there.

    I have an issue with designating different power dissipations: Pd, Pc. The power dissipation of the device is fixed for a given set-up conditions. That power will be dissipated in one way or the other, through board and case. Given that the dissipation through case is limited, using the board is more accurate. I would focus on using the theta-jB parameters for the thermal calculations. If a heat sink is used on the bottom, conceivably, the additional theta-bs (board to sink) and theta-sa (sink to ambient) could be used similar to equation 6.

    If the goal is to determine junction temperature and you know the power dissipation of the device and you can measure the board temp, then I think that you can use the standard theta-jb equations.

    --Russell

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