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TPS7A11: Thermal measurement when use heat sink on top of IC

Koji Ikeda
Expert 5725 points
Part Number: TPS7A11

Hello Support Team,

Customer inquires how to measure temperature and assume junction temperature when they use TPS7A11 with heat sink on top of IC, could you support? Referring SPRA953 application note, Psi-JT can't be used in this case, can they use Psi-JB when use heat sink case?

Thanks,

Koji Ikeda

  • 0
    TI__Genius 12365 points

    Hi Koji, 

    Thank you for sharing this information. 

    Psi-JB is related to the PCB temperature, therefore, if a heatsink is being used, then the following equations have to be used: 

    What is the package for TPS7A11 and conditions i.e Load, Vin, Vout? 

    Best, 

    Edgar Acosta

  • 0 in reply to Edgar Acosta1
    TI__Expert 5725 points

    Edgar-san,

    Thank you for your support.  The equation 5 looks like to use thermal impedance, I feel that this might not fit what they would like to do, they would like to measure temperature around IC and assume the junction temperature. Do you have any other way?

    Thanks,

    Koji Ikeda

  • 0 in reply to Koji Ikeda
    TI__Genius 12365 points

    Hi Ikeda-San, 

    Equation 5 uses Junction-to-case (Rjc), which is applicable when using a heatsink, R(sa) is related to the Sink-to-Ambient, and R(cs) is : 

    Therefore, this is all related to the heatsink. 

    What is the package being used and conditions? I didn't get this answered, this will help determine if this can be used as Rjc has to be smaller tha Rja. 

    If the DRV package is being used, then there is no use for this, if the YKA package is being used, then customer can still use Equation 5. 

    Also, if the DRV package is used, is there a reason why the customer wants to use a heatsink? Proper PCB layout should help reduce Rja and help improve thermal dissipation. 

    Best, 

    Edgar Acosta

  • 0 in reply to Edgar Acosta1
    TI__Expert 5725 points

    Edgar-san,

    Thank you for your support. But, I assume that if they know all exact thermal impedance, they can use equation 5. But, in this case, they might not have full impedance information, so they would like to know junction temperature by measurement. Could you tell me how to measure and know junction temperature?

    They uses DRV in tight condition, they don't need PCB layout discussion.

    Thanks,

    Koji Ikeda

  • 0 in reply to Koji Ikeda
    TI__Genius 12365 points

    Hi Ikeda-San, 

    Thank you for the package information. 

    Will the Heat sink be on top of the package or on the Bottom of the PCB? Placing at the bottom would yield better thermal performance. 

    When using a heat sink then psi js (Junction to the Heat sink) must be used. 

    Understood that the device will be used in a tight condition and perhaps PCB layout will not have a big impact, yet the majority of the thermal dissipation will be through the PCB due to the exposed pad being soldered down to the board itself.

    When using SMD devices, only small amounts of heat flow to the top. The temperature difference between the junction to the top is usually smaller than the junction to the PCB. 

    What are the Vin, Vout and load conditions? 

    Here are some references: 

    AN-2020 Thermal Design By Insight, Not Hindsight (Rev. C) (ti.com)

    How to Evaluate Junction Temperature Properly with Thermal Metrics (Rev. B)

    Best, 

    Edgar Acosta

  • 0 in reply to Edgar Acosta1
    TI__Expert 5725 points

    Edgar-san,

    Could you provide PSI JS if you recommend to use it?

    Thanks,

    Koji Ikeda

  • 0 in reply to Koji Ikeda
    TI__Genius 12365 points

    Hello Ikeda-San, 

    This parameter comes from the thermal compound (thermal paste) used in the case that the heatsink is installed on top of the device, and if it is not provided then it has to be calculated. 

    If the Heatsink is installed at the bottom of the PCB, then RθJC(bot) Junction-to-case (bottom) can be used as an estimation. 

    Please refer to the App notes below for further information: 

    AN-2020 Thermal Design By Insight, Not Hindsight (Rev. C) (ti.com)

    How to Evaluate Junction Temperature Properly with Thermal Metrics (Rev. B)

    Best

    Edgar Acosta, 

  • 0 in reply to Edgar Acosta1
    TI__Expert 5725 points

    Hello Edgar-san,

    They plan to use heat sink on top of IC. I'm not sure if you answered customer expectation. They would like to measure temperature on any point around IC and know the junction temperature. I feel that the application not looks like to show the method to check all thermal impedance and  assume junction, this looks like paper work. Do you mean that it's tough to know junction through measurement and using PSI factor? If not and application note show the method, please show the page number and how to measure and assume junction temperature clearly.

    Thanks,

    Koji Ikeda

  • +1 in reply to Koji Ikeda
    TI__Genius 12365 points

    Hi Ikeda-San, 

    Psi JB can be used, the issue is that using only PsiJB without taking into account the heatsink can yield to be inaccurate. 

    Besides the previous statement, to use PsiJB the temperature of the PCB has to be measured, which can be a challenge specially with very tight spaces it has to be at least at 1mm from the device and centered to the package edge. If customer can measure the temperature near the IC adjusting for emissivity, then using TJ = TB + ΨJB × PD could give some estimate. This is because the Heatsink will have some impact on the thermal dissipation, and this still needs to be included on the calculations. 

    This again, will be only an approximation, below is a model shown in one of the application notes for the thermal resistances when using a Heatsink. Thermal Resistances can be treated as electrical resistances (series/parallel combinations); this can serve as a starting point (AN-2020 Thermal Design By Insight, Not Hindsight (Rev. C) (ti.com) page 8 Section 3.5.1

    From one of the other app note (How to Evaluate Junction Temperature Properly with Thermal Metrics (Rev. B)): 

    In this case Rjc(top) should be used and this ties back to:

    Again, if PsiJB is used it can give a rough estimate of what the Tj is, but a more accurate answer would be using equation 6. 

    If the heatsink is known and customer has their layout, I can submit a request to our thermal modeling group, but I would need the PCB files and use case conditions like Vin, Vout, Ta, and Pd. 

    Best, 

    Edgar Acosta