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LMZ1X-LMZXEXTDESIGN-CALC: Lateral Thermal Resistance of FR4 (per square mm)

Marvin Cantos
Expert 6560 points
Part Number: LMZ1X-LMZXEXTDESIGN-CALC
Other Parts Discussed in Thread: LMZ12001, TPS92623-Q1

Hi Team, 

Good day. I am posting this inquiry on behalf of the customer.

Please see the inquiry below.

"I analyzed the document you made about Estimated Junction Temperature and I have some questions about the "Calculations" Sheet. In particular I don't understand formula:

Lateral Thermal Resistance of FR4   (per square mm) =IF(velocity_lfpm>0;1/(0.23*mm*0.221/10);1/(0.23*mm*0.221/3.75))

1) If air velocity is positive and >0 FR4 Lateral Thermal Resistance increases, instead if air velocity is 0m/s the thermal resistance decreases. this does not make sense.

2) why Lateral Thermal Resistance of FR4   (per square mm) doesn't take care of FR4 thickness ?!?! (the value 0.221 in the equation what is it)

If You can send me more INFO, I would be very pleasure."

Thank you for extending your help.

Kind regards, 

Marvin

  • 0
    TI__Mastermind 49370 points

    Hi Marvin,

    What is the document for estimated junction temperature? Could you send me that calculation sheet?

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    TI__Expert 6560 points

    Hi Nancy, 

    Thank you for your prompt response. Please see the link below.

    https://www.ti.com/lit/zip/snvu041

    Thank you.

    Kind regards, 

    Marvin

  • 0 in reply to Marvin Cantos
    TI__Mastermind 49370 points

    Hi Marvin,

    Lateral Thermal Resistance of FR4   (per square mm) =IF(velocity_lfpm>0;1/(0.23*mm*0.221/10);1/(0.23*mm*0.221/3.75))

    1. Which cell can I find above equation in the top sheet of excel tool? I could not find it.

    2. What is the reason customer would like to calculate the thermal resistance?

    3. For LMZ12001 module, we already have the thermal resistance junction to case and junction to ambient in the datasheet.

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    TI__Mastermind 49370 points

    Hi Marvin,

    I could not find this document in ti.com. Did it release to ti.com? How did customer get this document?

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    TI__Expert 6560 points

    Hi Nancy, 

    Please see the link below.

    https://www.ti.com/tool/LMZ1X-LMZXEXTDESIGN-CALC

    Thanks

    Kind regards, 

    Marvin

  • 0 in reply to Marvin Cantos
    TI__Mastermind 49370 points

    Hi Marvin,

    Thanks, could you answer my question as below?

    Lateral Thermal Resistance of FR4   (per square mm) =IF(velocity_lfpm>0;1/(0.23*mm*0.221/10);1/(0.23*mm*0.221/3.75))

    1. Which cell can I find above equation in the top sheet of excel tool? I could not find it.

    2. What is the reason customer would like to calculate the thermal resistance?

    3. For LMZ12001 module, we already have the thermal resistance junction to case and junction to ambient in the datasheet.

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    TI__Expert 6560 points

    Hi Nancy, 

    Please see the feedback from the customer.

    1) the lateral thermal Resistance of FR4 is calculated in cell C21 (Sheet : Calculations, this is an invisible sheet where you do all math calculations).
    2) I would like to calculate thermal resistance of TI LED Driver like TPS92623-Q1 to design PCB and Heatsink in my Project.
    Thank you.
    Kind regards, 
    Marvin
  • 0 in reply to Marvin Cantos
    TI__Mastermind 49370 points

    Hi Marvin,

    1. I could not see sheet "calculations" and C21.

    2. I am not sure if this tool is only for LMZ1xx modules and may not apply to TPS926123-Q1. You need to create a separate post for question regarding TPS92623-Q1.

    I am checking with the thermal team and it may take several days. Please be patience.

    Thanks,

  • 0 in reply to Nancy Zhang
    TI__Mastermind 49370 points

    Hi Marvin,

    1. How can customer see the invisible sheet and cell? How did they do to see internal equation? Please let me know.

    2. 1) If air velocity is positive and >0 FR4 Lateral Thermal Resistance increases, instead if air velocity is 0m/s the thermal resistance decreases. this does not make sense.

    As a default data in the tool, if I enter airflow in LFPM=0, then I get below value.

    If I change airflow in LFPM=100, then I get lower value. It does make sense. Why did customer get opposite value?

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    TI__Expert 6560 points

    Hi Nancy, 

    Please see the update from the customer. 

    1) The Excel file is unprotected, so hidden sheet are available, you need to set them as "visible" in VB tool
    2) I try to insert air velocity with lower value than 100[LFPM] for natural convection analysis and I got different values ( I also try with 100 LFPM and is alright)

    3) Another question is why changing Board Thickness (setting n° thermal vias to 0) doesn't change Junction Temperature. The equations you use to calculate FR4 thermal resistance don't take care of his thickness, instead the equations to calculate Copper Thermal Resistance use Copper Thickness . I attach you two images to let you see:
    Thank you for extending your help. 
    Kind regards, 
    Marvin
  • 0 in reply to Marvin Cantos
    TI__Mastermind 49370 points

    Hi Marvin,

    Can we discuss this offline? I will loop our thermal team. They can not see the equation as well. Could you give us the detail for "visible in VB tool"? I will close this post.

    My email: nancy_zhang@ti.com

    Thanks,

    Nancy