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TPS54521 thermal management problem

BX27
Genius 3900 points
Other Parts Discussed in Thread: TPS54521

Hi there.

We are designing a buck with TPS54521. Fixed Input 12V, and Output requirement is 5V/4A. We have to estimate the junction temprature to prevent overheat.

As simulated in WEBENCH, the efficiency is about 90%, so the power dissipation Pd=(5V*4A/0.9)*(1-0.9) = 2.22W. Assume the environment temperature Ta=25 degree. When I read thermal parameters on page 3 of datasheet, my questions are :

(1) why there are two θJA? one is 47.2 and another is 32 ? what's the difference?

(2) Which parameter should I use to estimate the junction temperatur? θJA or θJB? some one suggest using Tj=Ta+Pd*θJA, that seems reasonable, but I noticed that TPS54521's PowerPad is helpful to dissipate heat, since the thermal resistance of juction-to-board θJB is just 3.2 degree/watt, much less than θJA.

(3) According to datasheet, these thermal parameters are tested under certain conditions. How will these parameters change when it comes to the real product?

I am confused about this situation. Does TI has any learning material that can help me understand these thermal parameters and give some guidacne to make reasonable estimation in thermal management ?

Thanks a lot!

  • 0
    TI__Guru**** 191445 points
    1. The first Theta JA is for a JEDEC standard coupon. This value is is best used for comparison purposes. It allows direct comparison of thermal performance under identical conditions from part to part. The JEDEC coupon is not particularly effective at heat dissipation. The second number is for the PCB configuration in note 3. That is more of a best case condition where you have over 18.75 square inches of copper ground to dissipate the heat. So the two numbers listed represent a worst case and best case scenario. A well designed PCB should perform somewhere in between. Remember, you are operating near 90% efficiency, so only 10% of the power is dissipated in the TPS54521 and 90% in your (possibly) near by load. That power dissipation can greatly effect thermal performance at a board level.
    2. You can use Theta JB if you know the thermal parameters of your PCB. It is best used if you have thermal modeling software for your PCB.
    3. Yes the parameters for theta JA will change with the PCB. Others such as theta JB are intrinsic to the device and are not PCB dependent, assuming you are using the recommended footprint and thermal pad.

    You can get more info on this topic here:
    www.ti.com/.../spra953b.pdf
  • 0 in reply to JohnTucker
    Genius 3900 points

    Hi John!

    Thanks for your answer.

    1. Yes, 90% pwer is on load. Our load is 200 relays evenly distributed on PCB, each consumes 20mA current, so total current is 4A. Is there any software or practical method to simulate/estimate the heat condition of the whole PCB? So I could adjust my design to control the PCB temperaure rise in advance.

    2. I intend to use Theta JB to calculate Tj. So I need to konw the "PCB-to-ambient" thermal resistance, right? Where could I get its value if I already know PCB's material, area, copper thickness ?

    Thanks again!

     

    Yi

  • 0 in reply to BX27
    TI__Guru**** 191445 points

    Your thermal analysis software or possibly PCB layout tool should be able to calculate that for you.  It should also take into account the heating effects from other circuitry on your PCB.