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WEBENCH® Tools/TPS54620: default indductor

Yuto Sakai
Mastermind 6100 points
Part Number: TPS54620

Tool/software: WEBENCH® Design Tools

Hi,

Please tell me about Webench simulation of TPS 54620.
When designing under the following conditions, the load transient oscillates, but why? The circuit constant has not been changed.
Why is it because the inductor placed by default is too large (56 μH), why is it optimized with this value? Optimizing efficiency with top priority will exceed 100 μH.

--------------------
Vin=12V,Vout=2.5V,Iout=0.3A
Load transient_TPS54620.pdf
--------------------

Best Regards,
Yuto Sakai

  • 0
    TI__Guru**** 191445 points
    Sakai-san,
    I'm not sure what you mean. The load transient in the Webench report does not show any oscillation. I can comment on your design though. It all results from specifying a very low load current relative to the 6 A rating of TPS54620, only 5% of the capability. Because of this, Webench will generate a design with a very large inductor. The effective load resistance is also very large. This drives the compensation components to unusual values and gives a very low closed loop bandwidth. Designing for highest efficiency in Webench decreases the switching frequency which drives the output inductor value even higher. High value inductors have lots of wire turns and larger size and higher DCR which hurts efficiency. I would recommend that you set Webench to design for a higher output current such as 3-4 A.
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    Sakai-san,

    By the way, did you see my private email sent Monday? Please respond if you can.
  • 0 in reply to JohnTucker
    Mastermind 6100 points

    Hi John-san,

    Sorry, I made a mistake in attaching the file.

    TPS54620_oscillation.pdf

    Attach load transient when performing design simulation focusing on efficiency.
    Although oscillation is seen when shifting from heavy load to light load, I think that this phenomenon is because the inductance value is too large.
    Is it impossible to design the inductance value so that oscillation does not occur when designing with efficiency top priority?

    Also, why is it that a large inductance value (56 μH) is automatically used when designing with a very low load to the current capacity of the device like this time? I think that the inductor is automatically selected so that the ripple current of the inductor falls within about 30% of the set Iout, is it correct?

    Best Regards,
    Yuto Sakai

  • 0 in reply to Yuto Sakai
    TI__Guru**** 191445 points
    Sakai-san,

    That oscillation is activation of OVP feature.
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    Yes the inductor is calculated by a factor corresponding to a percentage of the load current. If you specify a light load you will get a large inductor. We intend to limit this in the future. I think the minimum AC ripple current will be 5 % of the maximum rating (5% of 6 A for TPS54620), not 30 % of the requested current.