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LM3481 - SEPIC link capacitor RMS current equation seems wrong

Fred P
Expert 2235 points
Other Parts Discussed in Thread: LM3481

In LM3481 datasheet, I think equation 58 for RMS current through the link capacitor (Cs) in SEPIC configuration is wrong.
It does not give the same result as the equation suggested in TI article "Designing DC/DC converters based on
SEPIC topology" from Analog Application Journal Q4 2008.

During Ton, current through the capacitor is the current through L2.
During Toff, it is the current through L1.
RMS value I find using the full integral formula on that piecewise function is always very near Iin*sqrt((1-D)/D) as suggested in the article.  Therefore, it looks right to me.

I am not quite sure where eq 58 comes from but it does not look right.

Frederic

  • 0
    Expert 2235 points
    Any comments on that?
  • 0 in reply to Fred P
    TI__Intellectual 1775 points

    Hi Fred,

    Thank you for the comment, I looked at the equation on the datasheet and it does look odd. The equation from the article you are referring to is valid as long as the inductor currents are assumed to be DC (inductor current ripple negligible).

    I looked into the RMS current  across the capacitor and I come up with :

    Since IL1 is equal to Iin and IL2 is equal to Iout. If the inductors have the same value as is usually the case and assuming the DC voltage across the capacitor is close to the input voltage, ΔIL1= ΔIL2 and the equation becomes:

    This is assuming the SEPIC capacitor is large enough so that the inductor ripples are linear.

    If the ripple currents are assumed small enough to be neglected, the equation simplifies to the one you mentioned:

           with:  coming from the input/output ratio of the SEPIC.

    And so:

      

    I tried to simplify equation 58 from the datasheet back into this simple form but I have not managed to get the same result. We'll keep looking into it and modify the datasheet as needed.

    Again thank you for pointing that out.

    Florent

  • 0 in reply to Fred P
    TI__Intellectual 1775 points

    Hi Fred,

    Sorry for the previous messages. This forum does not seem to support copying equations or images straight from Word. I hope this shows the equations properly:

    Thank you for the comment, I looked at the equation on the datasheet and it does look odd. The equation from the article you are referring to is valid as long as the inductor currents are assumed to be DC (inductor current ripple negligible).

    I looked into the RMS current  across the capacitor and I come up with:

    Since IL1 is equal to Iin and IL2 is equal to Iout. If the inductors have the same value as is usually the case and assuming the DC voltage across the capacitor is close to the input voltage, ΔIL1= ΔIL2 and the equation becomes:

    This is assuming the SEPIC capacitor is large enough so that the inductor ripples are linear.

    If the ripple currents are assumed small enough to be neglected, the equation simplifies to the one you mentioned:

           with:   coming from the input/output ratio of the SEPIC.

    And so:

      

    I tried to simplify equation 58 from the datasheet back into this simple form but I have not managed to get the same result. We'll keep looking into it and modify the datasheet as needed.

    Again Thank you for pointing that out.

    Florent