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Original question:

UCC28780: Worksheet Gripe

UCC28780: Need help with Mathcad worksheets

Gerrit Barrere
Expert 1245 points
Part Number: UCC28780

I am having trouble with calculations coming out wrong in the sluc644 Mathcad worksheets for this IC.  In another thread I was told that I should arbitrarily override a couple of the calculated variables.  This makes me really uncomfortable -- it means the worksheet calculations are wrong or incomplete, and I also don't know what the downstream effect would be of jamming in values in like this.

Can someone help me fix the worksheets so my ROPP no longer comes out negative?  I have been tracing back through the equations with no success.  The worksheets are a great help when they work, but when they don't it is really difficult to figure out where the problem lies.

I have made a number of changes to the worksheets and can provide my versions.  I have different operating conditions and components than the worksheets were originally written for.

  • 0
    TI__Guru 74095 points
    Let's clarify. The mentioned "another threshold" does not ask an arbitrary override but asks to manually change Vin_RUN, Rcs, and ROPP, for example, Vin_RUN = 200V, Rcs = 0.28-ohm then ROPP = 265-ohm, to acheive the curves in the two figures below ROPP to meet the targeted specs as much as possible. The current tool works in this way.
  • 0 in reply to Hong Huang
    Expert 1245 points

    Hong, please don't reply to this thread.  I am looking for help with the worksheets, which you aren't willing to provide.

    Vin_RUN is an input parameter which the user sets, so changing it is valid.  Manually changing RCS and ROPP is not.  Those values are the result of calculations which depend on many variables, and simply forcing them to an arbitrary number is proof that something is wrong with the preceding calculations.

  • 0 in reply to Gerrit Barrere
    TI__Guru 74095 points
    My reply is based on my conversation with the device team and the owner of the Mathcad file. The tool works in this way as I showed to you.
  • 0 in reply to Hong Huang
    Expert 1245 points
    Hong Huang, please don't respond to this thread.

    Here are some of the biggest worksheet problems I have found so far, in worksheets #1 and #2:
    > Have to override RCS and ROPP calculations in #2 to avoid negative ROPP and get proper curves for VCST and OPP.
    > OPP Curve in #2 doesn't seem to match data sheet figures. I suspect the fixed voltages in the VCST_OPP formula, perhaps other problems?
    > The curves for switching freq in #2 don't come near fSWmin from #1. Transformer inductance, losses, and other parameters depend on this, and one of the calculations appears to be wrong.
    > #1 says min switching freq occurs at max load & min line, but that isn't what the curves for switching freq in #2 show.
    > #1 recommends a TVS across the clamp cap, but the voltage is not calculated. I don't see this TVS in any reference designs for this IC either.
    > Clamp cap voltage and current aren't calculated in #1, which are important for selecting this part.
    > COSS for high- and low-side FETs are not calculated separately in #1, as they need to be if different FETs are used.
    > KZmax in #1 is not calculated properly when VBULKmin > VRfl.

    Ulrich Goerke from TI has been very helpful working with me to fix the last three items, but the others are just as important. The worksheets seem to have large errors for my design, which is not the common 90 - 260 VAC input. I'd be happy to work with someone to fix them.
  • 0 in reply to Gerrit Barrere
    TI__Guru 74095 points
    I will have to reply to this as we need to move on what ever can be. We have taken records on your questions and requests and we will make improvement on the existing design tool. In this sense, we will close this string.
  • 0 in reply to Hong Huang
    TI__Guru 74095 points
    Before I close this string, I need point out one important thing - this design tool uses Depletion FET BSS126 as part of the assumption for input from 85Vac to 265Vac. If the input voltage is > 265Vac, BSS126 voltage rating needs to be checked to see if still ok. In your design input 375Vac, BSS126 voltage rating may not be adequate. You may need to find a differnt depletion FET with adequate voltage rating for your design.