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Original question:

UCC28950EVM-442: UCC27714EVM-551 choice of H-bridge switching frequency

UCC28950EVM-442: SLUC222D spreadsheet provides different results compared to formulae used in SLUA560C

Nitish Agrawal
Expert 1595 points
Part Number: UCC28950EVM-442

Hello,

SLUC222D spreadsheet provides different results compared to formulae used in SLUA560C.  For instance, the secondary rectification FET (QE, QF) Vds spec in SLUA560C is calculated as 19.5V (equation 62), while in SLUC222D it is indicated as 39.05V (row 85).  Also, there is no margin for voltage overshoots (even though secondary voltage clamp is used).

There are other differences as well, such as transformer turns ratio calculation.

Would TI provide formulae used in the SLUC222D spreadsheet...as that appears to calculate more conservative numbers?

Kind regards,

Nitish

  • 0
    TI__Guru* 85926 points

    Hello,

    Let me study this

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 85926 points

    Hello,

    It looks like someone updated the spread sheet in 2018.  It use to agree with SLUA560C back in 2010 when the UCC28950EVM-442 was designed.  However, I believe the Excel calculator is correct.  

    2*Vinmax/a1 should be the maximum Vds voltage.  The application note is in error and I will have it corrected on the next revision.  Thanks for pointing this out to us.

    Regards,

  • 0 in reply to Mike O'
    Expert 1595 points

    Hi Mike,

    Thank you for clarifying the SR Vds calculation.

    Please also clarify the formula used in SLUC222D spreadsheet to calculate Dmax.  Also, is there is way to change the number of significant digits in the Dmax cell?

    Finally, is it possible for me to get the formulas used in the various cells in the spreadsheet?

    Regards,

    Nitish

  • +1 in reply to Nitish Agrawal
    TI__Guru* 85926 points

    Hello,

    The equations in the design calculator were the same as the SLUA560C application note.  It appears that that the SR Vds max was correct in the application note.  I believe the rest of the equations are unchanged.  So SLUA560C are the same equations used in the Excel spread sheet except the SR Vds max.

    It is not possible to change the significant digit of the Dmax at this time.  This does average down to 1% which should be good enough for most designs.  However, if more accuracy is required than the tool gives, you can use SLUA560C to calculate Dmax t more significant digits if you like.

    Excel is limited if you have MathCAD there is a design tool at the following link you can use. This will let you see all the equations and give your more accurate equations than excel.  https://www.ti.com/lit/zip/sluc210

    Regards,