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UCC28780: Meaning of a variable in Mathcad doc?

Gerrit Barrere
Expert 1245 points
Part Number: UCC28780

I'm using the Mathcad documents from sluc644 for a design with the UCC28780.  For the most part they're clear, but I'm stumped on the meaning of variable Vin_BUR in the document "Neutron calculation tool_45W GaN (v.4.1)_general.xmcd".

This variable is used in computing VCST_BUR, but not by a formula I can find in the data sheet or figure out.  It's also used in calculating a decoupling cap and in the frequency compensation network calculation.  The AC input range for the two worksheets in sluc644 is 85 - 265 VAC and Vin_BUR is set to 230V * sqrt(2).  Is there a relationship there?

Can anyone explain what Vin_BUR is, so I can set it correctly in my design?

  • 0
    TI__Mastermind 40490 points
    Hello Gerrit,

    In the UCC28780 datasheet, section 7.3.1 page 14, in the middle of the first paragraph a statement reads (in part) “…50% to 60% of output load at high line is the recommended…”. It is obscure, I agree, but relates to the fact that the burst threshold set by BUR should be set to correspond to the peak primary current (Ipp) in the nominal high-line condition. Ipp is lower at high line than at low line. In the Mathcad program, the nominal high line voltage is 230 Vrms. Ipp depends on the bulk voltage across the primary magnetizing inductance, hence Vin_BUR = root2*230V.
    Vin_BUR does not have to be always set at (1.414*230V), but more generally set to (1.414*Vnom_high-line) for any particular design.

    Further in the program, it is used to calculate stand-by power in one of the resistors. It should be noted that this stand-by power dissipation is valid only for the Vin_BUR level. In the EU, stand-by power is measured at 230Vrms input. If a different high-line value is selected for Vin_BUR, then Vin_BUR should really not be used for this stand-by power calculation. Stand-by power should be calculated at the designated stand-by voltage for any given country. In light of this, the Mathcad equation is too specific to the typical 85-265Vac input range and does not allow for deviations from this assumption. You are free to revise your copy of the program to reflect your specific design requirements.

    Even further, Vin_BUR is again used in a frequency compensation network calculation, again assuming the nominal high-line input of 230V. If an input voltage used for Vin_BUR differs from the input voltage used for nominal high line, then Vin_BUR would be incorrectly used in the compensation equation. More generally, this equation should use the nominal high-line value. Apparently, it was convenient to use Vin_BUR since it was defined as the typical nominal high line value, rather than define another variable. We’ll work to improve the Mathcad calculator tool to allow for a distinction.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Expert 1245 points
    Thanks again, Ulrich. The key is VBUR = (1.414*Vnom_high-line). I think I will define a variable in my copy of the worksheet to make this clear.
    All the Vin_BUR ramifications that you discuss demonstrate what a complex IC this is! And Vin_BUR is just one variable...