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UCC28950: Transformer Design Calculations

Mark Walters
Prodigy 100 points
Part Number: UCC28950

We are revisiting a project working with the UCC28950 designed for 2KW output power, 390V input, 200-360V output  and a switching frequency of 100kHz.  As we began working with our transformer manufacturer for some prototypes there has been some concerns raised about the transformer requirements generated by the calculation spreadsheet.  Specifically, the primary RMS current and the Primary Magnetizing Inductance.  The calculator using the above data at the 200V output requires around 1.8mH magnetizing inductance and 16.5A RMS current.  The manufacturer is indicating that a transformer to meet these requirements would be significantly larger than most transformer we have seen in this power range as well as it being gapped at nearly 3cm.  Are we missing something here?

Thanks

Mark

  • 0
    TI__Mastermind 25570 points

    Hello Mark

    There are a couple of relevant issues with the calculator -

    The turns ratio calculated in cell C25 is rounded to the nearest integer, If you write the integer value into cell C26 then subsequent calculations will use this integer. However, if you cut and paste cell C25 into cell C26 then the cell C26 shows the integer value BUT uses the original un-rounded value for calculations. The difference is small when Vin/Vout is large (400V/12V for example) but can be significant when the ratio is small (390 / 360V for example).

    The voltage stresses on the SRs calculated in cell C85 are half those in the actual circuit. The reason is that the calculator doesn't take account of the fact that the SR sees the total voltage across the whole of the centre tapped secondary winding.

    The transformer does not need to be gapped because there is no DC component to the primary current. If it's not gapped then the core size needed to achieve 1.8mH of inductance should be significantly smaller than they are assuming.

    Please re-post if you need any further information about this.

    Regards
    Colin