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UCC28700-Q1: Dot convention on transformer

Harshal Pawar17
Intellectual 260 points
Part Number: UCC28700-Q1

Hello,

I am using UCC28700-Q1 for DC-DC converter. In the typical application circuit of the datasheet (Page No. 17)  Primary & Auxiliary side of transformer are out of phase (as per dot conventions) but in attached design ( even in WEBBENCH design also) they are in phase.

So what should be the status of auxiliary?

And what precautions should be taken while constructing transformer with out of phase output ?in terms of start & end of winding, direction etc. 

  • 0
    TI__Expert 6080 points
    Hello Harshal,

    The secondary and Aux winding can be considered as two output windings, for a flyback topology primary winding should be out of phase to both the output windings.
    Attached is the application note we have published for transformer considerations for better efficiency and EMI.
    www.ti.com/.../slup338.pdf

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Intellectual 260 points
    If Aux winding is a output winding then as per typical application circuit what is the reason behind ground of primary and aux side (i.e. ground at aux winding, RS2, CVDD, RCBC, IC GND & RCS) is same?
  • 0 in reply to Harshal Pawar17
    TI__Expert 8210 points
    Harshal

    The term "flyback transformer" is a bit of a misnomer, since it is actually a coupled inductor. The main difference between a transformer and coupled inductor is that a coupled inductor can store energy while a transformer does not. One other difference between the two is that the dot polarity on a schematic between input and output winding is inverted. All the output windings have the same polarity since they deliver power at the same time, regardless of which ground they are connected with respect to. The ground that each winding is connected in reference to do not need to be the same, as long as there is a path for current to flow between the two windings of each output it will work.

    If you are interested in more information on flyback transformer design, section 5 (page 48) of the TI/Unitrode Magnetics Design Handbook www.ti.com/.../slup132.pdf goes into more detail on the basics. I also recommend reviewing our flyback fundamentals training material that's available here www.ti.com/.../support-training.html to help clarify any confusion on operation.

    Best Regards,
    Eric