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UCC29950: Power isolation step-down transformer model

Part Number: UCC29950

 Hello,

Can someone provide info on the step-down isolation transformer model I see in these Resonant LLC designs.

In particular, the transformer shows three windings (N:1:1)  Lm, and a 1:1 turns ratio; primary(Lp) to secondary(Ls)).

My question  concerns the primary-secondary side for correct simulations.

How does one calculate the primary/secondary coupling inductance (1:1)  for a correct simulation?

For instance, if Lm=160uH, and a turns ratio N = 3,  then a proper inductance for the coupling step-down transformer (1:1)

would equal Lm/N^2 for Lp and Ls.  But you can't put a low primary coupled inductor (Lp) in parallel with Lm. That would affect the current

through Lm. It would not be linear (i.e. triangle wave).

Would one use a very large primary Lp in parallel with Lm and Lp/N^2 for the secondary inductor value?

Can't find a good reference for this question. And transformer companies just provide turns ratio.

I guess they leave it up to us to characterize the value for Lm.

Thanks  Doug

  • Hello Doug

    The transformer turns ratio N:1:1 means that the primary has N turns and each of the two secondary windings has 1 turn. The datasheet contains a worked example of the design - see section 8.2.2 page 39.
    The value of magnetizing inductance is critical in an LLC design and transformer companies normally provide that information -

    Let me know if you need any further information
    Regards
    Colin
  • Hi Colin

    I see that. However the  Lp and Ls1/Ls2  values are just expressed as a turns ratio.

    If the Lm and Lp are wound on similar E-cores then both inductance values will be similar.

    For simulation purposes I initially chose a small Lp inductance so the current in Lm was not

    triangular.

    I will choose a PFC/LLC controller eventually.

    I think it makes a great combo part.

    Doug

  • Hi Doug

    For simulation purposes you could do worse than copy the design values in the UCC29950 DS -

    Regards
    Colin