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UCC29950: Power isolation step-down transformer model

douglas Sudjian
Prodigy 190 points
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 resonant

LLC frequency. Do I place a voltage-controlled voltage (vcvs) between Lm and the primary coupled inductance?

Or 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

  • 0
    Prodigy 190 points
    If two U cores were used to build the resonant inductor and gapped transformer then I guess Lp would be large, similar to Lm and Ls=Lp/N^2.
    Now the current is linear in Lm.
  • 0 in reply to douglas Sudjian
    TI__Mastermind 31580 points
    Hi Douglas,

    Sorry for delay in replying to your post, we appear to have missed it first time around. I have asked the apps engineer for the UCC29950 to respond to your questions, he is traveling internationally, but I hope he will be able to reply in a day or so.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 190 points
    Hi,
    That s OK. My question was based upon understanding the type of transformer construction needed for this application.
    I think a PFC/resonant LLC converter is a great combo part I will use in the near future.
    Doug
  • 0 in reply to douglas Sudjian
    TI__Mastermind 25570 points
    Hi Douglas

    The calculation of the turns ratio of the LLC transformer and other parameters of the resonant tank are given in the data sheet for the UCC29950. There is also an Excel calculator available for this part under the tools and software tab of the product page - at www.ti.com/.../toolssoftware This will allow you to size the components correctly.
    The LLC topology uses a series resonant inductor. The leakage inductance of the power transformer is sometimes used as the resonant element but the transformer needs to be carefully designed to achieve the required inductance - that is the leakage inductance of the finished transformer should be tested in the same way as its turns ratio and magnetizing inductance.
    It may be easier to have a separate resonant inductor and to wind the main transformer for minimum leakage inductance, in this case the transformer leakage inductance will add to the resonant inductance.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 190 points
    Hi Colin,
    Thanks for your update. I may have to learn how to test transformers.
    Also, I am curious why TI doesn't design a self-driven high voltage synchronous rectifier (120V/220V)?
    There is an excellent IEEE article about that (2010)
    I looked for something like that before and saw only transformer secondary control. I will look again.
    Doug