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PSFB Transformer Design.

Martin Chien
Expert 3590 points
Other Parts Discussed in Thread: UCC3895, UCC28950

Hello Team,

My customer is using TI's parts UCC3895 and UCC28950 and they need your explicit instructions about transformer design.

1. They refer to UCC28950 application report SLUA560B, but they don't see anything about calculate flux density to determine number of turns, can you help?

2. The equation 9 in SLUA560B is calculating minimum Lmag to ensure system operates in current mode, they need more detail principle of it, can you provide any document about the equation 9 background?

Thank you so much.

Martin

  • 0
    TI__Mastermind 25130 points

    Martin,

    The design of the magnetics is a function that the converter designer performs. This equation is a guide to that designer that he will need a primary side magnetizing inductance large enough that it will result in a change in primary current due to primary side magnetization that is less than the down slope of the current through the output inductor during the primary side freewheeling time when translated to the primary side.  If this magnetizing inductance is too small the primary side magnetizing current will get to be too big and result in a current mode controlled converter operating in voltage mode control.

    John

  • 0 in reply to John Bottrill
    Intellectual 940 points

    Sorry for opening an old post, but I was about to ask the same question.

    Is the formula correct?

    This is my interpretation of it:

    - The goal is having the ripple of the magnetizing current to be less than half of the output current ripple (referred to the primary)

    - the magnetizing current ripple is Vin*D/Lmag/fs (and NOT Vin*(1-D)/Lmag/fs)

    - - this needs to be less than delta_iLout/a1*0.5 (less than half...for unmentioned reasons. Could have been less than 0.2)

    Referring to John's answer, the downslope of the output inductor current while freewheeling is not Vin(1-D)/Lo/fs but Vo(1-D)/Lo/fs, so it has nothing to do with it. The output current is already been considered in formula (9) in the term delta_iLout/a1*0.5

    This is very confusing. I really believe that the formula is wrong and the application note mentions no references at all, just a very vague explanation regarding operating in voltage mode instead of current mode (almost understandable as the sensed current would have a big percentage belonging to the magnetizing current instead of to the output current).