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smps design

Discussion in 'Electronic Design' started by Steve, Jul 23, 2006.

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  1. Steve

    Steve Guest

    Hi all,
    I study smps design and I have the following question:
    When designing continous mode half-bridge and full-bridge designs; has
    the transformer core magnetization current (in primary) anything to do
    with maximum throughput power? I mean, must the magnetizing current
    have a minimum magnetude to achieve a certain output power level? I
    guess this would be a question for DNA / Genome, anyone seen him lately
    by the way?
  2. Eeyore

    Eeyore Guest

    He's been growing encrustations from his bottom apparently. I wouldn't advise
    meeting him at all !

  3. Tim Williams

    Tim Williams Guest

    I would suspect so. I think you have it backwards; the maximum inductive
    current is equal to saturation current, which depends on turns and core
    material. The current is, of course, recycled each cycle, being purely
    inductive, so you don't lose anything*.

    *Minus resistance, diode drops, and extra capacitor stress of course.

    You can, of course, reduce magnetizing current as much as you want, by using
    a hugely oversized transformer, so that resistance is low and inductance is

    In flyback or buck (half wave I suppose) designs, the peak transformer
    amp-turns must, of course, be less than saturation. There is no minimum
    limit, though.

  4. (snip)

    No direct connection. The magnetizing current is an artifact of
    having a non infinite permeability in the core.

    If you don't drive the primary through a capacitor or otherwise force
    the average current to zero, you may have to gap the core and that
    lowers its effective permeability more, raising the magnetizing current.

    If you can afford a larger core, you can raise the winding inductance
    and lower the magnetizing current.

    So the magnetizing current is one factor among many that you
    compromise when designing a transformer.
  5. Steve

    Steve Guest

    John Popelish skrev:

    Ok, so the best thing would be to have a very high inductance primary
    for extremely low magnetizing current to lower dissipation in switches.
    This will appearently have no effect upon maximum output power level?
    May I ask you another thing:
    In buck derived converters, the primary current is a result of
    secondary loading + the magnetizing current. To keep secondary voltages
    constant you need a certain duty cycle, but that has nothing to do with
    the primary current??
    If this doesn't make sense, it's probably because of my bad
    explanation. I try this one also:
    Imagine a full-bridge design outputting 100VDC on secondary. Load is
    very low. This results in a 50% duty-cycle. Primary current is low.
    Suddenly load increases sharply. Duty-cycle goes up to 75% to
    compensate for drops. Primary current is high.
    What I want to ask is there are no direct relationship between primary
    current and output voltage regulation, so how would a current mode full
    bridge work??
    Best regards,
  6. That is the general idea. In fact, if you lower the magnetizing
    current by using a larger core, the power output capability will go up
    while the magnetizing current goes down. The magnetizing current is
    essentially a process going on in parallel to energy transfer between
    windings. It is just necessary to produce volt seconds across each
    turn (even at zero power transfer).
    That's right. In a buck derived converter, the load current
    determines the primary current except for magnetizing current) and the
    primary voltage and the duty cycle determine the average voltage per turn.
    By this, I assume that you mean that the primary switches are
    supplying voltage to the winding only half of each half cycle, and
    zero the other half.
    That is a lot of drops, but okay. Usually, the drops need a much
    smaller correction, assuming the output filter has continuous current.
    Most of the duty cycle variation is usually reserved for changes in
    primary voltage.
    There has to be a monotonic relation between primary current and
    output voltage or current mode control will not work. But they don't
    have to be proportional under all conditions, with a fixed
    proportionality factor. But at any output load current, increasing
    the primary current must increase the output voltage.
  7. Genome

    Genome Guest

    You promised me a good face sitting session with Agnetha.

    I don't know what went wrong but perhaps you might write back to her and
    explain that it involved her buttocks and my face rather than the other way
    around. Obviously I shall buy some more Haddock and Chips with an extra
    portion of scraps along with mushy peas and curry sauce. I have learnt how
    to do gravy as well, ask her what flavour she wants from the Bisto range, I
    might be able to stretch to a bit of best if she wants to be picky.


  8. Steve

    Steve Guest

    Genome skrev:

    Ok...things seems to have collapsed for you...
    I remember you as a competent smps designer but obviously you've gone
    to other business.
    Good luck with Agnetha!

    --- Stefan
  9. Genome

    Genome Guest

    Sigh, I suppose you are right.

    It's a trade off between core losses and winding losses. The number of turns
    on your primary, and hence secondary, are set by the peak (or peak to peak)
    flux excursion you can accept.

    N = VIN.TON/Bp.Ae

    The greater the flux excursion the fewer turns you need so the lower the
    winding losses. Unfortunately the flux excursion causes core losses and the
    greater the flux excursion the greater the core losses. Of course operating
    frequency also affects core and winding losses.

    With fewer turns the primary magnetising inductance is smaller so the
    magnetising current is larger so that implies a higher magnetising current
    is associated with a higher throughput power. That's really a secondary
    effect though. The higher magnetising current also results in greater switch
    losses but the relative levels mean it's not so important.

    You can work out core losses by referring to the graphs given by
    manufacturers. Winding losses are a little bit harder but here's a program
    that works out what the AC impedance of your wires might be......

    Not gauranteed to give the right answer......

    So, yes, higher magnetising current implies greater power throughput.
    However it's all a trade off with other things.

  10. Steve

    Steve Guest

    Genome skrev:

    That's the 'DNA' I know of...
    Nice to hear from you again, I'm still reading your docs about slope
    matching and error amp compensation, subharmonic oscillation and how
    big waste the voltage mode topology is.
    Thanks for your reply. It indicates that I'm not that far out in

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