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Transformer and Frequency

Discussion in 'Electronic Basics' started by Thomas, Aug 17, 2004.

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  1. Thomas

    Thomas Guest

    Hello,

    Short question:
    Why is it not possible to use an "normal" transformer for high
    frequency like 20 kHz?
    Why should I use ferrite?

    I thought it should work even better with a higher frequency??

    Thaks a lot.
    Thomas
     
  2. Metal cores are effectively single shorted turns. This effect is much
    reduced by breaking the core into thin laminations that makes each
    lamination a fractional turn, but the effect is still there. Thin
    tape wound cores are better than laminations in this regard, and
    breaking the metal into powder and insulating the individual grains is
    even better, but the insulation lowers the effective permeability
    (flux per amp turn).

    There are also hysterisis losses that consume a fixed amount of energy
    each cycle, so that higher frequency of operation means higher total
    core loss. Higher frequency operation usually implies lower peak flux
    in each direction and this lowers the per cycle hysterisis losses, but
    the two effects do not completely cancel out. Ferrite has a much
    higher resistance than metal (and can be made with different
    resistances and permeabilities for different frequency ranges), so the
    core currents are much smaller than with metal cores, and some
    ferrites have very flat BH curves, for low hysterisis losses per
    cycle. Ferrites support lower peak flux than most metal alloys used
    for cores, but at higher frequency operation the peak flux is usually
    lower, anyway.
     
  3. John Larkin

    John Larkin Guest

    Most audio transformers have laminated iron cores. A transformer
    designed for 60 Hz may not work well at 20 KHz because of leakage
    inductance (higher because of insulation requirements), core loss
    (cheap, klunky laminations are good enough at 60 Hz), distributed
    capacitance (not a problem at 60 Hz) and maybe eddy-current losses in
    thick solid wire. All these things can be fixed at 20 KHz and more,
    but it costs more, so 60 Hz power transformers aren't usually very
    good up high.

    Ferrites are fine at high frequencies, but terrible for low freqs, so
    it's not common to use them for audio.

    John
     
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