Re: Transformer and Frequency

From: John Popelish (jpopelish_at_rica.net)
Date: 08/17/04 

Date: Tue, 17 Aug 2004 11:12:06 -0400

Thomas wrote:
>
> 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

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. 

-- 
John Popelish

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