Transformer and Frequency

[Thomas]

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

 

[John Popelish]

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

 

[John Larkin]

On 17 Aug 2004 04:41:00 -0700, th.ahndorf@gmx.net (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

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