Transformer and Frequency

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.

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

Short Answer:

Inductive Reactance.

Cheers! Rich