Thank you Jim for this most interesting brief explanation!
I say brief, because I have developed a vague feeling over the years for how
long it would have to be to cover every aspect we face just in the amateur
field... And I'm still a green novice when it comes to ferrite/powder iron
cores.
Paul-Henrik / OH1LSQ
Quoting James Moritz <[email protected]>:
> Dear John, LF Group,
>
> There is no strict limit on the frequency range of a particular type of
> ferrite, rather there is an optimum frequency range depending on the
> application.
>
> All ferrites have losses that increase with frequency. At low frequency this
> is mainly due to magnetic hysteresis in the core, which results in a loss
> that rises with operating frequency at a given level of magnetic flux, and
> at high frequencies things like eddy currents and dielectric losses increase
> in significance too. The lower permeability ferrite materials tend to have
> lower overall loss in the magnetic core material in the LF/MF/HF range, but
> require more turns of wire to achieve a given inductance, or a particular
> maximum level of flux in the core, resulting in higher losses in the
> windings. So there is a trade-off, favouring high permeability materials at
> low frequency where the loss due to hysteresis is relatively low and the
> smaller number of turns needed is a benefit, and low permeability materials
> at higher frequency where fewer turns are required.
>
> Whether a material is suitable at a particular frequency depends a lot on
> what it is being used for. If one looks at the impedance of a particular
> winding, the core losses result in a resistive component that increases with
> frequency, and an inductance that is constant at low frequencies, but
> decreases rapidly at high frequencies. At very high frequencies, the
> resistive component may also reduce. So there comes a crossover point when
> the coil impedance becomes mostly resistive, and at higer frequencies still
> the overall impedance of the coil actually reduces (this is ignoring the
> effect of stray capacitance, which will also cause the impedance to reduce
> at frequencies above resonance). The crossover tends to occur at higher
> frequencies for lower permeability materials. For a signal transformer, one
> does not usually care too much about the resistive component, provided the
> overall winding impedance is high, which favours high permeability cores.
> For a high Q coil in a tuned circuit, one wants to minimise the resistive
> component as much as possible, which tends to favour low permeability cores.
> For noise supression, one wants to maintain a large impedance over a wide
> frequency range, and a resistive impedance is actually quite useful in
> damping out resonances. Here the upper limit is where the overall impedance
> starts to decrease. For power applications such as SMPSUs and transmitters,
> the trade-off becomes more complicated, because one also must consider flux
> density, temperature rise, size and cost of the core, the effect of a DC
> bias current, etc.
>
> So the reccomended frequency range of a ferrite material is really rather a
> vague notion. Nothing terrible happens at low frequencies, but the windings
> tend to get unmanageably large. At high frequencies, there comes a point
> where the losses are too high for the circuit requirements. But in both
> cases, the frequency limits will depend a lot on what the core is being used
> for.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>
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