sci.physics.electromag

On 18 mar, 18:16, Theo Markettos
wrote:
> Imagine I have an air-cored transformer. This has some frequency response
> up to (say) 100MHz where it tails off.
>
> Then I insert a ferrite core. This increases the mutual inductance so I
> have a greater frequency response at low frequencies. But at high
> frequencies the response is (probably) worse than the air-cored case.
>
> What effect is causing this? Is it just as a result of adding the core that
> I increase the inductance of the transformer so I can't get much current to
> flow in the primary? Is it that hysteresis in the core is converting much
> more energy into heat - but there's still primary current flowing? Is it
> eddy currents (but air and ferrite are insulators)?
>
> What I'm trying to work out is what's going on in the core. I have a core
> with a given susceptibility. I apply a strong AC field (so we can neglect
> inductance). What's the quantity that controls how much magnetisation there
> is in the core at a given time? In other words, the magnetic domains inside
> the material need time to align. In air this is minimal (there's nothing to
> align, unless you count diamagnetism), while there quite a lot of alignment
> to be done in a ferr[o|i]magnetic material. So what sets this time constant
> and what is it called?
>
> Thanks
> Theo

Hello Theo,

Talking about traditional transformers (not transmission line types)
and wire length << 0.25 lambda, HF performance is dictated by
capacitance between turns of own windings and capacitance between the
two windings. Another one is the the leakage flux. It is the flux
generated by the primary that does not encircle the secondary.

The leakage inductance is the inductance that you measure when you
short circuit the secondary. The more air between the two windings,
the more the leakage (flux in between the two windings), the higher
the leakage inductance and the lower the high frequency roll-off. That
is the reason that in many HF transformers secondary and primary
winding are made simultaneously (so primary wire close to secondary
wire).

Adding a core generally does not greatly affect the leakage flux (so
the leakage inductance), but it does affect the magnetization
inductance. This is the inductance that you measure with open
secondary.

When driving a transformer with no ohmic losses with a constant
voltage amplitude and frequency, the total flux is not depended on
whether a core is present or not as flux is dictated by number of
turns and Vs product. Only the resultant current (at given drive
situation) to establish a certain flux is far lower in a coil/
transformer with appropriate core material.

What exactly happens in the core, is a different story. When you use
a low frequency MnZn power ferrite at (for example) 250 MHz. The
material will behave like a strong lossy capacitive medium. It can be
such that due to EM wave propagation there will be less flux in the
core (with respect to the situation withoug core). The core more or
less behaves as metal, and RF field will not penetrate to metal. So
using the wrong type of magnetic material may reduce the magnetization
inductance.

In a macroscopic DC view, Flux density (B, Vs/m^2) = Permeability (H/
m) * Magnetic field (A/m). In a macroscopic HF situation, it depends
on the properties of the magnetic material (that affects EM wave
propagation speed) and the size of the core with respect to wavelength
within the core material. For microscopic properties, you might check
various core producers like Epcos, ferroxcube/yageo, fairrite, etc.

Hope this helps a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl
remove abc from the mail address.