开云体育

Amidon documents maximum flux density guidelines in:


is there a reference to a technical document which derives these max
flux values?

John KN5L

John,

Found this:

Amidon documents maximum flux density guidelines in:


is there a reference to a technical document which derives these max
flux values?

W5USJ, EM22cv, QRPARCI #5422

Page 36 to 41 has more depth and calculations.

Hi All,

I may not have asked the question very well.

At the bottom of:


is a table of extrapolated AC flux density limits for both iron powder
and ferrite cores as a guideline to avoid excessive heating.

For example, at 7MHz, guideline limit is 57 gauss.

The question is how was 57 gauss, as a limit, derived?

Following pages discuss computing device gauss and temperature rise
equations.

Performing some computations using two Ferrite cores with equal size and
turn count:

15T FT114 -43 and -61, Ae=0.37, 100W 50 Ohm 70.7Vrms, B=41gauss

Computing Ferrite core loss, parallel resistance, using Fair-Rite
complex permeability data:
FT114-43 Rp=4.41kOhm = 1.13W
FT114-61 Rp=75.3kOhm = 0.066W

The -43 device will be much hotter than -61 device, though flux is equal.

John KN5L

John:

I have read in other MicroMetals literature that the maximum continuous
exposure temperature limit for the plastic binders that hold the iron
powder in shape as toroids is 128 degrees C.
A lot of the magnetic metals have a Curie temperature in the same general
temperature region.
If you hit the Curie temperature, the inductance value drops significantly,
until it cools down again.
So temperature rise is one of the controlling variables, probably dominant.

I expect that the flux limits are determined as some kind of temperature
rise limitation, probably empirically for each type of material.
Micrometals does not release the exact formulation, just some
generalization about what the general chemistry is.
So, you don't have the information to probably engineer the answer.

I suspect the answer is the same for the ferrites from Fair-Rite.

So, as a designer, you need to know the operating temperature limits, how
well you can cool them, your maximum ambient temperature, and derive the
maximum allowable temperature rise in operation.

--- Graham / KE9H

==