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Finding inductance and core loss for a powered iron inductor in a Two transistor forward SMPS?

On page 35 of DER484 by Power Integrations (as below) , there is a torroid inductor (Made of "kool mu" powered iron) for the output of a Two Transistor Forward converter.
This torroid is 77930-A7 by Mag-Inc. (as below) It is wound with 75 Turns.

Finding saturation flux density:
Looking at the B vs H curve here..

...Kool Mu appears not to saturate at 600mT....but its not possible to find out from the website what is the saturation flux density of Kool Mu?

Find Inductance at 4.6A:

I wish to first find the Inductance at its average current level of 4.6A.

I believe I have to do this by finding the new permeability at the current level of 4.6A, then find the Reluctance at this permeability level {using Reluctance = l/(uo.ur.A) }, then find inductance at 4.6A by L = (N^2)/Reluctance.

This is done by looking at the “Permeability vs DC Bias” curve of this…


…in fact , better still, there is an equation here stating “Percentage of initial permeability” is 1/(a+b.H^c) [H in Oersteds]

The result is that Inductance at 4.6A is 283uH. Would you agree?

Finding Core Loss:

We then have to find the core loss of this inductor. I take the peak and trough of inductor current as being 5.6A and 3.6A respectively. We then find the B at the peak and trough currents using B=uo.ur.H

The result is that the Bpkpk value is 0.013T. Is this correct? It seems kind of bizarre because the Flux density at 3.6A is greater than the flux density at 5.6A (!)
Then, taking this Bpkpk of 0.013T, we use it with the graph of “Core Loss Density curves” shown here…


..to find core loss..

The switching frequency is 66kHz, and from the above graph, the core loss is seen to be well under 30mW/cm^3 at 66kHz. (I.e 0.03W/1E-6 m^3) The torroid volume is 4150e-9 m^3. Therefore, the core loss is less than 120mW.

This seems ridiculously low for a powered iron core, which is known for having high core losses. Do you know what’s wrong here?

DER 484 document:

Torroid is 77930-A7 by Mag-Inc:

My workings are as attached



Well-Known Member
Most Helpful Member
There are plenty of rules of thumb used by people with limited knowledge of magnetic materials. Ferrites may have higher loss at high frequencies but you must first define "high" frequency. Also, define iron core. A stack of ferro-silicon plates or insulated powdered iron particles? What size and shape powdered iron? What kind of insulation?

Also, saturation current of powdered iron cores is much higher than ferrites (in general) so your core can be much smaller for a given current.

Also, your mu-metal isn't exactly an iron powder core, it is considered an alloy core - higher frequencies and lower saturation current than iron. Today there is more of a continuum of material choices than ferrite OR iron.


Well-Known Member
Most Helpful Member
Thanks, we see that realistically "kool mu" material can only be used up to about 550mT.
Thsi is seen from the Mag-Inc powder core catalog..page 48

For higher saturation, try Micrometals iron powder cores. The have about 10-different iron formulations (size of powder, binder content, insulation quality on each partial,...) and each has their own optimal frequency and saturation.

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