Z

#### zenerbjt

##### Guest

Hi

On page 35 of DER484 by Power Integrations (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.

Lookin at the B vs H curve here

...Kool Mu appears not to saturate at 600mT....

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…

https://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores/Kool-Mu-Material-Curves#kmpermvsdc

…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.

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…

https://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores/Kool-Mu-Material-Curves#kmcoreloss

..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.

DER 484 document:

Torroid is 77930-A7 by Mag-Inc:

My workings are as attached

On page 35 of DER484 by Power Integrations (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__Lookin at the B vs H curve here

### Magnetics - Kool Mu Material Curves

www.mag-inc.com

**?***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…

https://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores/Kool-Mu-Material-Curves#kmpermvsdc

…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…

https://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores/Kool-Mu-Material-Curves#kmcoreloss

..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

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