Estimate for Transformer leakage inductance?

a pity there is no sketch with dimensions to assist in the calculations

We get a good estimation with axisymmetric AC-magnetic simulation, e.g. in free FEMM. You don't necessarily need to model the core, leakage is effectively the same for an air-core transformer.

a pity there is no sketch with dimensions to assist in the calculations

Click to expand...

Thanks, i guess you mean the equation shown attached...from "Snelling, Soft Ferrites", page 356.
The equation seems odd that it does not ask for the permeability, or dimensions, of the magnetic core, which might be in there.

But thanks, also to FvM, about the air cored equivalence situation.

Also, the equation does not tell what is leakage L , when you have a layer stack as follows for a 2 coil winding of pri and sec....


Sec
Pri2
Pri1
bobbin

....where Pri1 and Pri2 are in series...ie, winding primary as one full llayer, then continuing back to give 2 layers, but its just one coil....the Snelling equation cannot deal with this.......do you know how to deal....Snelling only deals with M = 1, 2 , 4, 6 as shown.

No, I mean a sketch of the actual thing you are trying to estimate the leakage for

with dimensions ...

Thanks, its a ferroxcube PQ2625 bobbin wound with
Pri = 44 turns of 2 strands of 0.25mm enamelled Cu Wire (2 full layers)
then..
Sec = 12 turns of 7/0.26mm TEXELZ which has diam of 1.06mm.

The snelling equation from above calculates this as 8uH. (leakage inductance measured from the 44 turn primary)
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If the above is changed such that the sec is now interleaved between the two 22 turn primary "halves"...then the leakage inductance comes out as 2.5uH
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PQ2625
www.ferroxcube.home.pl/prod/assets/pq2625.pdf

The primary inductance is 8.5mH.....so it seems amazing that the leakage inductance can really be this low.
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It seems strange that the Snelling equation does not require the number of secondary turns........but just insists that all layers of turns go right across the whole bobbin, evenly.
Also, the equation does not require the permeability of the core material used.
Also the equation does not ask if the core is gapped or not, which seems a bit odd?

However, i also tried the above Snelling equation with an actual EPC17 wound transformer...for which i actually measured the leakage inductance........the EPC17 was wound with an 80 turn pri of 0.2mm wire (2 full layers)...there was then a 11 turn sec , wound evenly across the bobbin (2 strands of 0.2mm)........so i calculated the lekage indictance from the primary of this.....and Snellings equation gave 6.5uH...but the actual measurement was 20uH (and that was 20uh across 5 individual measured samples).

So the above Snelling equation seems to be a bit of an underestimate.

May i ask is there a better way to calculate?