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Transformer leakage inductance control
First some background:
>From the same series "I can't way no more", yesterday night I quickly wound a
50 turn primary and a >200 turn secondary on the N27 U-cores. As I had no
bobbin
formers, I used two round pieces of PVC pipe >5 cm diameter. Then I
short-circuited the secondary and checked the primary current: almost no
change!
>It must have been so that the coupling was too loose. (This is my first
transformer, after all :) )
>So I left my primary and wound a primary ON the same bobbin of the
secondary. I
shorted again the >secondary and now the current did increase: I measured 14 uH
of leakage inductance. Then I started >increasing the air-gap up to
removing one
of the U-cores and then both U-cores: there was a minimal >variation in leakage
inductance, about 0.05 uH.
>I guess this is due to the fact that the most of the magnetic field is coupled
through air and only a >little part through the core: remember, I have a huge
clearance between core and bobbin. It seems to >me I really need a kind of
bobbin former, so that the copper is very near to the core surface, like in
>commercial transformers. Next step will be to build a kind of bobbin former,
tonight.
I wound two new bobbins for my double U-core ferrite transformer. This time the
copper-core coupling was as thight as I could possibly get with commercial
bobbin formers.
I wound 50 turns for the primary and 150 for the secondary. I measured a
primary
inductance Lp of 10 mH and a leakage inductance Ll of 1 mH.
Since my target is a 160uH leakage inductance, I started introducing some air
gap. As I increased the air gap, both Lp and Ll decreased: with 1 cm (!) of air
gap I finally reached 160uH. Unluckly, that was the value for both Lp and Ll:
that is, the secondary did not influence any more the primary equivalent
inductance (no change in inductance shortening the secondary!).
In my opinion, what happened was that, by increasing the air gap, I reduced the
coupling between primary and secondary and together the Lp inductance. End
result was an Lp inductance of 160uH and a secondary with coupling coeff. near
to 0. Actually it should have gone the other way round: increase of air gap
should have increased Ll.
In my understanding, by mounting primary and secondary on two separate legs
there is already an intrinsic leakage inductance that can't be reduced without
winding both winding on the same bobbin.
The interesting point of this story comes now: as the leakage inductance is
directly related to the primary turn number (i.e. to Lp), I removed turns from
my primary to get to 25 turns only (secondary always 150 turns). Then I
measured
Lp = 2.5 mH Ll = 250uH. Notice that the ratio Lp/Ll remained constant (100) so
it must not be dependant on the turn ratio!
This means that I can test a core/bobbin assembly, calculate this factor (100),
then the required Lp value, the number of primary and secondary turns and
that's
it! The only constraint is not to saturate the core.
Comments?