The tolerances of Q-factor and AC-resistance

[maido!]

Hi!
I am winding high power chokes with the litz wire 120x0.1mm,30 turns, 77µH inuductance, 3.8mm centre gap in planar core E32.
I am measuring with HP impedace analyzer 4194A AC-resistanse on 65KHz in the range from 150mΩ to 220mΩ.
Please, can me somebody explain why the tolerance is so large.:?:

Thank you in advance

 

[flatulent]

This looks like a test setup problem. I am assuming that you take several measurements and they fall in the range you specified.

For a sanity check, measure the DC resistance using the four wire (Kelvin) test clips. This should give you a lower bound on the resistance at AC.

Since the test clips can only add resistance, I would suspect that the lower number you specify is the right value of the choke.

 

[maido!]

I would like to specify my problem. I wound about 30 chokes with the same spec as above. Some chokes has low AC-R and some high AC-R. The difference about 40%! By the way, DC-R mesurement results are nearly the same.
I guess it depends on layers geometry but I can't find out how to wind better.
Thanks!

 

[flatulent]

It is the capacitance between turns that is the cause. Try finding the self resonant frequency (sweep for maximum reactance) an you should find the ones with the greatest resistance have the lowest SRF.

Try the winding method of building up a "triangle' cross section at one end of the bobbin and move across. This makes the far end of the wire farthest apart. Another way is to use a two or three section bobbin. N section bobbins reduce the capacitance by a factor of N squared.