Max allowed airgap in Ferrite Cores for designing DC inductors

Hi all,

I wanted to know typical and maximum allowed values of air gap used for ferrite core for designing dc-inductors. I am getting values of 2.4 to 3mm. However, data sheet typically gives values of Al upto a maximum of 2mm.

Hi, it depends on the magnetic path length, and we must not have the gap more than say 10% of that generally.
Or say in an ETD core, you wouldnt want the gap more than 20% of the centre leg ferrite length.
But it depends on the problems caused by the leakage flux, which sprays out all over the place when the gap is big, and can cause heating in the coils of the inductor......so at high power the gap may need to be smaller so you dont overheat.
Though if its relatively low power , and the noise issue of the expansive flux field isnt a noise problem, then you can use any gap size you like , as long as you can achieve your inductance value with few enough turns....and with the wanted tolerance on inductance value...because the bigger the gap, generally the worse the inductance tolerance can be.

If you download standard ferroxcube core datasheets for eg ETD cores, PM cores, PQ cores, etc etc…you can see what size standard gap sizes they have off the shelf…so generally don’t have a gap any bigger than this, unless you have special analysis to say its ok.

There is no min, we have used RM14 with 12mm gap, and good litz wire in the middle - you can add gap by using spacers in the outer legs as well as in the middle - just be sure your wire is not close to the gap for gap sizes of 1 - 5 mm the fringing mag field heats the wire...

For large gaps - try to keep the Bmax ( Bpk) down to 80mT or less if your freq is > 100kHz

Thankyou treez for the reply,

Is there any relation between power level handled and maximum allowed core gap or we can consider general rule of 10 % of magnetic path length /20% of length of center leg of ferrite core.