Powder cores are distributed air gap cores that possess many outstanding characteristics--high resistivity, low hysteresis and eddy current losses, and excellent inductance stability under both DC and AC conditions. Magnetics’ powder cores are not pressed with an organic binder, and therefore do not exhibit any thermal aging effects, as observed with powdered iron cores.
All Magnetics® powder core materials are used in inductors, but each has its own advantages. For the lowest loss inductor, MPP material should be used since it has the lowest core loss. For the smallest core size in a DC Bias dominated design, High Flux should be used since it has the highest flux capacity. For reasonably low losses and reasonably high saturation at a low cost, Kool Mu® should be used since it has the lowest material costs. XFlux® offers a more economical, high saturation material (1.6 T) for use in low and medium frequency inductors and chokes. Its high saturation is advantageous in applications where inductance under load is critical. For more information on how the materials compare view Magnetics Powder Core Materials presentation.
Magnetics measures inductance in a Kelsall Permeameter Cup. Actual wound inductance outside a Kelsall Cup is greater than the value calculated due to leakage flux and flux developed in the winding. The difference depends on the core size, permeability, core finish thickness, wire size, and number of turns, in addition to the way windings are put on the core. The difference is negligible for 125µ and higher and turns greater than 500. The following table is a guide to the differences that one might experience
No. of Turns Actual L No. of Turns Actual L
100 0% 100 +3.0%
500 +0.5% 50 +5.0%
300 +1.0% 25 +8.5%
The following formula can be used to approximate the leakage flux to add the expected inductance. This formula was developed from historical data of cores tested at Magnetics. Be aware that this will only give an approximation based on evenly spaced windings. You might expect as much as ± 50% deviation from this result.
L LK = 292N 1.065 Ae
l e X 10 5
where L LK = leakage inductance (mH)
N = number of turns
A e = core cross-section (cm²)
l e = core magnetic path (cm)
Many cores can be pressed to different heights. Dies are made so that the cavities can accommodate these different heights. Each core size is different, however. Consult an Application Engineer for specific questions on the size of interest. One advantage this offers is the ability to produce other core sizes with the expense of additional tooling.
Loctite® ESP 109 is a one part epoxy adhesive that is recommended for use on Powder Core materials. This adhesive has good strength at room temperature and retains strength at high temperature.
Stacking core will increase the cross section (Ae) by the multiple of the number of cores in the stack. The magnetic path length (le) will remain constant. The AL can be estimated by the same method as for single sets, where a leakage adjustment is estimated based on the ratio of window area (WA) to core area (Ae). Because that ratio decreases as cores are stacked, the AL of n stacked sets is slightly less than n times the AL of a single set.
Soft saturation is a distributed gap material advantage over a ferrite. The DC Bias curve does not have the traditional saturation point that a ferrite core does, rather as the oersteds increase the permeability slowly rolls off in a predictable fashion.