Magnetics® soft ferrite cores are an oxide made from Iron (Fe), Manganese (Mn), and Zinc (Zn) which are commonly referred to as manganese zinc ferrites. They have a low coercivity and are also known as soft magnetic ferrites. Because of their comparatively low losses at high frequencies, they are extensively used in switched-mode power supply (SMPS) and radio frequency (RF) transformers and inductors. Ferrite cores for the high frequency power supply and high quality communication markets are produced in a variety of shapes and sizes for inductors, pulse transformers, high frequency transformers, and noise filters. Notable characteristics of Magnetics ferrite materials are high permeability, good temperature properties, and low disaccommodation. Magnetics offers nine materials. The materials range in permeability from 900µ to 10,000µ and are available in a variety of geometries including toroids, shapes and pot cores.

Permeability is flux density, (B), divided by drive level, (H). Power materials are generally used for high frequency transformer applications. Hence, the important characteristics are high flux density and/or low core losses. Permeability is of lower importance because of its variability over an operating flux range.

Disaccommodation, occurring in ferrites, is the reduction of permeability with time after a core is demagnetized. This demagnetization can be caused by heating above the Curie point, by applying an alternating current of diminishing amplitude, or by mechanically shocking the core. In this phenomenon, the permeability increases towards its original value, then starts to decrease exponentially. If no extreme conditions are expected in the application, permeability changes will be small because most of the change has occurred during the first few months after manufacture of the core. High temperature accelerates the decrease in permeability. Disaccommodation is repeatable with each successive demagnetization; thus, it is not the same as aging.

When calculating the core losses, it is assumed that the structure is homogeneous. In reality, when core halves are mated, there is leakage flux (fringing flux) at the mating surfaces, and the gap losses contribute to the total losses. Gap losses are caused by flux concentration in the core and eddy currents generated in the windings. When a core is gapped, this gap loss can drastically increase overall losses. Additionally, because the cross-sectional area of many core geometries is not uniform, local “hot spots” can develop at points of minimum cross section. This creates localized areas of increased flux density, resulting in higher losses at those points.

MnZn materials have a high permeability, while NiZn ferrites have a low permeability. Manganese-zinc ferrites are used in applications where the operating frequency is less than 5 MHz. Nickel-zinc ferrites have a higher resistivity and are used at frequencies from 2 MHz to several hundred megahertz. The exception is common mode inductors where the impedance of MnZn material makes it the best choice up to 70 MHz and NiZn is recommended from 70 MHz to several hundred GHz.

Permeability (and AL) varies with drive level. For power applications, there is no need to place a limit on the maximum AL. A minimum AL translates into maximum excitation current.

Generally, a recommended figure is about 700 kg/m2 (100 lbs./sq. in.) of mating surface. For specific recommended pressures for RM, PQ, EP, and pot cores, consult the Magnetics Ferrite Core Catalog.

Cores are flat-ground on the mating surface because of the uneven surface produced during the firing process. It is important for cores to mate with a minimum air gap to keep the gap losses low and to achieve optimum inductance.

Lapping is an additional production process used to improve the mating surface. It Is typically done on cores with material permeability of 5000 and greater in order to achieve the maximum AL value for a given material. A mirror-like finish is the result. The surface finish for a normally flat-ground surface is 0.5 to 1.0 microns and for a lapped core is 0.1 to 0.2 microns.

Due to limitations of the machine performing the gapping, as the gap dimension decreases it is increasingly difficult to hold tight tolerances. As AL increases the gap gets smaller and the tolerances increase. As the gap gets smaller, the mechanical tolerance becomes proportionately larger, in addition the influence of variation in the material permeability becomes greater. A gap specified by its AL value yields a tighter tolerance than a gap specified by its physical dimensions.

Gluing should be done with thermosetting epoxy resin adhesives. The available range is very large. Important factors in the choice are the required temperature and viscosity. The economic curing temperature must not be above the maximum temperature to which the assembly may be safely raised. High viscosity resin can be difficult to apply. Low viscosity resin may run out of a poorly fitted joint or may be absorbed by the porous ferrite material. Follow the manufacturer’s instructions for a particular resin. Take care not to thermally shock ferrites; raising or lowering the core temperature too rapidly is dangerous. Ferrites will crack if changes in temperature exceed 5-10ºC/min. In addition, care must be taken to match the adhesives’ coefficient of thermal expansion (CTE) to that of the ferrite material. Otherwise, the resin may expand or contract more quickly than the ferrite; the result can be cracks that will degrade core properties.