Customized ferrite cores … a few things to consider before introducing a new design
Custom cores do not necessarily follow published data values. Many parameters must be considered, for example, mechanical tolerances. A figure of +/-2% is a guide due to shrinkage from sintering. Linear shrinkage can be 16 – 20% whilst volume can be affected up to 40%.
Permeability is a published calculation with values using a ring core. Custom geometry can have a significant effect and therefore, feasibility must be checked by core shape. Further reading below on custom cores shows the design issues to consider.
Ferrites are soft-magnetic, ceramic materials which (due to their ferromagnetic structure) exhibit lower permeabilities and saturation flux densities than metallic magnetic materials. Their advantage against these materials is the much higher resisitivity, which allows operation up to high frequencies (from a few kilohertz to above 100 megahertz).
Kaschke produces 2 groups of ferrite materials (manganese-zinc ferrite and nickel-zinc ferrite) with approx. 25 different grades in total.
Both groups distinguish substantially in their properties: manganese-zinc ferrites have a resistivity of 0.05 up to 20 m, which leads to higher permeabilities (up to approx. 25000) and higher saturation flux densities (up to approx. 550 mT) with working frequencies up to several megahertz, while nickel-zinc ferrites have a resistivity between 104 … 108 m and, therefore, have lower permeabilities (up to 2000) and saturation flux densities (up to 450 mT), but working frequencies up to several hundred megahertz.
The production of ferrites is carried out with a typical ceramic process: raw oxides, dopants and additives are blended, milled, calcined and finally granulated.
Afterwards, this fabricated powder is brought into the desired shape either by dry pressing or by extrusion technique, and finally sintered in a temperature and atmosphere controlled furnace with temperatures between 1200 and 1400°C.
During the sintering, a transformation of the crystalline structure into a dense spinel structure takes place, which is accompanied by a shrinkage of the original volume of approx. 40 % (which corresponds with a linear shrinkage of 16 … 20%). For the construction of new pressing tools, this shrinkage must be known very precisely, to produce the sintered body exactly to the desired final geometry. Without subsequent machining a geometrical tolerance of ± 2% of the respective dimension has to be tolerated (guide value).
As the electromagnetic properties depend on the crystalline structure and the appropriate occupation of the lattice sites, and this is influenced by statistical parameters (diffusion, nucleation, grain growth etc.), bigger tolerances have to be accepted, too: for ring cores the initial permeability is generally tolerated with ± 25%, for highest permeabilities even ± 30% must be anticipated.
Mechanical stress generated by tight winding or unsuitable potting can reduce the permeabilty up to 50% in worst case. Therefore, a highly elastic material should be used for the potting.
The electromagnetic properties for magnetically sheared core shapes (i.e. cores with airgap) diminish with the degree of shearing (see databook or corresponding datasheets). The greater the airgap, the greater the effect on the electromagnetic properties of the core.
A mechanical finishing is possible to a certain extent: cores can be ground or cut, a coating with an electric insulating lacquer or powder or a joining to larger cores sizes by gluing is possible.
When the mating faces of a pair of cores is ground, a residual airgap (not equal to zero) remains, which leads to a magnetic shearing, and thereby to a reduction of the effective permeability. In extreme cases (small core sizes and high permeabilities) a residual airgap of 1 μm can effect a drop of permeability up to 50%. To avoid this, a polishing of the mating faces can be necessary.
During the processing of ferrite cores it is necessary to ensure that no mechanical or thermal shocks occur. Ceramics are brittle and have a high Young’s modulus, and therefore shocks as well as compressive and tensile stress can initiate or elongate cracks until the core eventually breaks.
Properties of ferrite materials which are determined on specified ring cores cannot be transferred readily to other geometries and sizes. For this, firm data have to be provided by the producer.
For the evaluation of feasibility of a new design it might be necessary to produce a prototype tool and to execute a sample production. Early specifications (e.g. on customer’s request or for the harmonization of geometrical parameters) are always preliminary. Only after confirmation of the feasibility can a valid specification be provided.
Kaschke Components GmbH