I - Choice of the Magnetic Core
| The fundamental purpose of an inductors' magnetic core is to provide an easy path for flux in order to facilitate flux linkage, so that the magnetic energy can be stored in a non-magnetic low permeability region. In inductors' development, the choice of the magnetic material is mainly related to the following parameters: working frequency, current waveform, maximum dimensions and environment conditions. As a matter of fact, the core selection is the first designing step and after that all other building details are defined: the number of turns and the conductor element (that can be copper foil, solid wire, litz wire), the isolation, the output connections. Several cores are available on the market: they are different because of chemical composition, production process and dimensions. These elements define the main features of the core, like for example saturation flux, frequency behavior, temperature behavior. |
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For high frequency applications, the most used materials are:
For lower working frequencies other materials are available:
By looking at the materials features, it is possible to make the choice of the magnetic material. |
Soft Ferrite Cores
Soft ferrite is the cheapest and the most used material in high frequency applications. As a matter of fact, this term refers to a wide range of ceramic materials, which are obtained by sinterization of Iron Oxide and other metal oxides. Materials available on the market are usually identified with a code or a commercial name and they possess different characteristics (for example permeability, saturation flux, specific losses, temperature behavior, …). The specific material, the shape and dimensions define core performances and what kind of inductive component is suitable for.
In high frequency power conversion system ferrite materials have saturation flux in the range of 0,3¸0,5 Tesla and some thousands of relative permeability. Therefore, ferrite materials has quite low saturation flux, that is the most critical limit for inductive components, especially in case of high DC currents (like output inductors) or high overcurrent risk. Ferrite materials for power application have the great advantage of low losses level on a wide frequency range.
Amorphous and Nanocrystalline Laminated Cores
Amorphous and nanocrystalline laminations are obtained by a special rapid quenching technology. Typical thicknesses of those laminations are few tens of μm. Material features are defined by the chemical composition (materials are normally classified in Iron based and Cobalt based) and the production process (that in case of nanocrystalline materials involves the annealing phase too).
For few tens of working frequencies and critical environment conditions those materials have to be preferred because of high saturation flux (more than 0,9 Tesla), high permeability (in the 10000¸100000 range) and very stable performances with temperature, even if the cost is much higher.
Amorphous cores are suitable for output inductors, when high DC current, low ripple current, extremely compact dimensions. In fact, high saturation flux level is suitable for high currents and low ripple current doesn't cause critical core losses.
Nanocrystalline cores have vary low losses, even for high variation of magnetic flux, so they can be used for switching transformers too.
Powder Cores
Powder cores differ in chemical composition. Powder cores are pressed with an organic or inorganic binder, that is responsible for storage of energy. As the non-magnetic area is distributed in the whole core's volume and it is not concentrated on a small region, they are called distributed airgap cores. Because of this peculiarity, powder cores are suitable for high direct current inductors (output inductors) and flyback transformers in critical applications. One of the most interesting feature of this kind of cores is the smooth and sweet permeability versus direct current curve, that underlines the robustness of this materials against overcurrents.
By changing the magnetic to non-magnetic parts ratio, the “equivalent” relative permeability alters in the range of 10 to 600; some standard values are quite common on the market (for example: 26, 60, 90, 125, 147, 300, 550). For a particular core, the inductance factor (AL) will depend on material, shape and dimensions.
Each material, identified by the producer with a code or a commercial name, possesses its own characteristics in terms of permeability, saturation flux, specific losses. They are typically very stable with the temperature and they can be used in critical environment applications. Powder cores are more expensive than ferrite cores, but normally cheaper than amorphous and nanocrystalline ones.