About Ferrite Cores
What are ferrites?
Ferrite cores are made of non-metallic magnetic materials with dense and homogeneous ceramic structure. It consists of iron oxide (Fe2O3) and oxide or carbonate compounds of one or several other metals (eg manganese, zinc, nickel, magnesium). The ferrite raw material is pressed, then sintered at a high temperature of 1300 degrees C, and finally machined to make a finished magnetic core that meets the application requirements. Compared with other types of magnetic materials, ferrite has the advantages of high magnetic permeability, high resistance and low eddy current loss in a wide frequency range. These material properties make ferrite an ideal material for applications such as high frequency transformers, broadband transformers, tunable inductors and other high frequency circuits from 10kHz to 50MHz. Ferrite cores can be selected in a wide range, and different materials and shapes can be selected according to different magnetic parameters.
How important is magnetic permeability for power materials?
Magnetic permeability is the value obtained by dividing the magnetic flux density (B) by the excitation power (H). Power materials are often used in applications such as high frequency transformers. Therefore, its important characteristics are: high magnetic flux density and low core loss. Permeability is less important because it changes with the operating flux range.
What is Permeability Decay?
The magnetic permeability attenuation phenomenon occurs in ferrite, which means that after the magnetic core is demagnetized, the magnetic permeability decreases with time. This phenomenon may be caused by the application of alternating current with gradually decreasing amplitude, which causes the temperature to rise beyond the Curie point; it may also be caused by mechanical impact on the magnetic core. In this case, the permeability will increase based on the original value and then decrease exponentially. If extreme conditions are not present in the application, the change in permeability will be small because most of the change occurs in the first few months after the core is produced. High temperature conditions will accelerate the reduction of magnetic permeability. The decrease in permeability can be repeated for each successive demagnetization process; therefore, it is different from aging.
Why is the actual core loss greater than the calculated core loss?
When calculating core loss, it is assumed that the structure of the core is uniform. However, in reality, when two core halves are paired, leakage flux (fringing flux) exists on the paired surfaces, and gap loss also increases the total loss. Gap loss is caused by the concentration of magnetic flux in the core and the generation of eddy currents in the windings. This gap loss can lead to a significant increase in total loss when the core is gapped. Additionally, due to the irregular cross-sectional area of many core geometries, localized "overheating" can occur at the point of minimum cross-sectional area. Losses at these locations increase because the localized small areas result in higher flux densities.
What is the difference between nickel zinc ferrite and manganese zinc ferrite?
MnZn material has high magnetic permeability, while NiZn ferrite has low magnetic permeability. MnZn ferrites can be used in applications operating at frequencies below 5MHz. Nickel zinc ferrite has a high resistivity and can be used in the frequency range of 2MHz to hundreds of megahertz. Except for common mode inductors, for applications below 70MHz, the impedance of manganese zinc material makes it the best choice; for applications from 70MHz to hundreds of gigahertz, nickel zinc material is recommended.
Why is only the smaller AL listed in the core datasheet?
Permeability (and AL) vary with excitation power. In power applications, there is no need to limit the maximum AL. Smaller AL translates into maximum excitation current.
What should be the proper clamping pressure?
Typically, the recommended figure is approximately 700 kg/m2 (100 lbs./sq. in.) of the mating surface. Consult the Magnetics Ferrite Core Design Handbook for specific pressure recommendations for RM, PQ, EP, and pot cores.
Why ground the mating surfaces of ferrite cores?
The reason for grinding the matching surface of the magnetic core is: the sintering process causes the surface to be uneven. The core surface must have a minimum amount of air gaps to reduce gap losses and obtain optimum inductance.
Why Grind Magnetic Cores? What is surface treatment?
Grinding is an additional production process that can be used to improve core mating surfaces. It is mainly applied to core materials with a magnetic permeability of 5000 or more to achieve the maximum AL value for a given material. The mating surface should be ground like a mirror surface. Typically, the ground surface has a thickness of 0.5 to 1.0 microns, and the ground core has a thickness of 0.1 to 0.2 microns.
Why can't the tolerance of ferrite gap be kept at ±3% all the time?
The smaller the gap size, the more difficult it is to maintain tight tolerances due to the mechanical limitations of machining the gap. As AL increases, the gap will decrease and therefore the tolerance will increase. As the gap decreases, mechanical tolerances increase proportionally, and the effect of changes in material permeability also increases. Clearances defined by AL values have tighter tolerances than clearances defined by physical dimensions.
How to bond ferrite cores?
Bonding should be done with a thermosetting epoxy adhesive. Its availability is very wide. Important factors to consider in selection include required temperature and viscosity. Its practical curing temperature must not exceed the maximum temperature for safe assembly. High viscosity resins may be difficult to apply. Low-viscosity resins, in turn, can cause weak joints to crack, or be absorbed by porous ferrites. For information on specific resins, refer to the manufacturer's instructions. Be careful not to thermally shock the ferrite; raising or lowering the core temperature too quickly is dangerous. If the temperature changes faster than 5-10ºC/min, it may cause the ferrite to crack. In addition, care must be taken to adapt the coefficient of thermal expansion (CTE) of the adhesive to the coefficient of the ferrite material. Otherwise, the resin may expand or contract faster than the ferrite; cracks may result, reducing the performance of the core.
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