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Posted on 02 May 2019

MnZn Ferrites with High Saturation Flux Density for High Frequency Transformers and Inductors

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MnZn ferrites are widely used in high frequency transformers and inductors. Requirements for ferrite cores used in the electronic components include soft magnetism, easy magnetization with a small external magnetic field, and low loss.

By Chong Yan, Yang-zhong Du, Su-ping Wang, Hengdian Group
DMEGC Magnetic Limited Company, Dongyang 322118, China

Soft magnetic materials are classified into soft metallic materials and soft oxide materials. Because the electric resistance of the soft metallic materials is generally lower, driving the high frequency transformers and inductors cause large eddy current loss. In order to suppress loss, magnetic oxide materials, especially MnZn ferrites, are used in high frequency transformers and inductors rather than metallic materials.

Compared with metallic powder core, the drawback of MnZn ferrites is their saturation magnetic flux density. In general, the saturation magnetic flux density of MnZn ferrites is about 500mT at room temperature, which is lower than those in metallic powder cores. The comparison of saturation magnetic flux density of MnZn and metallic powder core material is shown in figure 1. This means that a larger volume of ferrite core is required to produce the same amount of magnetic flux as metallic cores produce. In order to enhance saturation magnetic flux density of MnZn ferrites, a new MnZn ferrite material DMR28 based on the ternary MnO-ZnO-Fe2O3 system had been developed by DMEGC.

Comparison of saturation magnetic flux density of MnZn and metallic powder core material at room temperature

DMR28 material characteristics are listed in table 1. From the table 1, the saturation magnetic flux density of DMR28 material is 600mT at room temperature (25°C) and 490mT at high temperature (100°C), which is about 20% higher as DMR40 material and about 10%. Higher as DMR24 material. The comparison of saturation magnetic flux density of DMR28, DMR24 and DMR40 material is shown in figure 2. Possible methods to increase saturation magnetic flux density include: Increasing the density of the sintered body of the core, increasing the total of the magnetic moments of the constituent ions in the spinel matrix phase, and raising the Curie temperature.

DMR28 material characteristics

It is clear that enhancing the density of the core will result in an increase in saturation magnetic flux density. For this purpose, optimizing the sintering conditions and the kinds and amounts of additive trace elements is very important. In order to increasing the total of the magnetic moments and raising the Curie temperature, it is essential to enhance the contents of Fe2O3 in MnZn raw materials. The main composition of DMR28 material is the contents of about 65 mol% Fe2O3, 20 mol% MnO and 15mol% ZnO as oxide equivalents. The contents of Fe2O3 in DMR28 are more than in DMR40 and DMR24 material.

Comparison of saturation magnetic flux density of DMR28, DMR24 and DMR40 material

When ferrite cores are used in the design of chokes or filters, which are required to carry direct current, it is necessary to prevent the inductance degradation caused by the static field. When DC flows through the winding of a ferromagnetic device, it tends to pre-magnetize the core and reduce its inductance. The permeability of a ferrite material measured with superimposed DC might increase slightly for very low values of DC ampereturns, but then it progressively decreases as the DC field is increased and the core approaches saturation. This permeability is referred to as the incremental permeability μΔ. It is necessary to enlarge ΔB(ΔB=Bs-Br) for increasing μΔ. The B-H curves of DMR28 and DMR24 are shown in figure 3. The ΔB of DMR28 about 450mT is higher than that of DMR24 about 360mT at room temperature. The DC superposition characteristic of DMR28 exceeds that of DMR24's conventional high saturation magnetic flux density at 25°C is shown in figure 4. The test cores are the standard ϕ25×ϕ15×8 toroid cores without gap. DMR28 has a DC superposition characteristic superior to that of DMR24 by as much as 30% at 100A/m DC magnetic field.

B-H curves of DMR28 and DMR24 at room temperature

Conclusion

A new MnZn ferrite material DMR28 was developed by DMEGC, and exhibits 20% higher saturation magnetic flux density compared to that of the conventional DMR40 material. The saturation magnetic flux density of this ternary MnO-ZnO-Fe2O3 system ferrite is increased by increasing the Fe2O3 content and enhancing the sintering density of the core. The transformers using DMR28 can be downsized and the choke made of this material exhibits super DC superposition characteristics. The characteristics exceed those of conventional high saturation magnetic flux density DMR24 material by about 30% at room temperature.

 

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