Soft ferrites for power electronics

Ferrites are magnetic oxides with a special magnetism known as ferrimagnetism. Louis Néel, winner of the Nobel Prize in Physics in 1970, made a major contribution to explaining and understanding their properties.

There are two main families of industrial ferrites: soft ferrites and hard ferrites, found mainly in the polycrystalline state, in the form of solid ceramics. This name derives from the first technical magnets made from steel at the beginning of the 20th century. In fact, metallic iron, when it contains impurities such as carbon inclusions, has both a high coercive field and high mechanical hardness; when it is pure, its coercive field is low and its mechanical hardness low (soft material).

Hard ferrites are therefore used as permanent magnets [D 2 100] . Soft ferrites are used as low-loss magnetic cores for transformers and inductors in power electronics [E 2 130] . Their high electrical resistivity (> 1 Ω · m) and low manufacturing cost are behind the many industrial applications for these materials. Their saturation magnetic polarization J _s is less than 0.6 T, which is lower than that of metals such as iron or cobalt. The weakest coercive fields are close to 10 A/m, i.e. 10 times greater than the softest metal alloys. Despite this, their performance at high frequencies (f > 100 kHz) is far superior to that of all other magnetic materials.

There are three families of soft ferrites:

• the 1 ^re family are manganese-zinc spinel ferrites (Mn-Zn) used for frequencies ranging from 10 kHz to 1 MHz. These materials are mainly used in the fields of energy conversion or signal processing. Their electrical resistivity is typically 1 Ω · m ;

• the 2nd family are nickel-zinc and nickel-zinc-copper spinel ferrites (Ni-Zn and Ni-Zn-Cu) used between 1 and 500 MHz. Their electrical resistivity can reach 10 ⁸ Ω · m. Like Mn-Zn ferrites, they are used to make transformers or inductors ;

• finally, the 3rd family is made up of "microwave"...