Preparation Technology of Power Ferrite for High Frequency Switching Power Supply Transformer

With the development of power electronics technology, the demand for multi-function and high density of electronic equipment is further increased. As an indispensable switching power supply for electronic equipment, it is urgent to achieve small and lightweight. In order to miniaturize the switching power supply, it is necessary to miniaturize the switching power transformer first. PC44 and PC50 power ferrite materials and magnetic cores with higher operating frequencies have been developed to meet this demand.

The properties of ferrites are determined not only by their chemical composition and crystal structure, but also by their density, grain size, porosity and their distribution within and between grains. Therefore, formulation is the basis and sintering is the key to prepare high-performance power ferrite materials. Formulation and density determine the saturated flux density Bs (power ferrite cores usually work in the state of DC bias field, high Bs is needed to ensure high DC superposition characteristics) and Curie temperature (fc). Adding effective additives and matching with appropriate sintering process will determine the performance of ferrite and affect the process of solid state reaction. Degree and final phase composition, density and grain size make the microstructure of soft magnetic ferrite more effectively controlled, so as to ensure the harmony and unity of the main characteristic parameters of the material.

Selection of Main Formula for 1 High Performance Power Ferrite

In order to improve the efficiency of power conversion and avoid saturation, it is required that the power ferrite materials used in high frequency switching power supply transformers have high Bs, high initial permeability (_ i) and high amplitude permeability (_ a). At the same time, in order to avoid thermal breakdown of transformers at high frequency, the power loss (Rc) of materials should be as small as possible and negative temperature coefficient should be expected. It can be said that the three important magnetic properties of power ferrite materials are mui, Bs and Rc, and the frequency, temperature and time stability of these parameters. They are a contradictory unity. Some parameters are even in serious opposition. The general idea of their organic unity is to control the magnetocrystalline anisotropy constant K1-t curve and the microstructure of ferrite in formulation, formulation and composition. Additives and sintering process make K1 have a good temperature characteristic, adjust the minimum value of K1 to the appropriate position and make it tend to zero.

The size of Mui contributes most directly to the high inductance factor (AL) of the core. Therefore, it is necessary to ensure that the ferrite has a high Mui value. But on the other hand, mutually restricting between muti and material cut-off frequency fr, increasing the use frequency of materials and increasing Mui are antagonistic to each other, and can only give consideration to each other in practical materials.

As far as the Bs rat and Curie temperature Tc of power ferrite are concerned, they are determined by formula and density. For the main formula of power ferrite, domestic and foreign soft magnetic researchers have done a more in-depth systematic research, and made it into the phase diagram (without additives) shown in Figure 1 to make it more intuitive. After years of research, TDK Company in Japan has further delineated the value area in the phase diagram of Mn-zn ferrite composition. The formula of central position is about: FezO 3:MnO:znO=53.5:36.5:10 (mole fraction), which is basically consistent with the main formula of PC44 in many domestic enterprises: FezO 3:MnO:ZnO=53.3:36.5:1O.2 (mole fraction). As far as PC44 and PC50 are concerned, because their Bs are relatively high, it is necessary to adopt Fe formula because Fe2O3 content is within the range of (51-55) mO1%, and Bs increases with the increase of Fe2O3 content (conversely, excessive ZnO content will cause material high temperature, or the decrease of Bs and tc). The optimum formula combination can be determined by orthogonal process test combined with impurity addition and sintering process.

2. Selection of additives for high performance power ferrite

The chemical composition of power ferrite is not the only factor to determine the properties of ferrite. The distribution of cations and crystal point defects in crystal sites plays an important role. By adding additives and adjusting process to improve the microstructure of ferrite, it is more conducive to the harmony of the main characteristic parameters of the material. According to the basic theory of magnetism, the cut-off frequency fr of power ferrite material is related to the grain size of ferrite in right form (1).

In the formula, Ms is the saturated magnetization of the material.

Beta is the damping coefficient.

Formula (1) shows that it is inversely proportional to D (mu 1-1). Therefore, by adding additives and adjusting sintering process, the grain size can be refined and the cut-off frequency of materials can be increased. However, the infinite reduction of grain size necessarily increases power loss. On the other hand, the magnitude of FR is also related to the level of mu 1 (which is closely related to sintering temperature).

For PC44 and PC50 materials normally operating at several hundred kHz high frequencies, power loss mainly consists of hysteresis loss Rh and eddy current loss Pe. As hocBm3 (Bm is the working flux density), it can be seen that in order to reduce Ph, the Bs of the material should be high, and the homogeneity of the composition should be good (using high purity raw materials). At the same time, the consistency of grain size and the density of the material must be improved, so as to minimize the internal stress. The eddy current loss is expressed by formula (2).

Pe=(2/4). r2. lf2. Bm2/p(2)

In the formula, R is the average grain size.

P is the resistivity.

It can be seen that there are two main ways to reduce power loss of materials at high frequencies: increasing resistivity and controlling ferrite grains in the optimum state range (too small grains, Pe will become smaller, but Ph will increase).

The most effective way to control the grain size and resistivity is to mix the additives reasonably and improve the sintering process. It is well known that adding some beneficial additives such as Sn02, TiO2 and Co2O3 can further control the K1 value of the material and make it very small in a wide temperature range. Compound addition of CaO and SiO 2 can increase the resistivity of the material and reduce the power loss of the material. In fact, there are many useful additives to improve the properties of Mn-zn ferrites. Their main functions can be divided into three categories: the segregation of additives at grain boundaries affects grain boundaries resistivity; the change of microstructures during sintering can be affected by the control of sintering temperature and oxygen content. The microstructures can be improved by controlling sintering temperature, reducing power loss and increasing the temperature of material permeability. Degree and time stability, expansion frequency, etc. The third kind is solid solution in spinel structure, which affects the magnetic properties of materials. The additives of Ca and Si belong to the first and second categories, Bi, Mo, V and P belong to the second category, and the main roles of _Ti, Cr, C0, Al, Mg, Ni, Cu and Sn belong to the third category.

Fig. 2 shows the effect of six additives, MoO, CuO and so on, on the permeability of Mn-zn ferrite. Among them, mu 1 and mu 1 represent the permeability of ferrite without additives and fewer additives, respectively. Fig. 3 shows the effect of SiO 2 on the permeability of Mn-Zn ferrite. Fig. 4 shows the effect of the amount of titanium dioxide on the mu-t curve of Mn-Zn ferrite. Fig. 5 (a) and Fig. 5 (b) show the composite respectively. The effect of adding SiO 2 and CaO on the resistivity and specific loss coefficient of Mn-zn ferrite at 100 kHz (tan Delta 6/u i).

When developing SB-lM (equivalent to PC50) materials, researchers of Japan Northeast Metal Corporation found that part of the general compound additive SiO2 CaO would dissolve in the grains, thus increasing the hysteresis loss. Under the condition of 500 kHz-1 MHz, the effect of reducing power loss was not good. Therefore, they carried out fruitful research work, hoping to find out more hysteresis loss that did not increase. Additives that can effectively improve resistivity. Table l lists their research results. Among the eight additives, Al2O3, SnO2, and titanium dioxide are dissolved in grains, and the effect of increasing resistivity is almost invisible. Other additives are mainly free in grain boundaries. Among these additives, HfO2 has the most significant effect on increasing resistivity and reducing eddy current loss.

In the development of high-performance power ferrite materials, we should make full use of the achievements of predecessors and not waste too much energy on the exploration of formulations and additives. The general formulation and doping principle are to make the magnetocrystalline anisotropy constant K1 and magnetostriction constant lambda s approach zero as far as possible. Choosing additives should pay attention to the following principles:

1) The total amount of doped additives (wt%) should be controlled below O.2%.

2) CaO (or CaCO3) and SiO 2 are usually indispensable additives.

3) V2O5, Nb205, _Ti02, Ta2O5, HfO2, CO2O3 and other high-valent ions should be added in combination, and the components should not be excessive, preferably no more than 4 kinds. The weight of each additive should be generally controlled below 1000ppm.

4) In all the additives mentioned above, except Co3 + ions, the K1 value of other ions is negative. For example, the 3F3 material developed by Philips Company (a material between PC40 and PC50), the basic technical point is to add Ti4 + and 203 + simultaneously to control the temperature characteristics of the material and reduce the hysteresis loss, as shown in Figure 6.

Sintering Technology of 3 High Performance Power Ferrite

Sintering is the key process to prepare high-performance power ferrite materials. In the sintering process, the three key factors that must be strictly controlled are the heating and cooling rate, the maximum sintering temperature and the atmosphere in the furnace. They have great influence on the microstructure, chemical composition and electromagnetic properties of ferrite materials. Appropriate sintering process should be determined according to raw material formula and additives, pre-firing temperature, kiln structure and length, cooling mode, performance of power ferrite, and process verification and judgment through final performance of materials.

The heating rate is directly related to the density, grain size and homogeneity of ferrite products. Excessive heating rate will make the grain size uneven, and there are more pore inside. If the heating rate is too slow, the density of sintered ferrite will be low and the pore will obviously increase. In order to obtain ferrite with small and uniform grain size (PC40 material, grain size is about 10-14 micron, PC50 material, grain size is about 3-6 micron), few stomata, high density and no cracking defect, heating up below 600 C is not too fast, 600-900 C is faster, 900-l100 C is the initial stage of grain, it is appropriate to raise temperature smoothly, at the same time, densification measures are adopted, and the temperature above 1100 C can be slightly faster, the highest. Sintering temperature is not more than 1 350 C (to limit grain size), holding time is 3 to 4 hours, and then under the protection of nitrogen (N2) to choose the appropriate oxygen partial pressure cooling.

It is necessary to take densification measures at 900-1100 (?) C in order to reduce the porosity of ferrite. TDK Company in Japan pays special attention to the control of temperature rising rate and ambient atmosphere between 900 and 1100 C. He believes that this stage is the key to ensure the good microstructure of ferrite. The control of this stage is particularly important for the preparation of high-performance power ferrite such as PC44 and PC50. The usual densification measures are to steadily raise the temperature from 900 C to L100 C, then keep it warm for 1 h, and to control the partial pressure of oxygen by filling a proper amount of N2. This makes the apparent density of ferrite rapidly reach 99% of the true density, and most of the pore is on the grain boundary. Of course, it is also very important to ensure that there is enough oxygen content in the kiln and the exhaust gas pipeline is unblocked in the heating section below 1000 C.

During the cooling stage, ferrite will be oxidized or reduced. The partial pressure of oxygen in the kiln can be controlled by adding proper amount of N2 to protect the atmosphere. The purpose is to prevent the plasma valence of Mn, Fe, CoCu, desolvent and lattice change during the cooling process of ferrite. Excessive oxidation and reduction lead to the precipitation of a-Fez03, Fe0, Fe3O4 and Mn203, which leads to the rapid deterioration of magnetic properties. Fig. 7 is a phase diagram of equilibrium atmosphere of power ferrite with the formula Fe2O3:MnO:ZnO=51.9:26.8:18.3 (mol%). From Fig. 7, we can see the importance of atmosphere to the oxidation state of spinel phase and Fe2O3 phase boundary.  Particular attention should be paid to the fact that the growth kinetics is insensitive at the lowest temperature after cooling along the iso-component line, which minimizes the dissolution of a-Fe203 and the degree of oxidation and phase formation. Fig. 8 shows the typical sintering process curve of power ferrite.