Magnetic materials and magnetic circuits for switching power supplies. Reference data

Encyclopedia of radio electronics and electrical engineering / Reference materials

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Most often, in chokes and transformers of amateur radio and industrial high-frequency switching power supplies, magnetic cores made of 1000NM-2000NM ferrite are used. However, strictly speaking, their use in power supplies is not always correct, since these ferrites are designed to work in weak magnetic fields (in loop coils, matching transformers, etc.).

It is possible to significantly improve the energy characteristics of network transformers and chokes if magnetic cores made of ferrite of such grades as 2500NMS1, 2500NMS2, 3000HMC, 3000NMS1 are used. These low-frequency (H) manganese-zinc (M) ferrites with a relative magnetic permeability of 2500 and 3000, respectively, are designed to operate in strong fields (C). Ferrites of this group are designed specifically for high-power electronic devices and are able to operate normally at temperatures up to 125...150°C. Below are the main comparative characteristics of some common ferrites designed for operation in high magnetic fields.

Specifications:

• Specific volumetric magnetic losses, kW/m3, not more, at induction 0,2 T, frequency 16 kHz and temperature 25°C for 2500NMS1......170
• 2500NMS2......150
• Z000NMS......160
• Specific volumetric magnetic losses, kW/m3, no more, at induction 0,2 T. frequency 16 kHz and temperature 100°С for 2500НМС1......140
• 2500NMS2......110
• Z000NMS......160
• Induction, T, not less, at a field strength of 240 A/m and a temperature of 100°C for
• 2500NMS1......0,29
• 2500NMS2......0,33
• 3000NMS......0,27
• Temperature or Curie point, °С......200
• Density, g/cm3......4,7...4,9
• Electrical resistance, Ohm m......1

These ferrites have rather similar characteristics, and the specific volume losses not only do not increase with temperature, as in many other similar materials, but even decrease. This circumstance, and the fact that the Curie point of ferrites of the class under consideration is very high, allow us to classify them as thermally stable.

Rice. 1 illustrates the temperature dependence of the specific volumetric magnetic losses of two ferrites - 2500NMS2 and 2000NM1. It can be seen that at normal temperature the materials are practically not inferior to one another, and already at 100 ° C, which is quite realistic for a transformer or inductor operating in a power source, the losses in the 2000NM1 ferrite are almost 2,5 times greater than in 2500NMS2.

On fig. Figure 2 shows typical dependences of specific magnetic losses as a function of the magnetic field induction amplitude at two temperatures. It is known that the losses in the magnetic circuit are proportional to the square of the induction amplitude. Ferrites of the group under consideration, as the graphs show, are significantly superior to traditional ones, similar to 2000NM1, and in terms of the maximum allowable induction, especially at elevated temperatures.

Typical dependences of the magnetic induction B and the relative magnetic permeability μ on the strength H of the applied external field at normal temperature for the same two materials are shown in Figs. 3. A joint analysis of this and the previous figures allows us to conclude that. that "high-field" ferrites allow normal operation of the magnetic circuit with an induction amplitude greater by 30% compared to conventional ferrites in the entire operating temperature range.

With an increase in the temperature of the magnetic circuit, the permissible induction amplitude decreases, nevertheless remaining significantly greater than that of ferrites like 2000NM1. This is confirmed by the graphs in Fig. 4 taken for ferrite 2500NMS1 in two temperature regimes.

The range of types of magnetic circuits made from ferrites for strong fields is quite wide (Table 1). The industry has been producing most standard sizes for a long time, they are listed and described in detail in the reference book Sidorov I.N., Khristinin A.A., Skornyakov S.V. "Small-sized magnetic circuits and cores" - M .: Radio and communication. 1989.

An exception is the relatively new HF magnetic circuits. convenient for use in switching power supplies. The magnetic circuit KB consists of two identical parts (Fig. 5; one part is shown), fastened into a single whole with special spring ties. After assembly, an annular space is formed inside the magnetic circuit to accommodate the coil.

The main dimensions of the magnetic circuits of the entire manufactured series, made from the considered ferrites, are summarized in Table. 2. The KV14-5 magnetic circuit, unlike the others, has a central through hole with a diameter of 5 mm (dl).

The full designation of a ferrite magnetic circuit always begins with the letter M. This is followed by the brand of ferrite and through hyphens - the execution number, inductance coefficient, and type of magnetic circuit. Example: M2500NMS1 -15-250-KV8.

The inductance coefficient is the inductance in nanohenries of one turn laid in this magnetic circuit. Knowing this parameter, it is easy to calculate the inductance of the future coil if the number of its turns is known.

For magnetic circuits without a non-magnetic gap, the inductance coefficient is more than 1000, however, due to the fact that the spread of this parameter is very large, it is often not indicated. The introduction of a gap sharply reduces the inductance factor, but the tolerance for the value of this parameter also decreases (see Table 3; b / c - magnetic circuit without a gap).

Usually, a gap of one size or another is formed at the factory of the magnetic cores on special machine equipment. The gap is obtained by grinding the central protrusion on one or both parts of the magnetic circuit. In amateur conditions, a gap at a gapless magnetic circuit can only be formed by installing an annular gasket made of a solid non-magnetic material (getinax, textolite, fiberglass, etc.). When determining the thickness of the gasket, they proceed from the rule: half of its thickness is equal to the specified or calculated gap minus the factory gap (if any) of the existing magnetic circuit.

The characteristics of the magnetic circuits of the KV series, made of ferrite 2500NMS1, necessary for calculating the winding units of switching power supplies, are summarized in Table. 3.

In conclusion, it should be said that work on improving magnetic cores and creating new types of products continues. So. at the request of the customer, magnetic circuits of reduced height are produced, coil frames have been developed and their serial production has been established.

Author: A.Mironov, Lyubertsy, Moscow Region

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