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Type And Application Of Transformer Magnetic Core
Oct 20, 2017

Type and application of transformer magnetic core

I. Basic Characteristics of magnetic materials

Magnetization curves of 1. magnetic materials

The magnetic material is composed of ferromagnetic or ferrimagnetic material, in the magnetic field under the action of H, there will be magnetization M or magnetic induction intensity corresponding to the B, they change with the magnetic field strength H curve known as the magnetization curve (M ~ H or B ~ H curve). The magnetization curve is nonlinear in general, and has 2 characteristics: magnetic saturation and hysteresis.  When the magnetic field intensity H is large enough, the magnetization M reaches a certain saturation value Ms, continue to increase H, Ms remained unchanged; and when the value of M reached saturation, the external magnetic field H is reduced to zero, M does not restore to zero, but along the MsMr curve. The working state of the material is equivalent to a point on the M - H curve or B - H curve, which is often referred to as the working point.

Common magnetic parameters of 2. soft magnetic materials

Saturation magnetic induction intensity Bs: its size depends on the composition of the material, and its physical state corresponds to the neat arrangement of magnetization vectors inside the material.

Residual magnetic induction intensity Br: the characteristic parameter of hysteresis loop and the B value of H back to 0.

Rectangular ratio: Br / Bs

Coercivity Hc: the amount of magnetization that indicates the degree of difficulty of material magnetization, depending on the composition and defects of the material (impurities, stress, etc.).

Permeability: is the ratio of B to H at any point on the hysteresis loop, which is closely related to the working state of the device.

The initial permeability is I, the maximum permeability is m, the differential permeability is D, the amplitude permeability is a, the effective permeability is e, and the pulse permeability is p. Curie temperature Tc: the magnetization of ferromagnetic material decreases with the increase of temperature. When the temperature reaches a certain temperature, the spontaneous magnetization disappears and becomes paramagnetic. The critical temperature is the Curie temperature. It determines the upper limit temperature of the magnetic device.

Loss of P: hysteresis loss and eddy current loss Ph Pe P = Ph + Pe = AF + bf2+ C Pe / F2 T2 / P Ph method reduced hysteresis loss is to reduce the coercivity of Hc; reduce the eddy current loss method Pe is the resistivity of the thickness of T thin magnetic materials and progress. The relationship between the loss of the core and the temperature rise of the core in free static air is the total power dissipation (mW) / surface area (cm2)

Conversion between magnetic parameters of 3. soft magnetic materials and electrical parameters of the device

In the design of soft magnetic devices, the voltage and current characteristics of the device must be determined according to the requirements of the circuit. The voltage to current characteristics of the device are closely related to the geometrical shape and magnetization state of the core. The designer must be familiar with the magnetization process of the material and the relationship between the magnetic parameters of the grip material and the electrical parameters of the device. The design of soft magnetic device usually consists of three steps: the correct selection of magnetic materials; reasonable to determine the geometric shape and size of core magnetic parameters; according to the requirements, the electrical parameters of the corresponding simulation core working condition.

Two. Development and types of soft magnetic materials

Development of 1. soft magnetic materials

The application of soft magnetic materials in industry began in the late nineteenth Century. With the development of electric power and telecommunication technology, low carbon steel was used to manufacture motors and transformers. Fine iron powder, iron oxide and fine wire were used in the inductance coil cores of telephone lines. By the beginning of twentieth Century, the silicon steel sheet was developed instead of low carbon steel, which improved the efficiency of transformer and reduced the loss. Until now the steel in electric power industry with a soft magnetic materials are still in the first place. By 20s, the rise of radio technology promoted the development of high magnetic conductivity materials, such as permalloy and permalloy magnetic powder cores. From 40s to 60s, is the rapid development of science and technology, radar, television broadcasting, the invention of the integrated circuit, the soft magnetic material requirements are higher, producing soft magnetic ribbons and soft ferrite materials. In 70s, with the rapid development of telecommunication, automatic control, computer and other industries, has developed a soft magnetic alloy with the head, in addition to the traditional crystalline soft magnetic alloy, and the rise of another kind of material - amorphous soft magnetic alloy. Source:

2. types of common soft magnetic cores

Three ferromagnetic elements, iron, cobalt and nickel, are the basic components of magnetic materials.

Classification of products according to the main components, magnetic characteristics and structural features:

(1) powder core: magnetic powder core, including: iron powder core, iron silicon aluminum powder core, high magnetic flux powder core (High Flux), permalloy powder core (MPP), ferrite core

(2) band around iron core: silicon steel sheet, permalloy, amorphous and nanocrystalline alloy

Characteristics and application of three common soft magnetic cores

(1) powder cores

1. magnetic powder core

The magnetic powder core is a kind of soft magnetic material which is pressed by the ferromagnetic powder and the edge medium. Because of ferromagnetic particles is very small (high frequency use is 0.5 ~ 5 m), and by the nonmagnetic electrical insulation film material separated, therefore, on the one hand can be separated as materials for the high frequency eddy current; on the other hand due to the clearance effect between particles, resulting in materials with low magnetic rate and constant peimeability magnetic properties; and because of the small particle size, basically no skin phenomenon, the permeability variation is relatively stable. Mainly used in high frequency inductors. Magnetoelectric properties of magnetic core mainly depends on the powder material permeability, particle size and shape, the filling coefficient, content, insulating medium pressure forming and heat treatment process.

There are three kinds of magnetic powder cores: iron powder core, permalloy powder core and iron silicon aluminum powder core.

The effective permeability of the magnetic core is e and the inductance is calculated as: e = DL/4N2S * 109 source: transmission and distribution equipment network

Among them: D is the magnetic core uniform diameter (CM), L is inductance (enjoyment), N is winding turn number, S is magnetic core effective cross sectional area (cm2).

(1) iron powder core

Commonly used iron powder core is composed of carbon based iron magnetic powder and resin carbon based iron magnetic powder. The lowest price in powder core. The saturation magnetic flux density is about 1.4T; the permeability range is from 22~100; the initial permeability I varies with frequency; the DC current superposition performance is good; but the loss is high under the high frequency.

Variation of initial permeability of iron powder with DC magnetic field strength

Variation of initial permeability of iron powder with frequency

(2) permalloy powder core

The permalloy powder core mainly consists of Mo permalloy powder core (MPP) and high magnetic flux powder core (High Flux).

MPP is made up of 81%Ni, 2%Mo and Fe powder. The main characteristics are: saturation magnetic induction value is about 7500Gs; permeability range is large, from 14~550; in powder core has the lowest loss; temperature stability is excellent, widely used in the magnetic field

The magnetostrictive coefficient is close to zero, and no noise occurs when working at different frequencies. High quality factor Q filter, mainly used in the following 300kHz induction coil, load resonant circuit, LC circuit, commonly used in high requirements on the temperature stability of the output inductor, power factor compensation circuit, commonly used in AC circuit, the most expensive powder core.

High flux powder core HF is made up of 50%Ni and 50%Fe powder. The main characteristics are as follows: the saturation magnetic flux density is about 15000Gs; the permeability range is from 14~160; the highest magnetic induction intensity in the powder magnetic core, the highest DC bias capacity; the magnetic core volume is small. Mainly used in line filter, AC inductance, output inductance, power factor correction circuit, etc., commonly used in DC circuit, high DC bias, high DC and low AC more.  Price below MPP.

(3) Fe Si Al powder core (Kool M mu Cores)

The iron silicon aluminum powder core consists of 9%Al, 5%Si and 85%Fe powder. Is to replace the main iron core loss, 80% lower than the iron core, can be used at frequencies above 8kHz; saturation magnetic induction around 1.05T; permeability from 26 to 125; the magnetostrictive coefficient is close to 0, no noise generated at different frequencies; higher DC bias capability than MPP with performance; the best price ratio. Mainly used in AC inductance, output inductance, line filter, power factor correction circuit, etc.. Sometimes the air gap ferrite is replaced as the core of the transformer.

2. soft magnetic ferrite (Ferrites)

Soft ferrite is a kind of ferrimagnetic oxide with Fe2O3 as the main component. It is produced by powder metallurgy method. There are several kinds of Mn-Zn, Cu-Zn, Ni-Zn and so on, in which the Mn-Zn ferrite yield and the largest amount, Mn-Zn ferrite resistivity is low, 1~10 ohm - meters, generally below 100kHZ frequency use. The resistivity of Cu-Zn and Ni-Zn ferrite is 102~104 ohm - meter, and the loss in radio frequency range from 100kHz to 10 MHz is small, which is more widely used in radio antenna coil and radio intermediate frequency transformer. The shape of the core is rich in shape, including E, I, U, EC, ETD, square (RM, EP, PQ), tank shape (PC, RS, DS) and round, etc.. It's very convenient to use. Because of soft ferrite without the use of nickel and other scarce materials can get high permeability, powder metallurgy method and suitable for mass production, so the cost is low, and because is not sensitive to sinter hardness, stress, very convenient in application. Moreover, the permeability varies steadily with frequency and remains almost unchanged below 150kHz. With the emergence of soft ferrite, the production of magnetic powder core is greatly reduced. Many of the original magnetic powder cores are replaced by soft ferrite. Please login: power transmission and distribution equipment network browse more information

There are many manufacturers of ferrite at home and abroad. This paper introduces the application of Mn-Zn ferrite produced by Magnetics company in the United States as an example. They are divided into three basic materials: basic materials for telecommunications, broadband and EMI materials, power type materials.

Telecommunications with permeability ferrite from 750 to 2300, with low permeability loss factor, high quality factor Q, stable temperature / time, permeability at work a decrease in the slowest, about every 10 years, down from 3% to 4%. It is widely used in high Q filter, tuning filter, load coil, impedance matching transformer, proximity sensor. High permeability ferrite broadband ferrite is often said, permeability were 5000, 10000, 15000. It is characterized by low loss factor, high permeability and high impedance / frequency characteristics. It is widely used in common mode filter, saturable inductor, current transformer, earth leakage protector, transformer, signal and pulse transformer. It is widely used in wide band transformer and EMI. The power ferrite has a high saturation magnetic flux density of 4000 ~ 5000Gs. In addition, it has low loss / frequency relation and low loss / temperature relation. That is to say, with the increase of frequency, the loss increases little; with the progress of temperature, the change of loss is not big. Widely used in power choke, parallel filter, switching power supply transformer, switching power supply inductance, power factor correction circuit.

(two) ribbon wound core

1. silicon steel sheet iron core

Silicon steel sheet is an alloy, in the pure iron into a small amount of silicon (generally less than 4.5%) formed iron silicon alloy called silicon steel. The core has a saturation magnetization of the highest value is 20000Gs; because they have better and magnetoelectric properties, easy mass production, low price, mechanical advantage and small impact, in the power electronics industry have been widely applied, such as electric power transformer, distribution transformer and current transformer core. It is the largest material and output in soft magnetic materials. It is also the largest amount of material used in power transformers. Especially in low frequency, high power, the most applicable. There are commonly used cold rolled silicon steel sheet DG3 cold rolled non oriented electrical steel strip, cold rolled strip DQ DW orientation, applicable to all types of electronic systems, household appliances in low frequency and small power transformers and chokes, reactor, inductor core, this alloy can be good toughness, punching, cutting, folding with iron core type and winding type. But under high frequency, the loss increases sharply, and the general frequency is not more than 400Hz. From the application point of view, two factors should be taken into account for the selection of silicon steel: magnetism and cost. For small motor, reactor and relay, optional iron or low silicon steel sheet; for large motor, optional high silicon hot rolled silicon steel sheet, single orientation or cold rolled non oriented silicon steel sheet; the transformer used single oriented cold-rolled silicon steel sheet. In the use of power frequency, the thickness of the commonly used strip is 0.2~0.35 mm; under the use of 400Hz, often choose 0.1 mm thickness is appropriate. The thinner the thickness, the higher the price. Source: transmission and distribution equipment network

2. permalloy

Permalloy usually refers to Fe Ni alloy with nickel content in the range of 30~90%. It is a widely used soft magnetic alloy. By proper technology, can effectively control the magnetic properties, such as more than 105 of the initial permeability, more than 106 of the maximum permeability, low to 2 per thousand OE coercivity, close to 1 or close to 0 of the rectangular coefficient, and has a good plasticity with face centered cubic crystal structure of permalloy can be processed into 1 m thin strip and using various forms.  Commonly used alloys are 1J50, 1J79, 1J85 and so on. The saturation magnetic induction intensity of 1J50 is slightly lower than that of silicon steel, but the permeability is several times higher than that of silicon steel, and the iron loss is 2~3 times lower than that of silicon steel. Made of higher frequency (400~8000Hz) transformer, no load current is small, suitable for making below 100W small high frequency transformer. 1J79 has good comprehensive performance, suitable for high frequency, low voltage transformer, leakage protection switch core, common mode inductance core and current transformer core. The initial permeability of 1J85 can reach more than one hundred thousand and 105, which is suitable for low frequency or high frequency input and output transformers, common mode inductors and high precision current transformers for weak signals.

3. amorphous and nanocrystalline soft magnetic alloys (Amorphous and Nanocrystalline alloys)

Silicon steel and permalloy soft magnetic materials are crystalline materials, atom rules in three-dimensional space arrangement, the formation of lattice periodic structure, there are grains and grain boundaries, dislocations, interstitial atoms, magnetic anisotropy and other defects, unfavorable to soft magnetic properties. In terms of magnetic physics, the irregular arrangement of atoms, the absence of periodicity and the amorphous structure of grain boundaries are ideal for obtaining excellent soft magnetic properties.  Amorphous alloys and alloys are a new field of materials published in 70s.  The preparation technology is completely different from the traditional method, but the cooling rate is about one million degrees per second super rapid solidification technique, to strip a molding product from liquid steel, than the general cold rolling thin metal belt manufacturing process to reduce a lot of intermediate process, this new process is known as a the revolution of traditional metallurgy process. Due to the ultra rapid solidification, solidification time ordered atomic crystal, solid alloy are long-range disordering structure, there is no grain and grain boundary amorphous alloy, called amorphous alloy, is called a revolution in metallurgical materials. This amorphous alloy has many unique properties, such as excellent magnetic properties, corrosion resistance, wear resistance, high strength, hardness and toughness, high resistivity and electromechanical coupling properties. Because of its excellent performance and simple process, it has become the focus of research and development in the field of materials science at home and abroad since 80s. At present, the United States, Japan and Germany have a perfect scale of production, and a large number of amorphous alloy products gradually replace silicon steel and permalloy and ferrite to the market.

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Contact person: Elva Zhuang&Sunny Ni