There are many magnetic devices used in switching power supplies, among which the commonly used soft magnetic devices are: the main transformer (high-frequency power transformer) as the core device of the switching power supply, common mode choke, high-frequency magnetic amplifier, filter choke, spike signal suppressor, etc. Different devices have different performance requirements for materials. The table shows the performance requirements of various devices for magnetic materials.

(1) High frequency power transformer

The size of the transformer core depends on the output power and temperature rise. The design formula of the transformer is as follows:

P=KfNBSI×10-6T=hcPc+hWPW

Among them, P is the electric power; K is the coefficient related to the waveform; f is the frequency; N is the number of turns; S is the iron core area; B is the working magnetic induction; I is the current; T is the temperature rise; Pc is the iron loss; PW is the copper loss; hc and hW are coefficients determined by experiments.

It can be seen from the above formula: high working magnetic induction B can obtain large output power or reduce volume and weight. However, the increase in B value is limited by the Bs value of the material. The frequency f can be increased by several orders of magnitude, making it possible to significantly reduce the volume and weight. Low core loss can reduce temperature rise, which in turn affects the selection of frequency of use and working magnetic induction. Generally speaking, the main requirements for materials of switching power supplies are: low high-frequency loss, high enough saturation magnetic induction, high magnetic permeability, high enough Curie temperature and good temperature stability. Some applications require higher High rectangularity ratio, insensitive to stress, good stability, and low price. Because the core of a single-ended transformer operates in the first quadrant of the hysteresis loop, the magnetic requirements for the material are different from those of the main transformer mentioned above. It is actually a single-ended pulse transformer, so it requires a large B=Bm-Br, that is, the difference between the magnetic induction Bm and the residual magnetism Br must be large; it also requires high pulse permeability. Especially for single-ended flyback switching main transformers, or energy storage transformers, energy storage requirements must be considered.

The amount of energy stored in the coil depends on two factors: one is the working magnetic induction Bm value or inductance L of the material, and the other is the working magnetic field Hm or working current I. The energy storage W=1/2LI2. This requires the material to have a sufficiently high Bs value and a suitable magnetic permeability, usually a wide constant magnetic permeability material. For transformers working between ±Bm, the area of ​​the hysteresis loop, especially the loop area at high frequencies, is required to be small. At the same time, in order to reduce no-load losses and reduce excitation current, it should have a high magnetic permeability. The most suitable is a closed ring core, and its hysteresis loop is shown in the figure. This core is used in devices with double-ended or full-bridge working conditions.

Generally, it is not easy for metal crystalline materials to reduce iron loss at high frequencies. For amorphous alloys, they do not have magnetocrystalline anisotropy, metal inclusions and grain boundaries, and they do not have long-range ordered atomic arrangement. Their resistivity is 2-3 times higher than that of general crystalline alloys. In addition, the rapid cooling method forms amorphous thin strips with a thickness of 15-30 microns at one time, which is particularly suitable for high-frequency power output transformers. It has been widely used in iron cores of inverter arc welding power supplies, single-ended pulse transformers, high-frequency heating power supplies, uninterruptible power supplies, power transformers, communication power supplies, switching power supply transformers and high-energy accelerators. It is the best magnetic core material for transformers at frequencies of 20-50kHz and power below 50kW.

The new inverter arc welding power supply single-ended pulse transformer developed in recent years has the characteristics of high frequency and high power. Therefore, the transformer core material is required to have low high-frequency loss, high saturation magnetic induction Bs and low Br to obtain a large working magnetic induction B, so as to reduce the size and weight of the welding machine. The commonly used core material for high-frequency arc welding power supply is ferrite. Although it has low high-frequency loss due to its high resistivity, its temperature stability is poor, the working magnetic induction is low, and the transformer volume and weight are large, which can no longer meet the requirements of new arc welding machines. After using nanocrystalline ring core, due to its high Bs value (Bs>1.2T), high ΔB value (ΔB>0.7T), high pulse magnetic permeability and low loss, the frequency can reach 100kHz. The volume and weight of the core can be greatly reduced. In recent years, tens of thousands of nanocrystalline cores have been used in inverter welding machines. Users have reported that the welding machine made of nanocrystalline transformer core and amorphous high-frequency inductor is not only small in size, light in weight, easy to carry, but also has stable arc, small spatter, good dynamic characteristics, high efficiency and high reliability. This ring nanocrystalline core can also be used in medium and high frequency heating power supplies, pulse transformers, uninterruptible power supplies, power transformers, switching power supply transformers and high-energy accelerators. The core material can be selected according to the frequency of the switching power supply.

The ring nanocrystalline core has many advantages, but it also has the disadvantage of difficult winding. In order to facilitate winding when the number of turns is large, a high-frequency, high-power C-type amorphous nanocrystalline core can be used. The performance of the amorphous nanocrystalline alloy C-type core made by low-stress adhesive curing and new cutting process is significantly better than that of silicon steel C-type core. At present, this core has been used in inverter welding machines and cutting machines in batches. The inverter welding machine main transformer core and reactor core series are: 120A, 160A, 200A, 250A, 315A, 400A, 500A, 630A series.

(II) Pulse transformer core

Pulse transformer is a transformer used to transmit pulses. When a series of unipolar pulse voltages with a pulse duration of td (μs) and a pulse amplitude voltage of Um (V) are applied to a pulse transformer winding with N turns, at the end of each pulse, the magnetic induction intensity increment ΔB (T) in the core is: ΔB = Um td / NSc × 10-2 where Sc is the effective cross-sectional area of ​​the core (cm2). That is, the magnetic induction intensity increment ΔB is proportional to the area of ​​the pulse voltage (volt-second product). When outputting unidirectional pulses, ΔB=Bm-Br. If a demagnetizing winding is added to the pulse transformer core, ΔB = Bm + Br. In the pulse state, the ratio of ΔB of the dynamic pulse hysteresis loop to the corresponding ΔHp is the pulse magnetic permeability μp. The ideal pulse waveform refers to a rectangular pulse wave. Due to the influence of circuit parameters, the actual pulse waveform is different from the rectangular pulse and often distorted. For example, the rising time tr of the pulse front is proportional to the leakage inductance Ls of the pulse transformer, the distributed capacitance Cs caused by the winding and structural parts, the pulse top drop λ is inversely proportional to the excitation inductance Lm, and the eddy current loss factor will also affect the output pulse waveform.

Leakage inductance Ls of pulse transformer = 4βπN21 lm / h

Primary excitation inductance Lm of pulse transformer = 4μπp Sc N2 / l ×10-9

Eddy current loss Pe = Um d2td lF / 12 N21 Scρ

β is a coefficient related to the winding structure type, lm is the average turn length of the winding coil, h is the width of the winding coil, N1 is the number of primary winding turns, l is the average magnetic path length of the core, Sc is the cross-sectional area of ​​the core, μp is the pulse permeability of the core, ρ is the resistivity of the core material, d is the thickness of the core material, and F is the pulse repetition frequency.

From the above formula, it can be seen that, for a given number of turns and core cross-sectional area, the larger the pulse width, the greater the change in magnetic induction intensity ΔB of the core material required; for a given pulse width, increasing the change in magnetic induction intensity ΔB of the core material can greatly reduce the cross-sectional area of ​​the pulse transformer core and the number of turns of the magnetizing winding, and thus reduce the volume of the pulse transformer. To reduce the distortion of the leading edge of the pulse waveform, the leakage inductance and distributed capacitance of the pulse transformer should be minimized. To this end, the number of turns of the pulse transformer winding should be as small as possible, which requires the use of materials with higher pulse permeability. To reduce the top drop, the primary excitation inductance Lm should be increased as much as possible, which requires the core material to have a higher pulse permeability μp. To reduce eddy current loss, soft magnetic tape with high resistivity and as thin a thickness as possible should be selected as the core material, especially for pulse transformers with high repetition frequency and large pulse width.

The requirements of pulse transformers for core materials are:

① High saturation magnetic flux density Bs value;

② High pulse permeability, which can obtain sufficiently large excitation inductance with a smaller core size;

③ High-power unipolar pulse transformers require the core to have a large magnetic flux density increment ΔB and use low residual magnetic induction materials; when using additional DC bias, the core is required to have a high rectangular ratio and small coercive force Hc.

④ Low-power pulse transformers require the core to have a high initial pulse permeability;

⑤ Low loss.

Ferrite cores have high resistivity, wide frequency range, and low cost. They are widely used in small-power pulse transformers, but their ΔB and μp are both low, and their temperature stability is poor. They are generally used in places where the top drop and trailing edge requirements are not high.

(III). Inductor core

Iron core inductors are a basic component. In the circuit, inductors have an impedance effect on the change of current and are widely used in electronic equipment. The main requirements for inductors are as follows:

① When working for a long time at a certain temperature, the rate of change of the inductance of the inductor over time should be kept to a minimum;

② Within a given operating temperature range, the temperature coefficient of the inductance should be kept within the allowable limit;

③ The electrical loss and magnetic loss of the inductor are low;

④ The nonlinear spurious variation is small;

⑤ Low price and small size.

Inductor components are closely related to inductance L, quality factor Q, core weight W, and DC resistance R of the winding.

The ability of inductor L to resist AC current is expressed by the inductive reactance value ZL: ZL = 2πfL, the higher the frequency f, the greater the inductive reactance value ZL.