Selection and design of magnetic core in magnetic devices

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, etc. The transformer design formula 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 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 that 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, which may significantly reduce the volume and weight. Low core loss can reduce temperature rise, which in turn affects the selection of operating frequency and working magnetic induction. Generally speaking, the main requirements of switching power supplies for materials are: as low high-frequency loss as possible, sufficiently high saturation magnetic induction, high magnetic permeability, sufficiently high Curie temperature and good temperature stability. Some uses require a higher rectangular ratio, insensitivity to stress, good stability and low price. Because the core of a single-ended transformer works in the first quadrant of the hysteresis loop, the requirements for material magnetism are different from those of the aforementioned main transformer. It is actually a single-ended pulse transformer, so it is required to have a large B=Bm-Br, that is, the difference between the magnetic induction Bm and the residual magnetism Br should be large; at the same time, a high pulse magnetic permeability is required. Especially for single-ended flyback switching main transformers, or energy storage transformers, energy storage requirements should 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 stored 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 in double-ended or full-bridge working states.

Generally, it is not easy to reduce the iron loss of metal crystalline materials under high frequency. However, for amorphous alloys, since 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 an amorphous thin strip 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 the 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 transformers and high-energy accelerators. It is the best magnetic core material for transformers at frequencies of 20-50kHz and powers 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 the new arc welding machine. After adopting the 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 welding machines made of nanocrystalline transformer cores and amorphous high-frequency inductors are not only small in size, light in weight, and easy to carry, but also have stable arcs, small spatters, good dynamic characteristics, high efficiency, and high reliability. This type of ring-shaped 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.

(2) Pulse transformer core

A pulse transformer is a transformer used to transmit pulses.

When a unipolar pulse voltage of Um (V) is applied to a pulse transformer winding with N turns, at the end of each pulse, the magnetic flux density increment ΔB (T) in the iron core is: ΔB = Um td / NSc × 10-2 where Sc is the effective cross-sectional area of ​​the iron core (cm2). That is, the magnetic flux density increment ΔB is proportional to the area (volt-second product) of the pulse voltage. 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 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 rise time tr of the pulse front is proportional to the leakage inductance Ls of the pulse transformer and the distributed capacitance Cs caused by the winding and structural parts. The pulse top drop λ is inversely proportional to the excitation inductance Lm. In addition, the eddy current loss factor will also affect the output pulse waveform.

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

Primary excitation inductance of pulse transformer Lm = 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 turns of the primary winding, l is the average magnetic path length of the iron core, Sc is the cross-sectional area of ​​the iron core, μp is the pulse permeability of the iron core, ρ is the resistivity of the iron core material, d is the thickness of the iron core material, and F is the pulse repetition frequency.

It can be seen from the above formula 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; when the pulse width is given, 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 winding turns of the pulse transformer should be as small as possible, which requires the use of materials with higher pulse permeability. In order 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. In order to reduce eddy current losses, soft magnetic tape materials 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 for core materials of pulse transformers are:

① High saturation magnetic induction intensity 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 induction intensity increment ΔB and use low residual magnetic induction materials; when additional DC bias is used, 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 high initial pulse permeability;

⑤ Small loss.

Ferrite cores have high resistivity, wide frequency range, and low cost, and are widely used in small power pulse transformers, but their ΔB

Both μp and μp are low, and the temperature stability is poor. It is generally used in situations where the requirements for top drop and trailing edge are not high.

(III) Inductor core

Iron core inductors are a basic component that acts as an impedance to current changes in a circuit and are widely used in electronic devices. 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 the given operating temperature range, the temperature coefficient of inductance should be kept within the allowable limit;

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

④ The nonlinear divergence 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 ZL: ZL = 2πfL, the higher the frequency f, the greater the inductive reactance ZL