The core structure of electromagnetic components in power supply

The core is an important part of the electromagnetic component in the power supply and plays an important role in its performance. The design of the core of the electromagnetic component includes the following main contents:

(1) According to the circuit and operating frequency of the power supply, convert it into the requirements of the core for soft magnetic materials and select suitable soft magnetic materials;

(2) Select the appropriate core structure according to the performance indicators required by the power supply;

(3) Calculate and select the core size based on the transmitted power and input impedance (input inductance);

(4) Calculate the core and coil parameters based on the electromagnetic field mathematical model of the electromagnetic component;

(5) Convert the core heat dissipation area and operating temperature according to usage requirements.

If the operating frequency is within the audio frequency range of 10Hz to 20kHz, the requirements of the surrounding environment for audible noise must also be considered. In today's world where environmental protection is emphasized, audible noise pollution can cause considerable harm to people's physical and mental health. Therefore, it is very important to reduce audible noise and limit it to a certain range. If the indicators are not met, measures to reduce noise should be taken when designing the core structure.

For power transformers with iron cores, items (1) and (2) are decisive and have been introduced in Reference 1 and will not be repeated here. Regarding item (3), the more common method is to design and select the core size from the following three formulas.

ACAO=P2(1＋η)/KUJfΔBm(1)

ACLC=P2(1+η)/KUμOμfΔBm(2)

L1=μOμACN12/lC(3)

In the formula: AC is the actual cross-sectional area of ​​the core (including the core space factor KC); AO is the actual area of ​​the core window (including the window space factor KO), ACAO is the characteristic parameter of the core; P2 is the output power of the power transformer; η is the efficiency of the power transformer, P2(1+η) is equivalent to the apparent power of the power transformer; KU is the waveform factor, which is 2 for rectangular waves and 2.22 for sine waves; J is the current density; f is the operating frequency; ΔBm is the operating magnetic flux density variation range, which is 2Bm for power transformers with bidirectional changes in magnetic flux density and Bm-Br for power transformers with unidirectional changes in magnetic flux density; lC is the average magnetic path length of the core, ACLC is the effective volume of the core; L1 is the input inductance; μO is the vacuum permeability; μ is the effective permeability of the core at the operating frequency; N1 is the number of turns of the input (primary) coil.

Regarding item (4), under high-frequency conditions exceeding a certain operating frequency, the design of the power transformer should consider the one-dimensional, two-dimensional or three-dimensional mathematical model of the electromagnetic field, otherwise it will cause considerable errors. The guiding technical document SJ/Z2921-88 "Design Method of Switching Power Transformer" of the former Ministry of Electronics Industry is no longer applicable. A new guiding technical document should be formulated based on the now popular computer-aided design.

Regarding item (5), designers generally do not pay much attention to it, but make modifications based on the results of the temperature rise test of the trial samples. However, for high power (for example, above 100W) and high operating frequency (for example, above 100kHz), it is still necessary to calculate the core operating temperature first, so that measures can be taken in the design to prevent the core temperature from exceeding the specified value.

The above is a brief introduction to the design process of the power transformer core, not only to emphasize the importance of soft magnetic materials and core structure to the power transformer, but also to clarify some misleading practices in current design for reference by colleagues. Due to the incomplete collection of information, only three-dimensional core structures with a height of more than centimeters are introduced here, including composite core structures and multifunctional (magnetic integration) core structures. As for the planar core structure with a height of 1mm to 10mm and the thin film core structure with a height of less than 1mm, they will be introduced later.

2Silicon steel core

Most of the 50Hz~60Hz power frequency electromagnetic components and 400Hz~1000Hz medium frequency electromagnetic components use silicon steel core. The silicon steel core structure is divided into two types: laminated core and wound core.

The laminated core is a core made by cutting or punching silicon steel strips into core sheets, and then stacking them into a certain structural form. From the perspective of the development of the core sheet shape (Figure 1), the earliest was a single I shape, and later it was CI shape, EI shape and EE shape, the purpose of which is to facilitate stacking and reduce working hours. If the material is oriented silicon steel strip, it is necessary to pay attention to the direction of the magnetic lines of force in the magnetic circuit to be consistent with the orientation of the silicon steel, and not to be perpendicular to the orientation of the silicon steel, otherwise it will increase the core excitation energy and core loss. In order to solve the problem that the direction of the magnetic lines of force at the corner is not perpendicular to the orientation of the silicon steel, it later developed into a 45° beveled angle sheet. From the perspective of the development of the cross-section of the stacked core (Figure 2), it was originally square, and later developed into a three-step shape and a multi-step shape, so that the cross-section of the core gradually tends to be circular. This is on the one hand to reduce the average turn length of the coil, reduce impedance and copper loss; on the other hand, it is also to facilitate coil winding. From the development of the number of stacked core columns (Figure 3), the earliest was the two-column type used for single-phase transformers and reactors, and later developed into the three-column type used for three-phase transformers and the five-column type used for rectifier transformers with balancing reactors.

Figure 1. Silicon steel core sheet shape

(a) I shape (b) CI shape (c) EI shape (d) EE shape

Figure 2 Cross-section of silicon steel laminated core structure

(a) Square (b) Three-step shape (c) Multi-step shape

Figure 3 Number of columns in the silicon steel laminated core structure

(a) Two-column style (b) Three-column style (c) Five-column style

The wound core is a core made by cutting silicon steel strip into the required width and then winding it into a certain structural form. From the development of the shape of the wound core (Figure 4), it was originally a ring, and later developed into a square frame shape for the convenience of insulation structure design and coil winding. The square frame shape includes: single frame and double frame for single-phase power transformers, and three frame and four frame for three-phase power transformers. The three frame type is divided into two types: one is a composite, consisting of two small frames and a large frame; the other is independent, consisting of three square frames arranged at an angle of 120° to each other. In order to make the cross section of the core gradually tend to be circular, like the laminated core, the cross section of the wound core also develops from a rectangle, through three steps and multiple steps, to an R-shaped core with a basically circular cross section. The wound ring core with an R-shaped cross section is called an O-shaped core. It can make full use of the core material and reduce the average turn length of the coil, which is a relatively ideal wound core structure.

Figure 4 Silicon steel wound core structure

(a) Ring-shaped (b) Single frame (c) Double frame (d) Combined three-frame (e) 120° three-frame

Compared with the laminated core, the wound core can make the magnetic lines of force in the magnetic circuit completely consistent with the orientation of silicon steel, and there is no air gap, so the excitation energy and core loss will be reduced by 10% to 25%, and the noise is also lower. Its core processing technology is relatively simple, and it is easy to use mechanical processing instead of manual stacking. However, coil winding is more difficult than laminated core, and special winding equipment must be used. If the coil is damaged, the whole will be scrapped and cannot be repaired. In order to compensate for these shortcomings, the wound core is cut into two halves to become CD-shaped and XD-shaped cores (Figure 5). Although this structure has two or three air gaps, it still maintains the advantages of the wound core, and the excitation energy and core loss are not increased much, and the noise is not increased much. In addition to adding core cutting and air gap polishing processes, the core processing technology is not complicated and can still be processed mechanically. At the same time, it is easier to wind the coil like a laminated core. The coil damage is also easy to disassemble and replace. In addition, CD and XD cores are an ideal core structure for reactors that must have air gaps.

Figure 5 Silicon steel CD-shaped and XD-shaped core structures

(a)CD shape (b)XD shape

3Amorphous and microcrystalline alloy core

Iron-based amorphous alloys can be used as core materials for electromagnetic components in 50Hz-60Hz power frequency and 400Hz-20kHz medium frequency power supplies. In the late 1980s, the Osaka Transformer Factory in Japan concluded that the comprehensive performance of iron-based amorphous alloy cores above 150Hz is better than that of silicon steel cores. After more than ten years of research, iron-based amorphous alloy cores are expanding into the 50Hz-60Hz power frequency field and competing with silicon steel cores.

The structure of iron-based amorphous alloy core is also divided into two types: laminated and wound. The laminated core is a relatively early structure, which is a core with a certain structural shape after the iron-based amorphous alloy strip is cut into a certain core sheet. The thickness of the iron-based amorphous alloy strip is generally 20μm~40μm, and stacking is time-consuming and difficult to stack. In order to shorten the working hours and increase the strength of the core, several and dozens of thin core sheets are bonded together to form a core sheet with a thickness of 0.1mm~0.25mm, but the loss is also increased. There is no orientation problem in the magnetic properties of iron-based amorphous alloys, but the shearing process is difficult. Generally, the shape of the core sheet is a single I shape (Figure 6), and the cross-section of the core after stacking is a rectangle. The number of core columns is also divided into two types: two-column type for single-phase electromagnetic components and three-column type for three-phase electromagnetic components. Due to the large amount of working hours required, the stacked iron-based amorphous alloy core structure is rarely used now. However, after the 150μm iron-based amorphous alloy strip process matures, it is still possible to adopt a stacked core structure.

Figure 6 Iron-based amorphous alloy single I-shaped laminated core structure

Figure 7 Iron-based amorphous alloy lap core structure

The wound core is a core made by cutting or spraying the iron-based amorphous strip into a certain width and then winding it into a certain structural form. It was originally a ring-shaped core, and later developed into a square frame shape for the convenience of winding, including single frame, double frame, three frame and four frame shapes. Later, in order to simplify the winding and loading and unloading process and facilitate the replacement of coils, it developed into CD and XD shapes. Different from the silicon steel wound core structure, a new type of overlapping square frame core structure appeared in the iron-based amorphous alloy core in the early 1990s (Figure 7). In the joint part of the core, the core strips overlap each other, and the joint part is not in a straight line, so the air gap is smaller than that of the CD core. The excitation energy and core loss are basically the same as those of the wound square frame core. However, it can be opened layer by layer, and then closed layer by layer after the coil is loaded. Coil winding, loading and unloading and replacement are relatively easy. It is now generally believed that this overlapped square frame core structure is a better core structure that combines the advantages of the wound square frame and CD core structures. It can be used not only for low frequency, but also for medium and high frequency electromagnetic components. It has been widely used in distribution transformers, which not only shortens the core processing and assembly time, but also can give full play to the excellent performance of amorphous alloy materials.

Cobalt-based amorphous alloys and iron-based microcrystalline alloys are used for electromagnetic components in medium and high frequency power supplies of 20kHz to 500kHz, mainly in the form of a wound toroidal core structure, with some using a CD-shaped core structure. The CD-shaped core structure is mainly used for electromagnetic components of 20kHz to 50kHz. When the frequency exceeds 100kHz, due to the small number of coil turns, the toroidal core structure is mainly used. In large-capacity power supplies, electromagnetic components of 20kHz to 50kHz may use a lapped core structure in the future.

4. High magnetic permeability alloy (Permalloy) core

In order to give full play to the high magnetic conductivity of high magnetic alloy, a wound toroidal core structure is generally used. Since high magnetic alloy is sensitive to stress, the toroidal core should be placed in a protective box after heat treatment, and it must be handled with care during the winding and insulation treatment process to avoid the impact and stress on the performance of high magnetic alloy. In the past, below 1kHz, there were also individual cases where a laminated core structure was used, and the core punching was EE or EI shaped. It is relatively rare now. Permalloy has strong environmental adaptability and has expanded its frequency range, and its use in power supplies has increased. However, the core structure is still a wound toroidal core structure.

5Soft ferrite core

There are many core structures of soft ferrites. This is because the hot pressing process is used, which makes it easier to process into various shapes. There are EI, EE, EER, EP, UF, UYF, RM, PM, PQ, Q (can), T (ring) and LP shapes, etc. (Figure 8). The EI shape is marked by size A (E-shaped core width), and the EE shape is marked by size A (E-shaped core width) and 2×B (E-shaped core length). It has formed EI10~50, EE8.3/8.0~110/80 series, with many varieties, simple and mature manufacturing process, good heat dissipation, easy to lead out wiring, and low cost. The disadvantage is that the cross-section of the middle column of the core is square, which brings trouble to the coil winding. At the same time, there is no shielding, which is easy to generate stray magnetic field interference. The EER type is also marked by the dimensions A (E-shaped core width) and 2×B (E-shaped core length), and has formed the EER25/33~54/50 series. The cross-section of the middle column of the core is circular, which is more convenient for winding. At the same time, the winding length is shortened by 11% compared with the square cross-section, thereby reducing copper loss. However, there is still no shielding. The EP type core is marked by the core height dimension E, and has formed the 7 to 30 series. The middle core column is circular, and there is a shield on one side and a notch on the other side, which is convenient for lead-out wiring. The UF type core is marked by the width A of the U-shaped core, forming the 9.8~25 series. It can be wound with two columns, has good heat dissipation, and is convenient for lead-out wiring, but the core cross-section is rectangular and there is no shielding. The UYF type core is marked by the thickness C of the U-shaped core (sometimes including the height 2×B of the U-shaped core), and has formed the 10~18 series. Some of the two core columns are circular, and some of the core columns are circular (single-sided winding), and the cross-section of one core column is square. The RM core is marked by the diameter C of the middle core column, and has formed 4 to 14 series. There are two types: center hole and no center hole (marked by G). The middle core column is round, with shielding on both sides and notches on both sides for easy lead-out wiring. The PM core is marked by the maximum outer diameter A, and has formed PM50 to 114 series. Its shielding on both sides is wider than that of the RM core, and the effect is better. The Q-shaped (can-shaped) core is marked by the maximum outer diameter A and height C, and has formed Q7/4 to Q40/29 series. It has the best shielding effect and high inductance per unit space, but the notch is small, which makes it inconvenient to lead out wiring. It has a center hole for easy installation. The PQ core is marked by the maximum outer diameter A and height F, and has formed PQ20/16 to 50/50 series. It has a larger notch than the Q core, which is convenient for lead-out wiring, and has a large back heat dissipation area. It is the core structure with the best comprehensive performance for high-frequency power transformers. LP cores are characterized by the core height 2D and the smaller notch length E on one side, and have formed the LP23/8~LP32/13 series, which are also suitable for high-frequency power transformers. T-shaped (ring) cores are characterized by the outer diameter × inner diameter × height dimensions, and have formed the 6×3×2~124×62×40 series. The cross section is rectangular, the magnetic circuit has no air gap, the inductance value is large, and the leakage flux is small. If the cross section is changed to a circular structure, it becomes a structure similar to the silicon steel O-shaped core, and its winding process and performance are improved.

Figure 8 Soft ferrite core structure

(a) EI shape (b) EE shape (c) EER shape (d) EP shape (e) UF shape (f) UYF shape (g) RM shape

(h) PM shape (i) PQ shape (j) Q shape (can shape) (k) T shape (ring shape) (l) LP shape

6Composite core and magnetic powder core

Composite core refers to a core structure composed of two or more soft magnetic materials. If a core is divided into two (or three) cores to match the performance of the core in order to achieve good consistency, this cannot be considered a composite core structure because they use the same soft magnetic material, not two or more soft magnetic materials. A typical example of a composite core structure is the core for a pulse transformer. In order to ensure a short rise time and a small top drop, a core composed of two soft magnetic materials, Permalloy and soft ferrite, is used. There are ring-shaped and laminated cores.

However, the most commonly used composite core structure is the magnetic powder core, which is a composite of soft magnetic materials and non-magnetic materials. Although magnetic powder cores are not used in power transformers but in inductors, in order to make the core structure more comprehensive, it is also introduced to readers in this article.

Magnetic powder cores are made by mixing metal soft magnetic material powder and insulating material and then pressing them. They are generally toroidal cores. In order to indicate the type and performance of soft magnetic materials, they are also painted with colors (red, yellow, green, blue) as marks. Since the metal soft magnetic material powder is surrounded by soft magnetic materials, a dispersed air gap is formed, which greatly reduces high-frequency eddy current losses and has anti-saturation performance.

The national standard GBn251-85 "Nickel-iron magnetic powder core" only targets magnetic powder cores made of high-permeability nickel alloy materials. Now ferromagnetic powder cores, sendustine-aluminum magnetic powder cores, amorphous and microcrystalline magnetic powder cores have been developed, so the national standard needs to be expanded and revised.

The indicators proposed for magnetic powder cores include saturation flux density Bs, effective magnetic permeability μe, effective quality factor Qe, effective magnetic permeability temperature coefficient αμe, Curie point, specific gravity, etc. The performance parameters of the main magnetic powder cores are shown in Table 1. The national standard model of iron-nickel-molybdenum magnetic powder core is FN81, the model of Shanghai Iron and Steel Research Institute is SN, and the model of Arnold Company of the United States is MPP. It has high magnetic permeability, a wide range of working environment adaptability, low loss, but expensive. The national standard model of iron-nickel high magnetic flux magnetic powder core is FN50, and the model of Arnold Company of the United States is HF. It has high saturation flux density, medium magnetic permeability, low loss, and a relatively cheap price. Recently, domestic and foreign countries have paid more attention to research and use. The model of Shanghai Iron and Steel Research Institute for iron-silicon-aluminum magnetic powder core is SA, and the model of Arnold Company of the United States is MS. It is a magnetic powder core with a lower price and better comprehensive performance indicators. Ferromagnetic powder core Shanghai Iron and Steel Research Institute model SF, foreign general model IP, high magnetic permeability and saturation flux density, but large loss at high frequency, only suitable for low frequency and medium frequency below 20kHz. Due to its low price, it can be processed into large magnetic powder cores to replace silicon steel as filter reactors for large-capacity DC power supplies.

Table 1 Main properties of magnetic powder core

In addition to the properties listed in Table 1, the relationship curve between the effective magnetic permeability μe and the frequency f is generally given. Similar to soft ferrites, the effective magnetic permeability will drop rapidly after exceeding a certain limit operating frequency. At the same time, the external magnetic field strength at which the effective magnetic permeability μe drops by 50% is also given, that is, the constant magnetic range. Table 2 is the constant magnetic range of iron-nickel-molybdenum magnetic powder cores and high-flux iron-nickel magnetic powder cores. In addition, for magnetic powder cores used in output filters, AC and DC simultaneous magnetization curves must be given to indicate the changes in the effective magnetic permeability of the magnetic powder core under large DC magnetization conditions.

Table 2 Constant magnetic range of iron-nickel-molybdenum magnetic powder core and high flux iron-nickel magnetic powder core

7Multifunctional core (integrated magnetic core)

The multifunctional core structure refers to a core that simultaneously functions as a transformer and a reactor, a transformer and a magnetic switch. When Professor Cook proposed the Cook circuit, he called this multifunctional core structure an "integrated magnetic core", which means that it integrates several functions together like a semiconductor integrated circuit. The term "integrated magnetic core" has been used by some people until now. However, now there are technologies similar to semiconductor integrated circuit processes to manufacture thin-film cores and electromagnetic components for their applications, which some people call "micromagnetic devices" and others "integrated magnetic devices". Therefore, I think that using the integrated magnetic core again will easily confuse people, and they will think that two completely different core structures are the same thing, so the original integrated magnetic core is renamed "multifunctional core".

In fact, the earliest multifunctional core appeared in the ferromagnetic resonance voltage stabilizer, which included a transformer and a reactor composed of a magnetic shunt. The constant voltage transformer was developed from the ferromagnetic resonance voltage stabilizer, and the core also belongs to the multifunctional core structure.

Common multifunctional core structures are mainly in the form of planar layout (Figure 9). Figure 9 (a) is a core structure of a transformer and a reactor. Figure 9 (b) is a core structure of a transformer and two reactors. Figure 10 cleverly uses two soft magnetic materials to form a multifunctional core structure. The outer frame core of Figure 10 (a) uses high magnetic permeability alloy material to form a transformer, and the middle uses low magnetic permeability material to form a reactor. In addition to this frame plus middle column structure, there is also a bridge-type multifunctional core structure in Figure 10 (b), where the four bridge arms are made of high magnetic permeability material, and the bridge diagonal is made of low magnetic permeability material or an air gap.

Figure 9 Multifunctional core structure in planar layout

(a) One transformer and one reactor (b) One transformer and two reactors

Figure 10 Multifunctional core structure composed of two soft magnetic materials

(a) Cabinet core (b) Bridge core

Since the late 1980s, more and more people have been studying the three-dimensional multifunctional core structure composed of vertical cores. The earliest one was formed by rotating half of the C-shaped core by 90°. It was found that the windings on the C-shaped core that did not rotate 90° still had the transformer function. A winding was wound on the C-shaped core that rotated 90°, and the inductance of the winding on the C-shaped core that did not rotate 90° could be changed after current excitation (see Figure 11 (a)). Later, the core was removed to become Figure 11 (b), and it still had this function. Figure 11 (c) is a multifunctional core structure that combines two transformers and two controllable inductors.

Figure 11 Multifunctional core structure composed of vertical cores

(a) A C-shaped iron core rotated 90° with a controllable inductor (b) Evolution of (a), controllable inductor

(c) Combination of two transformers and two controllable inductors

Using this vertical multifunctional iron core structure, AC voltage-stabilized power supplies, inverter power supplies, voltage resonant and current resonant switching power supplies have been developed. They have the advantages of high reliability (mainly copper and iron materials), automatic stepless adjustment, good harmonic elimination, and high efficiency. It is a special iron core structure that is worth further development.

8 Conclusion

(1) Various soft magnetic materials can form different core structures due to their different application areas. When selecting the core structure, we should first pay attention to fully utilizing the advantages of soft magnetic materials, and then pay attention to the complexity of the process and the utilization rate of the materials, that is, the cost. Comprehensive consideration should be made based on the market and user requirements. We should not only pay attention to performance but also ignore the other side of cost and price.

(2) Each core structure has its own advantages and disadvantages. We cannot elevate one structure and deny the others because of subjective factors such as preference. We cannot follow the crowd and take an absolutely negative attitude towards a core structure because most people oppose it. For example, some people overly appreciate the R-shaped winding core structure and are unwilling to pay attention to some problems that exist in this core structure. Another example: Most people think that the 120° three-frame core structure cannot be used in power transformers, but some people have used it in power transformers that pursue small size and achieved good results.

(3) The current core structure is relatively mature in low-frequency and medium-frequency power transformers. There is still a lot of work to be done in high-frequency power transformers, which no longer belong to the scope of three-dimensional core structure, but to the scope of planar core structure and thin-film core structure. These two structures are becoming the research hotspots of high-frequency power transformers. According to reports, the height of high-frequency switching power supplies using planar core structure transformers was greater than 5mm in 1995. By 1998, the height of high-frequency switching power supplies using low-height planar core structure transformers was between 5mm and 3mm. It is predicted that by 2002, transformers using thin-film core structure and integrated process technology will integrate magnetic components, semiconductor devices and capacitors on a single chip to form a new type of monolithic high-frequency switching power supply, the height of which can be less than 3mm, or even reach 1mm. In just ten years, the three-dimensional core structure has leapt from the planar core structure to the thin-film core structure. The rapid development is amazing!

(4) In terms of using multifunctional core structures to develop DC and AC power supplies with unique performance, some personnel from Sony Corporation of Japan did not follow the crowd and insisted on researching vertical multifunctional core structures, and achieved results with their own intellectual property rights. This innovative spirit and unyielding efforts have given us a good inspiration. I hope that my country's power supply technology workers can develop products with their own characteristics and occupy a place in the new field of power supply technology.