Miniaturization of magnetic components: Application of new soft magnetic materials in electronic transformers

According to the above analysis, the soft magnetic materials in electronic transformers are mainly silicon steel in the power frequency and medium frequency ranges, and soft ferrite in the high frequency range. Now silicon steel is facing the challenge of amorphous nanocrystalline alloys, and soft ferrites are facing the challenge of amorphous nanocrystalline alloys and the competition of soft magnetic composite materials. In the challenges and competitions, not only new soft magnetic materials are developing rapidly, but also silicon steel and soft ferrites are developing. The application of newly developed soft magnetic materials in electronic transformers improves the performance of electronic transformers and reduces costs. It also gradually solves the difficulty encountered in the transformation of power supply technology to short, small, light and thin - the problem of miniaturization of magnetic components.

The following introduces some new developments in the application of silicon steel, soft ferrite, amorphous nanocrystalline alloy, and soft magnetic composite materials in electronic transformers . Thin film soft magnetic materials are not introduced here, which are a new generation of soft magnetic materials used for high-frequency small electronic transformers above 1MHz, and will be introduced in a special article later.

Silicon Steel

3% oriented silicon steel is widely used in industrial frequency electronic transformers in power supply technology . The thickness is now generally reduced from 0.35mm to 0.27mm or 0.23mm. The 0.23mm thick 23Q110 3% oriented silicon steel produced in China has a saturation flux density Bs of 1.8T and a P1.7/50 of 1.10W/kg; the 0.27mm thick 27QG095 3% Hi-B oriented silicon steel has a Bs of 1.89T and a P1.7/50 of 0.95W/kg. The 0.23mm thick 3% oriented silicon steel produced in Japan has a Bs of 1.85T and a P1.7/50 of 0.85W/kg. It is not much different from domestic products. However, the 0.23mm thick 3% oriented silicon steel has been specially treated, that is, the surface is polished to a mirror surface by electrolysis, then a tension coating is applied, and finally the magnetic domain is refined, which can reduce P1.7/50 to 0.45W/kg. At the same time, for electronic transformers that require low losses, Japan has further reduced the thickness to 0.15mm. After special treatment, P1.3/50 can be reduced to 0.082~0.11W/kg, which is basically the same as the level of iron-based amorphous alloys.

Japan also uses a new high-temperature annealing process in a temperature gradient furnace to make the Bs of 0.15mm thick, 3% oriented silicon steel reach 1.95~2.0T. After special treatment, P1.3/50 is 0.15W/kg and P1.7/50 is 0.35W/kg. The new three-time recrystallization process is used to make thinner silicon steel, with Bs of 2.03T, P1.3/50 of 0.19W/kg (0.075mm thick), 0.17W/kg (0.071mm thick) and 0.13W/kg (0.032mm thick).

In addition to using 0.20~0.08mm thick, 3% oriented silicon steel, Japan has adopted 6.5% non-oriented silicon steel for medium frequency (400Hz to 10kHz) electronic transformers in power supply devices. 6.5% silicon steel has approximately zero magnetostriction and can be made into low-noise electronic transformers with a magnetic permeability of 16000~25000. ρ is twice as high as 3% silicon steel, and the medium frequency loss is low. For example: 0.10mm thick 6.5% non-oriented silicon steel P1/50 is 0.6W/kg, P1/400 is 6.1W/kg, P0.5/1K is 5.2W/kg, P0.1/10k is 8.2W/kg, and Bs is 1.25T. 6.5% oriented silicon steel can be produced by warm rolling, and Bs is increased to 1.62~1.67T. 0.23mm thick 6.5% oriented silicon steel P1/50 is 0.25W/kg. Japan has used 6.5% silicon steel to make 1kHz audio transformers. At 1.0T, the noise is 21dB lower than 3% oriented silicon steel, and the iron loss is reduced by 40%. 6.5% silicon steel is also used to replace 3% oriented silicon steel in 8kHz welding machines, and the weight of the iron core is reduced from 7.5kg to 3kg. 6.5% silicon steel has been produced in small batches in China . While developing 6.5% silicon steel, Japan has also developed silicon steel with a gradient silicon content.

1) Medium-high frequency low-loss gradient silicon steel, the surface silicon content is 6.5%, the resistivity is high, the magnetic permeability is high, the magnetic flux is concentrated on the surface, the eddy current is also concentrated on the surface, and the loss is small. The internal silicon content is less than 6.5%. The total loss is lower than 6.5% silicon steel. For example: the P0.1/10k of 0.20mm thick 6.5% silicon steel is 16W/kg, and the gradient silicon steel is 13W/kg; P0.05/20k6.5% silicon steel is 14W/kg, and the gradient silicon steel is 9W/kg. Because the total average silicon content is less than 6.5%, Bs is higher than 6.5% silicon steel, which can reach 1.90T. The ductility and processability are also better than 6.5% silicon steel. This gradient silicon steel has been used to make inductors for home appliance inverters. Due to the high Bs and low loss, it is small in size and generates less heat.

2) Low remanent magnetism gradient silicon steel, the surface silicon content is high, the magnetostriction is small, the center silicon content is low, the magnetostriction is large. The difference in magnetostriction between the surface and the center layer causes stress. The elastic energy that appears leads to low remanence. Generally, the saturation flux density Bs is 1.96T, and the remanence Br is 0.34T. ΔB=Bm-Br exceeds 1.0T (Bm is the working flux density). The loss is also low, P1.2/50 is 1.27W/kg. It can be used for pulse transformers, unidirectional flux change power transformers, etc. When used as the core of the power transformer, it can also suppress the sudden current surge when closing the switch.

最近报导，日本开发出用于中高频电子变压器的硅钢新品种——添加铬(Cr)的硅钢。在4.5%硅钢中，添加4%铬，电阻率可达82μΩ·cm，而一般3%取向硅钢电阻率为44μΩ·cm，牌号为“HiFreqs”.0.1mm厚添加铬的硅钢损耗低，P0.2/5k为20.5W/kg，P0.1/10k为10W/kg，P0.05/20k为5W/kg;延伸性即加工性好，与3%硅钢一样，可以进行冲剪，铆固加工;耐腐蚀性好，在盐水和湿气中，不涂层也不腐蚀.已用这种添加铬的硅钢制成25kHz开关电源用滤波电感器，铁芯损耗为22W/kg，比6.5%硅钢(36W/kg)和铁基非晶合金(29W/kg)小.还用它制成70kHz感应加热装置的电子变压器，比0.1mm厚3%取向硅钢发热显著减少，寿命延长4倍以上.

Soft Ferrite

The characteristics of soft ferrite are: low saturation flux density, low magnetic permeability, low Curie temperature, low medium and high frequency loss, and low cost. The first three lows are its disadvantages, which limit its scope of use, and efforts are being made to improve them. The last two lows are its advantages, which are conducive to entering the high-frequency market, and efforts are being made to expand them.

Taking the loss at 100kHz, 0.2T and 100℃ as an example, TDK's PC40 is 410mW/cm3, PC44 is 300mW/cm3, and PC47 is 250mW/cm3. TOKIN's BH1 is 250mW/cm3, and the loss is constantly decreasing. JP4E produced by Jinning in China also reaches 300mW/cm3.

Continuously improving the operating frequency is another direction of effort. The operating frequency of TDK's PC50 is 500kHz to 1MHz. FDK's 7H20 and TOKIN's B40 can also work at 1MHz. Philips' 3F4, 3F45, and 3F5 all have operating frequencies exceeding 1MHz. Domestic Jinning's JP5 and Tiantong's TP5A all have operating frequencies of 500kHz to 1.5MHz. The operating frequency of DMR1.2K from Dongmei even exceeds 3MHz, reaching 5.64MHz.

Magnetic permeability is the weak point of soft ferrite. The current domestic products are generally around 10,000. The H5C5 of TDK Company and the 3E9 of Philips Company reach 30,000 and 20,000 respectively.

The research on the synthesis of MnZn ferrite materials by SHS method is worth noting. The experimental results of this method show that the energy consumption and cost of ferrite manufacturing can be greatly reduced. There are reports of successful experiments in China. Amorphous and nanocrystalline alloys

Iron-based amorphous alloys are competing with silicon steel in the industrial frequency and medium frequency fields. Compared with silicon steel, iron-based amorphous alloys have the following advantages and disadvantages.

1) The saturation magnetic flux density Bs of iron-based amorphous alloy is lower than that of silicon steel. However, under the same Bm, the loss of iron-based amorphous alloy is smaller than that of 3% silicon steel with a thickness of 0.23mm. It is generally believed that the reason for the low loss is that the iron-based amorphous alloy strip is thin and has a high resistivity. This is only one aspect. The more important reason is that the iron-based amorphous alloy is amorphous, the atomic arrangement is random, there is no magnetocrystalline anisotropy caused by the directional arrangement of atoms, and there is no grain boundary that produces local deformation and composition offset. Therefore, the energy barrier that hinders the movement of domain walls and the rotation of magnetic moments is very small, and it has unprecedented soft magnetism, so the magnetic permeability is high, the coercive force is small, and the loss is low.

2) The filling factor of the iron-based amorphous alloy core is 0.84~0.86, compared with the filling factor of silicon steel 0.90~0.95. The volume of the iron-based amorphous alloy core of the same weight is larger than that of the silicon steel core.

3) The working magnetic flux density of the iron-based amorphous alloy core is

1.35T~1.40T, silicon steel is 1.6T~1.7T. The weight of the iron-based amorphous alloy power frequency transformer is about 130% of the weight of the silicon steel power frequency transformer. However, even though it is heavy, for power frequency transformers of the same capacity, the loss of the core made of iron-based amorphous alloy is 70%~80% lower than that of silicon steel.

4) Assuming that the load loss (copper loss) of the power frequency transformer is the same and the load rate is 50%, then, to make the silicon steel power frequency transformer

If the iron loss of the transformer is the same as that of the iron-based amorphous alloy power-frequency transformer, the weight of the silicon-steel transformer is 1.8 times that of the iron-based amorphous alloy transformer. Therefore, the weight, cost and price of the iron-based amorphous alloy power-frequency transformer, which are generally accepted by the domestic public, are 130% to 150% of the silicon-steel power-frequency transformer, regardless of the loss level of the transformer. This does not meet the performance-price ratio principle required by the market. Two comparison methods have been proposed abroad. One is to find the weight and price of the copper and iron materials used in the two power-frequency transformers under the same loss conditions and compare them. The other method is to reduce the wattage of the loss of the iron-based amorphous alloy power-frequency transformer and convert it into currency for compensation. The no-load loss per watt is converted into US$5 to US$11, equivalent to RMB 42 to RMB 92. The load loss per watt is converted into 0 .7~1.0 USD, equivalent to RMB 6~8.3. For example, a 50Hz, 5kVA single-phase transformer uses silicon steel core, the price is 1700 yuan/unit; no-load loss is 28W, calculated at 60 yuan/W, it is 1680 yuan; load loss is 110W, calculated at 8 yuan/W, it is 880 yuan; then, the total evaluation price is 4260 yuan/unit. Using iron-based amorphous alloy core, the price is 2500 yuan/unit; no-load loss is 6W, equivalent to RMB 360; load loss is 110W, equivalent to RMB 880, the total evaluation price is 3740 yuan/unit. If the loss is not considered and the quotation is calculated alone, the 5kVA iron-based amorphous alloy power frequency transformer is 147% of the silicon steel power frequency transformer. If the loss is considered, the total evaluation price is 89%.

5) The core material loss of the industrial frequency power transformer is tested under a sine wave voltage with a distortion of less than 2%. The actual industrial frequency power grid distortion is 5%. In this case, the loss of the iron-based amorphous alloy increases to 106% and the loss of silicon steel increases to 123%. If the high-order harmonics are large and the distortion is 75% (such as an industrial frequency rectifier transformer), the loss of the iron-based amorphous alloy increases to 160% and the loss of silicon steel increases to more than 300%. This shows that the iron-based amorphous alloy has a stronger ability to resist power waveform distortion than silicon steel.

6) The magnetostriction coefficient of iron-based amorphous alloy is large, which is 3 to 5 times that of silicon steel. Therefore, the noise of iron-based amorphous alloy power frequency transformer is 120% of the noise of silicon steel power frequency transformer, which is 3 to 5 dB larger.

7) In the current market, the price of iron-based amorphous alloy strip is 150% of 0.23mm3% oriented silicon steel and about 40% of 0.15mm3% oriented silicon steel (after special treatment).

8) The annealing temperature of iron-based amorphous alloy is lower than that of silicon steel, and the energy consumption is small. In addition, the iron-based amorphous alloy core is generally manufactured by specialized manufacturers. Silicon steel core is generally manufactured by transformer manufacturers.

According to the above comparison, as long as a certain production scale is reached, iron-based amorphous alloys will replace part of the silicon steel market in electronic transformers within the industrial frequency range. In the medium frequency range of 400Hz to 10kHz, even if new silicon steel varieties appear, iron-based amorphous alloys will still replace most of the silicon steel market with a thickness of less than 0.15mm.

It is worth noting that Japan is vigorously developing FeMB series amorphous alloys and nanocrystalline alloys, whose Bs can reach 1.7~1.8T, and the loss is less than 50% of the existing FeSiB series amorphous alloys. If used in industrial frequency electronic transformers, the working magnetic flux density can reach more than 1.5T, and the loss is only 10%~15% of silicon steel industrial frequency transformers, which will be a more powerful competitor to silicon steel industrial frequency transformers. Japan expects to successfully trial-produce FeMB series amorphous alloy industrial frequency transformers and put them into production in 2005.

Amorphous nanocrystalline alloys are competing with soft ferrites in the medium and high frequency fields. In 10kHz to 50kHz electronic transformers, the working magnetic flux density of iron-based nanocrystalline alloys can reach 0.5T, and the loss P0.5/20k≤25W/kg, so they have obvious advantages in high-power electronic transformers. In 50kHz to 100kHz electronic transformers, the loss P0.2/100k of iron-based nanocrystalline alloys is 30~75W/kg, and the P0.2/100k of iron-based amorphous alloys is 30W/kg, which can replace part of the ferrite market. After more than 20 years of promotion and application, amorphous nanocrystalline alloys have been proven to have the following advantages :

1)不存在时效稳定性问题，纳米晶合金在200℃以下，钴基非晶合金在100℃以下，经过长期使用，性能无显著变化;

2) The temperature stability is better than that of soft ferrite. In the range of -55℃ to 150℃, the magnetic properties change by 5%~10%, and it is reversible;

3) Impact and vibration resistance: no performance deterioration occurred during the vibration test of the power supply unit under 30g;

4)铁基非晶合金脆性大大改善，带材平整度良好，可以剪切加工，也可以制成搭接式卷绕磁芯，经过5次弯折或拆卸，性能无显著变化.

Soft magnetic composite materials

After debate, a consensus has been reached on magnetic powder cores, which is that they belong to soft magnetic composite materials. Soft magnetic composite materials are formed by evenly dispersing magnetic particles in non-magnetic materials. Compared with traditional metal soft magnetic alloys and ferrite materials, it has many unique advantages: magnetic metal particles dispersed in non-conductor objects can reduce high-frequency eddy current losses and increase application frequency; it can be processed into powder cores by hot pressing, or it can be made into complex-shaped magnets by injection molding using current plastic engineering technology; it has the advantages of low density, light weight, high production efficiency, low cost, good product repeatability and consistency. The disadvantage is that since the magnetic particles are separated by non-magnetic bodies and the magnetic path is blocked, the magnetic permeability is generally less than 100. However, with the use of nanotechnology and other measures, there have been reports of magnetic permeabilities exceeding 1000 abroad, and the maximum can reach 6000.

The magnetic permeability of soft magnetic composite materials is affected by many factors, such as the composition of magnetic particles, particle shape, size, filling density, etc. Therefore, it can be adjusted according to the operating frequency.

Magnetic powder core is a typical example of soft magnetic composite materials. It has now replaced some soft ferrites in inductors with a frequency of 20kHz to 100kHz or even 1MHz. For example, the sendust magnetic powder core has a silicon content of 8.8%, an aluminum content of 5.76%, and the rest is iron. The particle size is 90~45μm, 45~32μm and 32~30μm. Silicone resin is used as an adhesive and about 1% stearic acid is used as a lubricant. Under a pressure of 2t/cm2, a ring-shaped magnetic core of 13×8×5 is made. It is annealed in hydrogen at 673°K, 773°K, and 873°K to make the magnetic permeability reach 100, 300, and 600. It has low loss at 100kHz and has replaced soft ferrite and MPP magnetic powder cores for inductors.

Some people have developed and researched the soft magnetic composite material - magnetic powder core for inductors of high-power power supplies. Below 20kHz, the magnetic permeability remains basically unchanged. At 1.0T, the magnetic permeability is about 100. The loss is small from 50Hz to 20kHz, and large magnetic cores weighing more than 100kg can be made. Moreover, in the audio range at 20kHz, the noise is 10dB lower than that of toroidal ferrite cores. It can replace silicon steel and soft ferrite in high-power power supplies.

Some people use cobalt/silicon dioxide (Co/SiO2) nanocomposite soft magnetic materials to make large-sized magnetic cores that are different from thin films. The average size of cobalt particles is 30μm, and the filling degree is 40% to 90%. After stirring, annealing forms Co/SiO2 nanocomposite powder, which is then pressed into a toroidal magnetic core. The magnetic permeability can reach 16 below 300MHz. The magnetic permeability of nickel-zinc ferrite is 12, and it drops rapidly after 100MHz. This proves that at high and ultra-high frequencies, soft magnetic composite materials can also replace part of the ferrite market.