1 Introduction
The power supply generally contains an electromagnetic device composed of a soft magnetic core. In a broad sense, electromagnetic devices in electronic devices and electronic circuits are called electronic transformers. Most of the transformers or electromagnetic devices in the power supply belong to the Electronic Transformer. However, in some power supplies, the transformer also has high voltage insulation requirements. For example: large-capacity DC power supply and large-capacity uninterruptible power supply, the rectifier transformer does not input from the general 380V or 220V, but from the 10KV or 6.3KV input, it is very different from the general electronic transformer, and is more similar to the power transformer. Therefore, the transformers in the power supply discussed in this paper include both electronic transformers and Power Transformers.
This article discusses the requirements and technical parameters of transformers in power supplies, and their relationship to core materials and conductive materials, in order to gain a deeper understanding of the other two articles "Recent Trends in Transformer Core Materials" and "Recent Trends in Transformer Conductive Materials" The content introduced in the article, so that the three articles form an organic whole. The purpose of writing these three articles is to better understand the development trend of transformers in power supply by understanding the recent trends of core materials and conductive materials, and to provide reference for friends in the power industry, electronic transformer industry and power transformer industry. If there is something wrong, please correct me.
2 General requirements
The transformer in the power supply, as a commodity product, the general requirement is to complete the specific function under the specific use conditions, and pursue the highest performance-price ratio. Starting from the general requirements, four general requirements are proposed: use conditions, complete functions, improve efficiency, and reduce costs. It includes both technical and economic indicators.
2.1 Conditions of use
Conditions for use of the transformer in the power supply, including reliability of use and use of electromagnetic compatibility.
The reliability of use means that the transformer can work normally until the service life under the specific conditions of use. The most important factor affecting the transformer in the conditions of use is the ambient temperature. It is the Curie point that determines the strength of the core material to be affected by temperature. The core material has a high Curie point and is less affected by temperature. The core material has a low Curie point and is greatly affected by temperature. The Curie point of the MnZn soft ferrite is generally only 215 ° C, which is relatively low, and the magnetic flux density, magnetic permeability and loss all change with temperature. In addition to the normal temperature of 25 ° C, we also give various parameter data at 60 ° C, 80 ° C, 100 ° C, iron core made of MnZn ferrite, the general working temperature is limited to below 100 ° C, that is, at ambient temperature 40 At °C, the temperature rise is only allowed to be below 60 °C. The Curie-based amorphous alloy has a Curie point of 205 ° C and is also low, and the use temperature is also limited to 100 ° C or less. The iron-based amorphous alloy has a Curie point of 370 ° C and can be used at 150 ° C - 180 ° C or lower. The Curie point of the iron-based nanocrystalline alloy is 600 ° C, and the Curie point of the silicon steel is 730 ° C, which can be used below 300 ° C.
It is not the copper wire that determines the working temperature of the conductive material, but the heat resistance level of the outer insulating material. For example, QZ polyester enameled wire has a heat resistance rating of B and a maximum temperature of 130 °C. QY polyimide enameled wire, heat resistance grade C, the maximum working temperature is 220 °C.
The use of electromagnetic compatibility means that the transformer does not generate electromagnetic* to the outside world and can withstand external electromagnetic*. Electromagnetic* includes audible audio beeps and inaudible high frequency noise. The main reason for the electromagnetic generation of the transformer is the magnetostriction of the core, and the core material with a large magnetostriction coefficient, which generates a large electromagnetic *. The magnetostrictive coefficient of iron-based amorphous alloy is (27-30) & TImes; 10-6, the largest, the measures to reduce noise suppression* must be taken when making iron core, and the magnetostriction coefficient of MnZn soft ferrite is 21 & TImes ; 10-6 or so, it is also easy to produce electromagnetic *. The magnetostriction coefficient of 3% oriented cold rolled silicon steel is (1-3) & TImes; 10-6. The magnetostriction coefficient of iron-based nanocrystalline alloy is (0.5-2) & TImes; 10-6, which is relatively easy to produce electromagnetic*. The magnetostriction coefficient of 6.5% non-oriented silicon steel and cobalt-based amorphous alloy is about 0.1×10-6, and it is not easy to generate electromagnetic*. The frequency of the electromagnetic* generated by the core material is generally the same as the operating frequency of the transformer. If there is an electromagnetic* below or above the operating frequency, it is caused by other reasons. Conductive materials do not produce electromagnetic*. A coil wound from a conductive material may generate electromagnetic*, not caused by a conductive material, but by an action between the conductive materials and a coil structure.
2.2 Completion function
The electromagnetic devices in the power supply are functionally distinguished mainly by transformers and inductors. The transformer performs three functions: power transfer, voltage conversion, and isolation. There are two inductor completion functions: power transfer and ripple rejection. The completion function of the transformer in the power supply is discussed here, as well as the completion function of the inductor in the power supply.
The power transfer of the transformer is accomplished by applying an alternating voltage across the primary winding of the transformer to create a flux change in the core that causes the secondary winding to induce a voltage that is output to the load, thereby transferring electrical power from the primary to the secondary of the transformer. The magnitude of the transmission power is determined by the induced voltage, that is, the amount of change in the magnetic flux density per unit time ΔB. ΔB is independent of magnetic permeability and is related to saturation magnetic flux density Bs and residual magnetic flux density Br. The saturation magnetic flux density of silicon steel is 1.5-2.03T, the saturation magnetic flux density of iron-based amorphous alloy is about 1.58T, the saturation magnetic flux density of iron-based nanocrystalline alloy is 1.2-1.45T, and the saturation magnetic flux density of cobalt-based amorphous alloy is 0.5-0.8T. The MnZn soft ferrite has a saturation magnetic flux density of 0.3 to 0.5T. As a core material for transformers, silicon steel is dominant, followed by iron-based amorphous alloys, and MnZn soft ferrites are at a disadvantage.
The power transfer of the inductor is accomplished by energizing the inductor windings, causing the core to be excited, magnetically stored, and then demagnetized into electrical energy for release to the load. The amount of transmission power is determined by the energy storage of the core, which is determined by the inductance of the inductor. The amount of inductance is not directly related to the saturation flux density, but to the permeability. The magnetic permeability is high, the inductance is large, the transmission energy is large, and the transmission power is large. The magnetic permeability of the cobalt-based amorphous alloy is (1 - 1.5) × 106, the magnetic permeability of the iron-based nanocrystalline alloy is (5 - 8) × 105, and the magnetic permeability of the iron-based amorphous alloy is (2 - 4) × 105. The magnetic permeability of silicon steel (2-9)×104, and the permeability of MnZn soft ferrite is (1-3)×104. As a core material for inductors, cobalt-based amorphous alloys and iron-based nanocrystalline alloys predominate, and silicon steel and MnZn soft ferrites are at a disadvantage.
The amount of transmit power is also related to the number of transfers per unit time, which is related to the operating frequency of the transformer and inductor. The higher the operating frequency, the greater the transmission power under the same size of core and the same number of coils.
The voltage conversion is done by the turns ratio of the primary and secondary line sets of the transformer. Regardless of the transformer power transfer size, the turns ratio of the primary and secondary windings is equal to the voltage conversion ratio of the input and output.
Insulation isolation is accomplished by the insulation structure of the primary and secondary windings of the transformer. The higher the applied voltage and the converted voltage, the more complicated the insulating structure. Generally, the applied voltage of the electronic transformer is less than 1kV, and the insulation structure is relatively simple. The applied voltage of the power transformer exceeds 6kV, and the insulation structure is relatively complicated. In addition to the power frequency test voltage, it is also required to withstand the short-term impact test voltage.
The ripple rejection of the inductor is achieved by the self-inductance potential. As long as the current flowing through the inductor changes, the magnetic flux generated by the coil in the core also changes, causing a self-inductance potential at both ends of the inductor coil, the direction of which is opposite to the applied voltage, thereby preventing the current from changing. The ripple frequency is higher than the operating frequency (basic frequency) and is therefore more suppressed by the self-inductance potential generated by the inductor. The ripple rejection is determined by the magnitude of the self-induced potential, which is determined by the amount of inductance. The inductance is related to the magnetic permeability of the core material. From the viewpoint of the inductor's ability to suppress the ripple, the cobalt-based amorphous alloy and the iron-based nanocrystalline alloy with high magnetic permeability are better as the core material, and the silicon steel with low magnetic permeability is The MnZn soft ferrite is relatively poor as a core material.
2.3 Improve efficiency
Increasing efficiency is an important requirement for transformers in power supplies. One reason is that energy conservation has become an important task in the contemporary era due to rising energy prices such as oil and coal. Many electronic devices, including power supplies, not only require energy consumption when assessing loads, but also require energy consumption when assessing standby (near no-load). The loss of the transformer in the power supply is a major part of the power consumption of the power supply. Another reason is that the number of transformers in the power supply is huge, although from the perspective of a transformer in a single power supply, the loss is only a few watts, not much. But in the hundreds of thousands, millions of power transformers, the total loss can reach several tens of thousands of watts, a few million watts, considerable. Also, in many power supplies, the transformer has been running for a long time, and the total annual loss is by no means a small amount. Therefore, transformers in power supplies must increase efficiency and reduce losses becomes an important requirement.
Transformer losses in the power supply include core losses and coil losses. As long as the transformer in the power supply is put into operation, the core loss is always the main part of the transformer no-load loss. When designing and manufacturing a transformer core, it is necessary to select a core material with a relatively low loss. The loss of core material is related to the working flux density and operating frequency of the transformer core. Therefore, the loss of the core material must be noted. For example: P1.4/50 is the loss at a working magnetic flux density of 1.4T and an operating frequency of 50HZ. P1.0/400 is the loss at a working magnetic flux density of 1.0T and an operating frequency of 400HZ. P0.25/100K is the loss at a working magnetic flux density of 0.25T (250mT) and an operating frequency of 100kHZ.
The core material loss includes hysteresis loss, eddy current loss and residual loss, and the eddy current loss is related to the core material resistivity. The greater the resistivity, the smaller the eddy current loss. The MnZn soft ferrite has a resistivity of 108-109μΩcm, and has low eddy current loss at high frequencies. It is dominant in high-frequency transformers in power supplies. The resistivity of iron-based amorphous alloys is 130-150μΩcm, and the resistivity of silicon steel is 20. — 40μΩcm, 106-107 times smaller than MnZn soft ferrite, and eddy current is high at high frequencies. If it is to be applied in a high-frequency transformer in a power supply, measures must be taken, such as reducing the thickness of the metal core material. The thickness of the metal core material used in the transformers of various working frequencies is generally: 0.50 for the power frequency 50HZ-60HZ. -0.23mm (500-230μm), medium frequency 400HZ to 1kHZ with 0.20-0.08mm (200-80μm), 1kHZ to 20kHZ with 0.10-0.025mm (100-25μm), medium-high frequency 20kHZ to 100kHZ with 0.05-0.015mm (50) -15 μm), 0.02-0.005 mm (20-5 μm) for high frequency 100 kHZ to 1 MHz, and less than 5 μm for 1 MHz or more. Iron-based amorphous alloys are generally 40-25 μm thick due to the spray belt equipment, and can be used at a power frequency of 50 Hz to an intermediate frequency of 400 Hz to 20 kHz. Used in medium-high frequency and high-frequency iron-based nanocrystalline alloys, the strip thickness is generally less than 18μm. Previously, people thought that the filling factor of the core is related to the strip thickness of the metal core material, and proposed a calculated empirical formula, taking the thickness of the core material as the only factor determining the core filling factor. It now appears that this empirical formula for calculating the core fill factor is not entirely true. Because the core filling factor is not only determined by a factor of the thickness of the core material, but also by other factors such as coating thickness, strip flatness and strip uniformity. According to the empirical formula, when the thickness of the iron-based amorphous alloy is 25μm, the filling factor is less than 0.80. However, the transformer core processed with 25μm thick iron-based amorphous alloy strip is generally more than 0.86, even reaching 0.90.
Transformer coil losses in the power supply are a major part of the load loss. The coil loss is determined by the resistivity of the conductive material. Most of the conductive materials of the transformer in the power supply now use copper. Instead of aluminum, the reason is that the resistivity of copper is small, resulting in low coil loss. In some small-sized high-frequency planar transformers and thin film transformers, the conductive material also uses gold and silver with lower resistivity. This is because the transformer has a small volume, a small heat dissipation area, and requires less coil loss to ensure that the coil temperature rise of the Planar Transformer and the thin film transformer does not exceed the specified allowable value.
2.4 Reduce costs
Reducing costs is an important requirement for power transformers as a commodity, and sometimes even a decisive requirement. Because the price-performance ratio is the main indicator of the product in the competition of commodities. Not paying attention to reducing costs, not paying attention to lowering prices, will often be eliminated in the competition of commodities.
Transformer costs in power supplies include material costs, manufacturing costs, and management costs. Material costs typically account for 40% to 60% of total cost and are the most important part. The cost of core materials and conductive materials accounts for about 80% of the material cost. Therefore, the market trend of core materials and conductive materials, the price changes have a significant impact on the cost of transformers in the power supply. Reducing material costs is also related to design. When designing the transformer in the power supply, the ratio of the amount of core material used for the transformer to the amount of conductive material (copper-iron ratio) should be adjusted according to the price of the core material and the conductive material, so that the material cost can be minimized under the existing conditions. When using a computer to design a transformer in a power supply, the pursuit of the lowest cost should be a major constraint.
Manufacturing costs are also related to design and process. When designing a transformer in a power supply, not only the price and amount of core material and conductive material should be considered, but also the structure of the core and the coil and the overall structure of the transformer are easy to process and assemble. How much labor time does it take? How many equipment and tools are needed? What testing equipment and instruments are needed to control quality? These are the considerations that transformer designers should consider.
Management costs are determined by the full use of human and financial resources. Making full use of manpower means improving the utilization of working hours and reducing the proportion of managers and workers. Making full use of financial resources means shortening the production cycle, reducing inventory, and accelerating the flow of funds. These are mainly the responsibility of the management personnel. But it also has a relationship with the transformer designer. If the designed transformer is easy to machine and assemble, the production cycle can be shortened. The raw materials and accessories used are easy to source and can reduce inventory. These are all beneficial to reduce management costs.
Therefore, a good power transformer designer, in addition to understanding the transformer theory and design methods, but also to understand the price of core materials, conductive materials, insulation materials, structural materials and market trends, but also to understand the core, coil and transformer overall Processing and assembly processes, but also to understand the quality control parameters and equipment, as well as production management knowledge and transformer market trends. Only a well-informed transformer designer can design a transformer product with good performance and price.
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