The role of electronic transformers in power supply technology-EEWORLD

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Introduction
　　Power supply devices, whether DC power supply or AC power supply, must use electronic transformers (soft magnetic electromagnetic components) made of soft magnetic cores. Although there are already air-core electronic transformers and piezoelectric ceramic transformers that do not use soft magnetic cores, up to now, the electronic transformers in most power supply devices still use soft magnetic cores. Therefore, discussing the relationship between power supply technology and electronic transformers: the role of electronic transformers in power supply technology, the requirements of power supply technology for electronic transformers, and the impact of the use of new soft magnetic materials and new core structures in electronic transformers on the development of power supply technology will surely arouse the interest of friends in the power supply industry and the soft magnetic material industry. This article puts forward some views in order to facilitate dialogue between the power supply industry and the electronic transformer industry and the soft magnetic material industry on issues related to electronic transformers and soft magnetic materials, so that they can communicate with each other and develop together. 1 The role of electronic transformers in power supply technology 　　Electronic transformers, semiconductor switching devices, semiconductor rectifier devices, and capacitors are called the four major components in power supply devices. According to the role in the power supply device, electronic transformers can be divided into: 　　1) power transformers, power transformers, rectifier transformers, inverter transformers, switching transformers, pulse power transformers for voltage 　　and power conversion; 2) broadband transformers, audio transformers, and medium-frequency transformers for transmitting broadband, audio frequency, and medium-frequency power and signals; 　　3) pulse transformers, drive transformers, and trigger transformers for transmitting pulses, drive, and trigger signals; 　　4) isolation transformers for insulating and isolating the primary and secondary sides, and shielding transformers for shielding; 　　5) phase conversion transformers for converting single-phase to three-phase or three-phase to single-phase, and phase conversion transformers (phase shifters) for changing the output phase; 　　6) frequency multiplication or frequency division transformers for changing the output frequency; 　　7) matching transformers for changing the output impedance to match the load impedance; 　　8) voltage stabilizing transformers (including constant voltage transformers) or current stabilizing transformers for stabilizing the output voltage or current, and voltage regulating transformers for adjusting the output voltage; 　　9) filter inductors for AC and DC filtering; 　　10) Electromagnetic interference filter inductors for suppressing electromagnetic interference, noise filter inductors for suppressing noise; 　　11) Absorption inductors for absorbing surge current, buffer inductors for slowing down the rate of change of current; 　　12) Energy storage inductors for storing energy, commutation inductors for helping semiconductor switches to commutate; 　　13) Magnetic switch inductors and transformers for switching; 　　14) Controllable inductors and saturation inductors for regulating inductance; 　　15) Voltage transformers, current transformers, pulse transformers, DC transformers, zero flux transformers, weak current transformers, zero-sequence current transformers, Hall current and voltage detectors for converting voltage, current or pulse detection signals. From the above list, it can be seen that whether it is a DC power supply, an AC power supply, or a special power supply, it is inseparable from an electronic transformer. Some people define power supply as a DC power supply and an AC power supply that have been transformed by high-frequency switching. When introducing the role of soft magnetic electromagnetic components in power supply technology, various electromagnetic components in high-frequency switching power supplies are often cited as examples. At the same time, various transformers occupy a dominant position among the soft magnetic electromagnetic components used in electronic power supplies. Therefore, transformers are used as representatives of soft magnetic components in electronic power supplies and are called "electronic transformers."
　　

　　2 Requirements of power supply technology for electronic transformers
　　The requirements of power supply technology for electronic transformers, like all products as commodities, are to pursue the best performance-price ratio in completing specific functions under specific conditions of use. Sometimes it may focus on price and cost, and sometimes it may focus on efficiency and performance. Now, light, thin, short and small have become the development direction of electronic transformers, emphasizing cost reduction. Starting from the general requirements, four specific requirements can be drawn for electronic transformers: conditions of use, completion of functions, improvement of efficiency and reduction of costs.
　　2.1 Conditions of use The conditions of use of electronic transformers include two aspects:
　　reliability and electromagnetic compatibility. In the past, only reliability was paid attention to. Now, due to the increased awareness of environmental protection, electromagnetic compatibility must be paid attention to. Reliability means that under specific conditions of use, the electronic transformer can work normally until its service life. The ambient temperature is the biggest influence on electronic transformers in general conditions of use. The parameter that determines the intensity of temperature influence on electronic transformers is the Curie point of soft magnetic materials. Soft magnetic materials with high Curie points are less affected by temperature; soft magnetic materials with low Curie points are more sensitive to temperature changes and are greatly affected by temperature. For example, the Curie point of manganese-zinc ferrite is only 215℃, which is relatively low. The magnetic flux density, magnetic permeability and loss all change with temperature. In addition to the normal temperature of 25℃, various parameter data at 60℃, 80℃ and 100℃ are also given. Therefore, the operating temperature of manganese-zinc ferrite core is generally limited to below 100℃, that is, when the ambient temperature is 40℃, the temperature rise must be lower than 60℃. The Curie point of cobalt-based amorphous alloy is 205℃, which is also low, and the use temperature is also limited to below 100℃. The Curie point of iron-based amorphous alloy is 370℃, which can be used below 150℃～180℃. The Curie point of high magnetic permeability Permalloy is 460℃ to 480℃, which can be used below 200℃～250℃. The Curie point of microcrystalline and nanocrystalline alloy is 600℃, and the Curie point of oriented silicon steel is 730℃, which can be used at 300℃～400℃. Electromagnetic compatibility means that the electronic transformer neither generates electromagnetic interference to the outside world nor can withstand electromagnetic interference from the outside world. Electromagnetic interference includes audible audio noise and inaudible high-frequency noise. The main reason for the electromagnetic interference generated by the electronic transformer is the magnetostriction of the magnetic core. Soft magnetic materials with large magnetostriction coefficients generate large electromagnetic interference. The magnetostriction coefficient of iron-based amorphous alloys is usually the maximum (27~30)×10-6, and measures must be taken to reduce noise and suppress interference. The magnetostriction coefficient of high magnetic permeability Ni50 Permalloy is 25×10-6, and the magnetostriction coefficient of manganese-zinc ferrite is 21×10-6. The above three soft magnetic materials are materials that are prone to electromagnetic interference, so they should be paid attention to in application. The magnetostriction coefficient of 3% oriented silicon steel is (1~3)×10-6, and the magnetostriction coefficient of microcrystalline and nanocrystalline alloys is (0.5~2)×10-6. These two soft magnetic materials are materials that are relatively easy to generate electromagnetic interference. The magnetostriction coefficient of 6.5% silicon steel is 0.1×10-6, the magnetostriction coefficient of high magnetic permeability Ni80 Permalloy is (0.1～0.5)×10-6, and the magnetostriction coefficient of cobalt-based amorphous alloy is below 0.1×10-6. These three soft magnetic materials are materials that are not easy to generate electromagnetic interference. The frequency of electromagnetic interference generated by magnetostriction is generally the same as the operating frequency of the electronic transformer. If there is electromagnetic interference below or above the operating frequency, it is caused by other reasons.
　　2.2 Completed functions Electronic transformers are mainly divided into two types: transformers and inductors in terms of function.
　　The functions completed by special components are discussed separately. Transformers have three functions: power transmission, voltage conversion and insulation isolation. Inductors have two functions: power transmission and ripple suppression. There are two ways to transmit power.

The first is the transformer transmission method, that is, the alternating voltage applied to the primary winding of the transformer generates a change in magnetic flux in the magnetic core, causing the secondary winding to induce voltage, which is applied to the load, so that the electric power is transmitted from the primary side to the secondary side. The size of the transmitted power is determined by the induced voltage, that is, the flux density variable ΔB per unit time. ΔB has nothing to do with the magnetic permeability, but is related to the saturation flux density Bs and the residual flux density Br. From the perspective of saturation flux density, the order of Bs of various soft magnetic materials from large to small is: iron-cobalt alloy is 2.3-2.4T, silicon steel is 1.75-2.2T, iron-based amorphous alloy is 1.25-1.75T, iron-based microcrystalline nanocrystalline alloy is 1.1-1.5T, iron-silicon-aluminum alloy is 1.0-1.6T, high magnetic permeability iron-nickel permalloy is 0.8-1.6T, cobalt-based amorphous alloy is 0.5-1.4T, iron-aluminum alloy is 0.7-1.3T, iron-nickel-based amorphous alloy is 0.4-0.7T, and manganese-zinc ferrite is 0.3-0.7T. As the core material of electronic transformers, silicon steel and iron-based amorphous alloys are dominant, while manganese-zinc ferrite is at a disadvantage. The second type of power transmission is the inductor transmission mode, that is, the electric energy input to the inductor winding excites the magnetic core, converts it into magnetic energy and stores it, and then converts it into electric energy through demagnetization and releases it to the load. The size of the transmitted power is determined by the energy storage of the inductor core, that is, the inductance of the inductor. The inductance is not directly related to the saturation flux density, but to the magnetic permeability. The higher the magnetic permeability, the larger the inductance, the more energy storage, and the larger the transmission power. The magnetic permeability of various soft magnetic materials is in the following order from large to small: Ni80 Permalloy is (1.2~3)×106, Cobalt-based amorphous alloy is (1~1.5)×106, Iron-based microcrystalline nanocrystalline alloy is (5~8)×105, Iron-based amorphous alloy is (2~5)×105, Ni50 Permalloy is (1~3)×105, Silicon steel is (2~9)×104, and Manganese-zinc ferrite is (1~3)×104. As the core material of the inductor, Ni80 Permalloy, Cobalt-based amorphous alloy, and Fe-based microcrystalline and nanocrystalline alloy are dominant, while silicon steel and manganese-zinc ferrite are at a disadvantage. The size of the transmitted power is also related to the number of transmissions per unit time, that is, it is related to the operating frequency of the electronic transformer. The higher the operating frequency, the greater the power transmitted under the same size of the core and coil parameters. The voltage conversion is completed by the turns ratio of the primary winding and the secondary winding of the transformer. Regardless of the power transmission size, the voltage conversion ratio between the primary and secondary sides is equal to the turns ratio of the primary winding and the secondary winding. Insulation isolation is completed by the insulation structure of the primary winding and the secondary winding of the transformer. The complexity of the insulation structure is related to the size of the applied and transformed voltage. The higher the voltage, the more complex the insulation structure. Ripple suppression is achieved through the self-inductance potential of the inductor. As long as the current passing through the inductor changes, the magnetic flux generated by the coil in the magnetic core will also change, causing a self-inductance potential to appear at both ends of the coil of the inductor, and its direction is opposite to the direction of the applied voltage, thereby preventing the change of current. The frequency of ripple change is higher than the fundamental frequency, and the current frequency of current ripple is greater than the fundamental frequency. Therefore, it can be more suppressed by the self-inductance potential generated by the inductor. The ability of the inductor to suppress ripple depends on the size of the self-inductance potential, that is, the size of the inductance, which is related to the magnetic permeability of the magnetic core. Ni80 Permalloy, cobalt-based amorphous alloy, and iron-based microcrystalline and nanocrystalline alloy have large magnetic permeabilities and are at an advantage. Silicon steel and manganese-zinc ferrite have small magnetic permeabilities and are at a disadvantage.
　　2.3 Improving efficiency Improving efficiency is a general requirement for power supplies and electronic transformers.
　　Although, from the perspective of a single electronic transformer, the loss is not large. For example, when the efficiency of a 100VA power transformer is 98%, the loss is only 2W, which is not much. However, when there are hundreds of thousands or millions of power transformers, the total loss may reach hundreds of thousands of W, or even millions of W. In addition, many power transformers have been in operation for a long time, and the annual total loss is considerable, which may reach tens of millions of kW·h.

Obviously, improving the efficiency of electronic transformers can save electricity. After saving electricity, fewer power stations can be built. After fewer power stations are built, less coal and oil can be consumed, less CO2, SO2, NOx, waste gas, sewage, smoke and ash can be emitted, and the pollution to the environment can be reduced. It has the dual social and economic benefits of saving energy and protecting the environment. Therefore, improving efficiency is a major requirement for electronic transformers. The losses of electronic transformers include core loss (iron loss) and coil loss (copper loss). Iron loss exists as long as the electronic transformer is put into operation, and it is the main part of the loss of electronic transformers. Therefore, selecting the core material according to iron loss is the main content of electronic transformer design, and iron loss has also become a major parameter for evaluating soft magnetic materials. Iron loss is related to the working magnetic flux density and working frequency of the magnetic core of the electronic transformer. When introducing the iron loss of soft magnetic materials, it must be stated at what working magnetic flux density and what working frequency the loss is. For example, P0.5/400 means iron loss at a working magnetic flux density of 0.5T and a working frequency of 400Hz. P0.1/100k represents the iron loss at the working magnetic flux density of 0.1T and the working frequency of 100kHz. Soft magnetic materials include hysteresis loss, eddy current loss and residual loss. Eddy current loss is inversely proportional to the resistivity ρ of the material. The larger ρ is, the smaller the eddy current loss is. The order of ρ of various soft magnetic materials from large to small is: manganese-zinc ferrite is 108-109μΩ·cm, iron-nickel-based amorphous alloy is 150-180μΩ·cm, iron-based amorphous alloy is 130-150μΩ·cm, cobalt-based amorphous alloy is 120-140μΩ·cm, high magnetic permeability Permalloy is 40-80μΩ·cm, iron-silicon-aluminum alloy is 40-60μΩ·cm, iron-aluminum alloy is 30-60μΩ·cm, silicon steel is 40-50μΩ·cm, and iron-cobalt alloy is 20-40μΩ·cm. Therefore, the ρ of manganese-zinc ferrite is 106-107 times higher than that of metal soft magnetic materials, and the eddy current is small in high frequency, so it is advantageous in application. However, when the operating frequency exceeds a certain value, the insulator inside the magnetic particles of manganese-zinc ferrite is broken down and melted, ρ becomes quite small, and the loss rises rapidly to a very high level. This operating frequency is the limiting operating frequency of manganese-zinc ferrite.

Reference address：The role of electronic transformers in power supply technology

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