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I. Requirements for Magnetic Components and Soft Magnetic Materials Under the Trend of High Power and High Frequency
1.1 Third-Generation Semiconductor Power Devices Show a High-Frequency, High-Power Development Trend
High-power, high-density fast-charging power supplies are gradually becoming the main market trend. Various power electronic devices based on silicon materials are approaching their theoretical limits and are difficult to use in specific environments such as high voltage, high temperature, and high frequency. At present, third-generation wide-bandgap semiconductor materials represented by gallium nitride (GaN) and silicon carbide (SiC) possess unique properties such as a wide bandgap, high breakdown electric field, high thermal conductivity, high electron saturation drift velocity, and low dielectric constant. They exhibit superior characteristics compared with traditional power devices and are being applied at a high growth rate in many fields, including photovoltaic inverters, new energy vehicles, charging piles, energy storage, server power supplies, data centers, and 5G base stations. SiC devices have smaller dimensions and lower switching frequency losses. Compared with Si-based MOSFETs of the same specification, SiC-based MOSFETs can be significantly reduced in size to as little as 1/10 of the original, and on-resistance can be reduced to at least 1/100 of the original. According to the power density formula, smaller dimensions are more conducive to achieving high frequency and further realizing high power density. In addition, compared with Si-based IGBTs of the same specification, the total energy loss of SiC-based MOSFETs can be greatly reduced by 70%.
For most IGBT modules, the switching frequency in the photovoltaic, new energy vehicle, and charging pile fields is often at the level of 100 kHz. When third-generation semiconductors begin to be used, the switching frequency will increase to the level of several hundred kHz or even megahertz, while voltage rises and power density increases, allowing higher temperatures to be tolerated. At present, the limitation is that the cost of silicon carbide is still relatively high, but the smooth promotion and application of SiC MOSFETs in many industries are basically driven by cost in a broad sense, that is, after adopting this type of device, the economic benefits brought at the system level will exceed the increased procurement cost. Yole predicts that the market size of silicon carbide power devices will grow from USD 1.09 billion in 2021 to USD 6.297 billion in 2027, with a compound growth rate of 34%. In the future, the market space for silicon carbide will gradually expand, and the development from IGBTs to wide-bandgap semiconductors is an inevitable trend.
1.2 Corresponding Requirements for Magnetic Components and Soft Magnetic Materials: High Resistivity, High Power, Miniaturization, and Energy Saving
The development of semiconductor power devices is determined by the gradual opening of the bandgap and the increase in power density across the first, second, and third generations of semiconductor materials, which also imposes higher performance requirements on the supporting magnetic components. At present, semiconductor devices are considered on the cost side; in reality, what limits power supply performance is the technological progress of supporting magnetic components, which lags behind the development of semiconductor devices. Magnetic components are electronic components that use the principle of electromagnetic induction to convert electrical energy and magnetic energy into each other, thereby achieving energy conversion and transmission. They are mainly divided into two categories: transformers and inductors. A transformer is a static electromagnetic device that realizes electrical energy conversion or transfers electrical energy from one circuit to another. An inductor converts electrical energy into magnetic energy and stores it; its main function is to pass DC and block AC, isolate and filter AC signals, or form resonant circuits with capacitors, resistors, and other components.
With the development of semiconductor power devices and technological progress, magnetic components are developing toward high frequency, high power, miniaturization, and energy saving: 1) High frequency: increase in switching frequency; 2) High power: increase in power density brought by higher voltage; 3) Miniaturization: increased switching frequency and power density reduce requirements on volume and weight; 4) Energy saving: lower energy consumption and higher conversion efficiency.
Magnetic components are mainly composed of magnetic core materials plus external windings (copper, aluminum). The determining factor of performance is the internal magnetic core material, which is made of soft magnetic materials. Under the development trend of high frequency, high power, miniaturization, and energy saving of magnetic components, the requirements for soft magnetic cores are: 1) High resistivity: eddy current losses under high frequency must be greatly reduced, that is, high resistivity is required; 2) High power: high power density requires the material to resist saturation and have good DC bias performance; 3) High saturation magnetic flux density: miniaturization of devices requires small core dimensions, meaning more magnetic moment per unit volume; 4) Low energy consumption: high efficiency and energy saving require lower material energy consumption.
II. Metal Powder Cores Are the Best Soft Magnetic Material Under the High-Frequency, High-Power Trend
2.1 Key Upstream Materials for Magnetic Components; Soft Magnetic Products Continue to Iterate
Soft magnetic materials refer to a class of magnetic materials with low coercivity that are easy to magnetize and demagnetize, featuring low hysteresis loss, high permeability, and high saturation magnetic induction. As the magnetic cores of magnetic components, soft magnetic materials need to develop toward high resistivity, high power, high saturation magnetic flux density, and low energy consumption. According to different performance and usage functions, soft magnetic cores are mainly divided into several categories: silicon steel sheets, soft magnetic ferrites, alloy powder cores, and amorphous/nanocrystalline materials.
The main magnetic parameters for evaluating the performance of soft magnetic materials are permeability, saturation magnetization, and magnetic loss. High saturation magnetization is conducive to obtaining high initial permeability. High permeability enables soft magnetic materials to obtain relatively large magnetic induction and high-density magnetic flux under a relatively low external magnetic field strength. During alternating magnetization, soft magnetic materials generate eddy currents under the action of the magnetic field, causing energy loss and reducing the effective working magnetic flux of the material. There are two ways to reduce eddy current loss: one is to reduce particle size, and the other is to increase resistivity, that is, reduce conductivity.
From the history of soft magnetic materials, they have gone through four development stages: traditional metal soft magnetic materials, ferrite soft magnetic materials, amorphous and nanocrystalline soft magnetic materials, and soft magnetic composite materials (metal powder cores). Metal powder cores combine the advantages of traditional metal soft magnetic materials and ferrite soft magnetic materials. Their application fields continue to expand, and their degree of industrialization has gradually surpassed that of other soft magnetic materials.
1) Traditional Metal Soft Magnetic Materials
Pure iron is low in price and abundant in reserves and was the earliest soft magnetic material put into use, but its resistivity is very low, so it can only work at low frequency or under DC. Silicon steel was successfully developed at the end of the 19th century. Its resistivity is several times that of electrical pure iron, and it is easy to mass-produce, inexpensive, and stable. However, its application frequency is limited to several hundred hertz, and it is currently mainly used for transformer cores. Permalloy has high initial permeability, but it is expensive, has low resistivity, and involves a complex manufacturing process, resulting in limited application scope.
2) Ferrite Soft Magnetic Materials
Ferrite soft magnetic materials came out in 1935. They are made by configuring and sintering magnetic Fe2O3 with other metal oxides. They have relatively high resistivity and permeability and successfully solved the problem of high losses under high-frequency conditions. However, their low initial permeability and saturation magnetization lead to poor magnetic energy storage capacity, limiting their application in scenarios requiring high magnetic energy density and high power and making them more suitable for high-frequency and low-power scenarios.
3) Amorphous and Nanocrystalline Soft Magnetic Materials
Amorphous and nanocrystalline soft magnetic materials were first industrially produced in the United States and Japan in the 1960s. Amorphous alloys are made by adding glass-forming elements (silicon, boron, carbon, etc.) during the metallurgical process of producing metal soft magnetic materials and using rapid quenching technology to make them amorphous. While retaining the high saturation magnetic induction and high permeability of metal soft magnetic materials, they increase resistivity and reduce eddy current loss. They are preferred materials for power transmission in the low- and medium-frequency fields and are currently mainly used in the field of distribution transformers. Thanks to advantages such as high saturation magnetic flux density and high permeability, nanocrystalline materials have significant advantages over ferrite soft magnetic materials in scenarios pursuing miniaturization, lightweight, and complex temperatures, but their price is tens of thousands to hundreds of thousands of yuan per ton, resulting in relatively high cost.
4) Soft Magnetic Composite Materials (Metal Powder Cores)
Soft magnetic composite materials (SMC), also known as metal powder cores, were industrialized in the 1980s. Metal powder cores are magnetic cores obtained by coating the surface of ferromagnetic powder particles with an insulating medium and then pressing them into the required shape using powder metallurgy processes. They feature distributed air gaps, good temperature characteristics, low loss, excellent DC bias characteristics, and high saturation magnetic flux density. They combine the advantages of traditional metal soft magnetic materials and ferrite soft magnetic materials and can meet the requirements of miniaturization, high power density, high frequency, and integration of power electronic devices. They are used as power factor correction (PFC) inductors, output filter inductors, resonant inductors, EMI differential mode inductors, and flyback transformer cores in photovoltaic inverters, energy storage inverters, new energy vehicles, charging piles, server power supplies, UPS power supplies, 5G communications, and other fields.
2.2 Combining the Advantages of Traditional Metal Soft Magnetic Materials and Ferrite Soft Magnetic Materials, Suited to High-Frequency and High-Power Application Scenarios
Metal powder cores mainly include five categories: iron powder cores, iron-silicon powder cores, iron-silicon-aluminum powder cores, high-flux powder cores, and molybdenum permalloy powder cores. Among them, iron powder cores are gradually being eliminated; iron-silicon powder cores have low loss and low cost and are the most widely used. Moreover, according to different downstream performance requirements, there are many subdivided products. The domestic listed company POCO Magnetic has continuously iterated and upgraded from "Iron-Silicon Generation 1" to "Iron-Silicon Generation 4," establishing a metal powder core system covering applications in the 5 kHz–2 MHz frequency range that can meet the performance requirements of many application fields. The company has also launched a new NPV series of powder cores for the silicon carbide era, retaining excellent DC bias capability while greatly optimizing core loss, making a huge contribution to saving copper wire and improving efficiency for power modules.
From a structural perspective, soft magnetic composite materials (metal powder cores) can be understood as a multiphase system that increases resistivity and reduces eddy current loss by coating at least one insulating layer on the surface of matrix ferromagnetic particles. Metal powder cores combine the characteristics of ferrite soft magnetic materials and metal soft magnetic materials and simultaneously feature low loss, high permeability, high saturation magnetic induction, high resistivity, excellent magnetic and thermal isotropy, and a relatively wide operating frequency range. They not only overcome the shortcoming of low saturation magnetic induction of ferrites but also compensate for the defect of large eddy current loss under high frequency of metal soft magnetic alloys, showing broad application prospects at medium and high frequencies.
From the perspective of the preparation process, metal powder cores are obtained by coating the surface of magnetic powder particles with an insulating medium and then pressing them into the required shape using powder metallurgy processes. The process includes several steps: magnetic powder particle preparation, particle size proportioning, insulation coating, press forming, and heat treatment.
1) Magnetic powder particle preparation: Particle morphology directly affects flowability, density, insulation coating, forming effect, and strength, and has an important impact on the performance of powder cores. The main processes are mechanical crushing and atomization. Mechanical crushing is simple and low-cost. Irregular magnetic powder particles facilitate insulation coating and forming but are easily crushed, leading to performance deterioration. Atomization is divided into water atomization and gas atomization. Gas-atomized powder has good sphericity and low impurity content; water-atomized powder has an irregular shape and lower cost. Iron powder produced by the reduction method and carbonyl method has small and dense pores, fine powder, and high purity, but at the same time higher cost.
2) Particle size proportioning: This affects the internal air gaps of the powder core, thereby affecting the microstructure and performance of the powder core. A reasonable particle size ratio can improve the DC bias performance of the powder core and reduce loss. Due to the presence of air gaps inside the magnetic powder, a relatively large demagnetizing field appears inside the powder core, making it difficult for the powder core to be magnetized to saturation. The larger the air gap, the stronger the demagnetizing field, and the better the DC bias performance of the powder core. As the proportion of fine powder in the magnetic powder increases, the effective permeability decreases, the DC bias performance becomes better, low-frequency loss increases, and high-frequency loss decreases.
3) Insulation coating: The main purpose is to reduce eddy current loss and adjust the air gap size to improve DC bias performance. According to the type of insulating layer, it is mainly divided into organic coating and inorganic coating. The organic coating layer can be selected from thermosetting resins such as epoxy resin, acrylic, polyester, and polyurethane, and the inorganic coating layer can be selected from oxides, phosphates, sulfates, and the like. Compared with organic coating, inorganic coating such as phosphating passivation technology has greatly improved the performance of powder cores. Chemical coating insulation layers are relatively uniform and easy to control, with lower production costs and simple processes, and are currently the most widely used insulation coating process in industry.
4) Press forming: After uniformly mixing the insulated magnetic powder with a release agent and a binder, it is pressed into the required compact. In industry, mold pressing is generally used. The greater the forming pressure, the higher the permeability and tensile strength of the powder core, and the lower the hysteresis loss.
5) Heat treatment: The main purpose is to relieve residual internal stress introduced during the forming process, also known as stress relief annealing. After annealing, the residual internal stress inside the formed compact caused by factors such as particle deformation, defects, and voids is relieved, and the corresponding magnetic performance is further improved.
According to the current technical path of metal powder cores, magnetic powder preparation and particle size proportioning are the most important links and determine the performance of powder cores; the technical barrier for insulation coating is relatively high, and the added amount, thickness, and uniformity of the coating layer still need continuous optimization and improvement; the pressure for press forming may be thousands of times that of other traditional powder metallurgy materials, and mold design is the main R&D direction; standard powder cores can no longer meet demand with the development
