The Japanese government is promoting the development of soft magnetic materials for pure EVs

    Power electronic devices are products used to control power in a variety of devices such as air conditioners, photovoltaic power generation systems, and pure electric vehicles. Power semiconductors are responsible for their miniaturization and high efficiency. In addition to the widely used silicon (Si) semiconductors (FETs and diodes), power semiconductors now also include semiconductors made of SiC (silicon carbide) and GaN (gallium nitride). In fact, magnetic materials are also important for the miniaturization and efficiency of power electronic devices. The following mainly introduces two soft magnetic materials that are being developed by Japanese government projects.

    Soft magnetic materials are mostly used for the iron core and magnetic shield of coils. They are indispensable magnetic materials for transformers, analog filters and motors. The main raw materials are iron alloys such as silicon steel and iron-nickel alloy (Figure 1).

    Hard magnetic materials are materials that retain their magnetism after being magnetized in a magnetic field, while soft magnetic materials tend to lose their magnetism after being magnetized. Permanent magnets such as neodymium magnets, which have attracted much attention due to the rare earth issue, are made of hard magnetic materials. The reason why the iron core of an electromagnet can be magnetic when current flows through the coil, but loses its magnetism when the current is stopped, is because the iron core is made of soft magnetic materials.

Compared with semiconductors, magnetic components are a bigger bottleneck

    Professor Toshihisa Shimizu (Department of Electrical and Electronic Engineering, Graduate School of Science and Engineering) of Tokyo Metropolitan University in Japan believes that "power electronic devices cannot evolve with SiC and GaN alone." Professor Shimizu said that over the past 40 years, the power density of power electronic products has been increasing at a rate of 10 times every 15 years. However, the technical limit of power density improvement is about to be reached.

    The power density here refers to the value obtained by dividing the output power of power electronic equipment by its volume. Professor Shimizu pointed out that although the efficiency of power semiconductors is increasing year by year, and new semiconductors such as SiC and GaN are gradually being put into use, there are still technical limits because "the volume of (other components) dominates". Taking the latest power electronic equipment (inverter) with an output power of 10kW as an example, in the entire product, the volume of power semiconductors does not exceed 15%, noise filters and smoothing capacitors each account for 30%, and high-frequency transformers account for 25%. The development of components other than power semiconductors can effectively improve the power density of products.

    In view of this situation, Professor Shimizu emphasized that the key to improving the power density of power electronic equipment in the future is to promote the development of passive components that support power semiconductors (Figure 2). He believes that magnetic devices such as transformers and filters are particularly important among passive components.

Material development to reduce iron loss is becoming more active

    The development of soft magnetic materials with low iron loss is likely to promote the evolution of magnetic devices. When a sine wave, rectangular wave, or current with voltage variation is passed through the coils of transformers and filters, the iron core will produce loss, which is iron loss. Large iron loss will generate heat, and it is difficult to reduce the size of electronic products in order to dissipate heat, which hinders the improvement of power density.

    Furthermore, when the switching frequency is increased to achieve higher efficiency and smaller size of inverters that control power consumption of electronic equipment, iron loss increases. The reason is that among the hysteresis loss, eddy current loss, and residual loss that constitute iron loss, hysteresis loss is proportional to the frequency, and eddy current loss is proportional to the square of the frequency (Figure 3).

    Therefore, it is necessary to develop soft magnetic materials that are less likely to produce iron loss even at high frequencies, because "soft magnetic materials have a lot of room for reducing iron loss" (Shimizu). However, there was no evaluation method for soft magnetic materials before, so they have never been optimized. Iron loss changes with the shape of the core, the waveform of the current flowing through the coil, the current and magnetic field previously applied, and the size of the crystal particles.

    Previously, the development of materials to reduce iron loss was conducted under conditions where material manufacturers had conducted tests, but in the future, it will be conducted under actual use conditions. The two projects led by the Japanese government, the "Tohoku Region Material Technology Leading Project" and the "High-efficiency Motor Magnetic Material Technology Research Group", began developing soft magnetic materials in 2012. Koji Takano, director of the technical department of Iwatsu Measuring Co., Ltd., which sells material magnetic measuring instruments, said, "Recently, research on soft magnetic materials has become increasingly popular," and the company launched a magnetic measuring instrument for soft magnetic materials in August 2013.