Metal magnetic powder cores for high-frequency applications in electric vehicles and other new power electronic devices

Published: 2023-06-06 Views: 次

1 What Are Soft Magnetic Composites?

Soft magnetic composites (SMC), also known as powder cores, are a new type of magnetic material made from soft magnetic metals through powder production, insulation treatment, bonding, pressing, and heat treatment. They offer advantages such as magnetic isotropy, high permeability, low coercivity, a high Curie temperature, and low loss.

Soft magnetic composites combine the advantages of metallic and ferrite soft magnetic materials. Their resistivity is significantly higher than that of soft magnetic metals, which effectively reduces eddy current loss, and their saturation magnetization is higher than that of soft magnetic ferrites, making them better suited to the miniaturization and integration requirements of power electronic devices.

Soft magnetic composites can be pressed into various complex shapes such as toroids, E-cores, and U-cores, enabling integrated component production. As a result, soft magnetic composites have become the fastest-growing magnetic material in terms of development and application. They are used to produce key power electronic components such as various inductors, filters, chokes, and transformers, and are widely applied in energy, information, transportation, national defense, and other fields, making them a key foundational material for the national economy and national defense construction.

Image source: KDM Official Website

2 Classification of Powder Cores

Depending on their composition and properties, soft magnetic composites can be classified into alloy powder cores, iron powder cores, and others.

2.1 Alloy Powder Cores

Alloy powder cores are a soft magnetic material made from iron-based alloy powder that is surface-insulated and coated, mixed with an inorganic binder, pressed, and then subjected to high-temperature treatment. Depending on their alloy composition, they are mainly divided into five categories: Sendust Cores, Si-Fe Cores, Neu Flux Cores, High Flux Cores, and MPP Cores.

2.2 Iron Powder Cores

Iron powder cores are a soft magnetic material made from high-purity reduced iron powder or base iron powder that is surface-insulated and coated, mixed with a binder, and pressed; they are not saturated under a soft magnetizing field. They offer good DC bias characteristics. Their production process is simple, and their price is the lowest among all types of metal powder cores. They are currently the most widely used and highest-volume type of metal powder core.

Table 1 Properties of Different Soft Magnetic Composites

3 Preparation Methods of Soft Magnetic Powders

In the preparation of soft magnetic composites, the magnetic powder particle size selected is generally less than 100 μm or even smaller. To achieve higher permeability, particle size grading is also required. Currently, the main methods for preparing metal magnetic powders are mechanical crushing and atomization.

Image source: YUEAN New Materials Official Website

3.1 Mechanical Crushing Method

This method mainly uses a ball milling process to prepare metal magnetic powder. Through a high-speed rotating ball mill, hard metal balls continuously bombard, collide with, stir, and grind the raw material, ultimately crushing the alloy into micron-sized metal magnetic powder.

The ball milling powder production process is simple and low in cost, making it the most commonly used powder production process for soft magnetic composites. It can be divided into wet milling and dry milling. In wet milling, a liquid milling medium such as deionized water or anhydrous ethanol is generally added to the ball mill jar. During milling, the medium isolates the magnetic powder from the air and prevents oxidation, making this method suitable for soft magnetic alloys that are prone to oxidation.

3.2 Atomization Method

Liquid metal is directly broken into droplets with a diameter of less than 150 μm, which then condense into metal magnetic powder. This mainly includes water atomization and gas atomization.

Gas atomization usually uses an inert gas such as argon or helium as the atomizing medium, which can effectively reduce oxygen content and alloy oxidation. Because gas as an atomizing medium cools the metal droplets relatively slowly, the droplets do not solidify immediately but gradually contract into spheres as they fall; therefore, gas atomization readily produces spherical metal magnetic powder.

Water atomization uses water as the cooling medium, so the cooling rate is faster than that of gas atomization. However, because the alloy is prone to oxidation during water atomization, this method is only suitable for alloys with relatively stable compositions. Clearly, the equipment and process for atomization powder production are relatively complex, the powder production cost is high, and the output of metal magnetic powder is relatively low.

4 Analysis of Difficulties

The preparation of magnetic composites includes composition design, powder production, insulation treatment, bonding, pressing, and high-temperature heat treatment. The process flow chart is shown in Figure 3.

Figure 3 General Process Flow Chart for Preparing Soft Magnetic Materials

(Image source: KDM Official Website)

There are two prominent contradictions in the preparation process:

(1) To ensure the density of the magnetic core as much as possible and thereby increase permeability, a relatively high pressing pressure is used, which increases the internal stress of the soft magnetic composite and increases hysteresis loss;

(2) To fully eliminate internal stress and reduce the hysteresis loss of the soft magnetic composite, a relatively high annealing temperature is required, but this causes some insulation coating layers with poor thermal stability to decompose or be damaged, resulting in a rapid decrease in resistivity and an increase in eddy current loss.

Therefore, it is necessary to balance three conditions during preparation: magnetic core density, pressing pressure, and annealing temperature. At present, detailed and comprehensive research work can be carried out through the following means: designing and developing new alloy compositions to reduce magnetostriction and the magnetocrystalline anisotropy coefficient; improving the resistivity of the alloy through alloying to reduce eddy currents within the magnetic powder particles; and preparing a thin, uniform, electrically insulating coating film with high heat resistance, using the high-resistivity insulating layer to reduce eddy currents between particles.

5 Applications of Soft Magnetic Composites

In photovoltaic inverters, metal soft magnetic composites are mainly used in Boost inductors and AC output filter inductors. In the automotive power supply industry, they are mainly used in on-board chargers (OBC), on-board DC/DC converters, 48V power systems, and others. In the power supply industry, their main applications include PC power supplies, server power supplies, UPS power supplies, and charging pile module power supplies. In the home appliance industry, their main applications include inverter air conditioners, induction cookers, washing machines, and others.

6 Development Prospects of Soft Magnetic Composites

Over the past 20-plus years, China's metal powder cores have developed rapidly. In particular, with the rapid development of new energy and new energy vehicles, many Chinese enterprises have also begun research and development work on metal powder cores. Product quality and production scale have greatly improved, and the gap with international levels has been greatly narrowed.

As cost-effective metal powder cores, Sendust and super Sendust alloy powder cores have been fully localized by domestic manufacturers, replacing international counterparts, and are widely used in new energy photovoltaic inverters, UPS power supplies, charging piles, high-end home appliances, and other fields.

Currently, the soft magnetic composite industry is on the eve of an industry boom. Countries around the world are investing enormous effort to increase research, development, and demonstration applications of metal powder core soft magnetic materials. The rapid development of modern information technology and the power electronics industry is strongly driving the development of soft magnetic composites.

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