AUTOMOTIVE Development of Low-Iron-Loss Powder Magnetic Core Material for High-Frequency Applications Tomoyuki IshImIne*, Asako WATAnAbe, Tomoyuki Ueno, Toru mAedA and Terukazu TokUokA Recently, there has been a growing trend toward reducing society's carbon footprint. In the field of energy, development of clean power generation systems, such as solar and wind generators, have been promoted. Electric vehicles are also replacing petrol vehicles in the automotive industry, and energy-saving electrical appliances are in demand more than ever. Due to this trend, small and powerful power-supply devices with better conversion efficiency are increasingly demanded, and accordingly, high performance inductors are required. Thus far, the authors have engaged in the development of powder magnetic cores, which are made by press compacting soft magnetic powder coated with insulating film, for use in motors and solenoid valves. In this study, the technology accumulated in the development of these cores has been successfully applied to that of an inductor core. The developed inductor core shows superior properties to ferrite and dust cores which are commonly used for inductors. In this paper, the advantageous properties of the developed core and the result of application to the inductor are reported. Keywords: soft magnetic material, inductor, reactor, choke coil, converter, inverter, eco-friendly vehicle 1. Introduction a low-iron-loss powder magnetic core material for high-fre- quency applications with characteristics superior to ferrite There has been a shift in recent years toward green or general-use dust cores previously used in these applica- power characterized mainly by solar- and wind-generated tions.(3),(4) In this study, the various characteristics of the power in the energy field, with an aim toward reducing so- developed material and result of application to power coils ciety's carbon footprint. The automotive industry is tran- are reported. sitioning from internal combustion engines using fossil fuel to electric-power-driven systems, and there is also a movement to reduce the consumption of electricity in the field of household appliances. Power storage, energy re- 2. Current Technologies Aimed at Development covery and voltage control technologies are critical to mov- Objectives and Issues ing these trends forward, and electric power devices that efficiently convert energy to the required power are in de- From the viewpoint of downsizing and high electric mand. Switching power supplies have recently become the power capacity, the iron core material used in power coils mainstream from the viewpoint of conversion efficiency should show a high saturation flux density and a high elec- and component downsizing. The choke coils, inverter tromagnetic conversion efficiency. Table 1 shows the typical coils used in transformer circuits and rectifier circuits, coil soft magnetic materials used in high-frequency applications, components such as power inductors, electric reactors and compares their power characteristics. Soft magnetic comprising an iron core and a winding greatly affect the materials generally used in iron cores for processing power performance of power devices. There is also a shift from included ferrite and general-use dust cores. Ferrite shows conventional energy systems, wherein power is conducted low loss (iron loss), and superior power efficiency, but as in one-way systems from the power generation source or shown in Fig. 1, because of its low saturation flux density, it storage battery to the electronic device, to two-way systems lacks the operation flux density required for high-capacity that recover the excess energy generated by power-con- power supplies, and so the device becomes large in size and suming devices. To this end, power devices and power requires much copper for use in the winding. General-use coils are needed to show complex transforming and recti- dust cores, on the other hand, are obtained by compres- fying functions, to be capable of handling greter power sion-molding magnetic metal powder in the same way as levels, and also to be smaller in size. powder magnetic cores, and they show superior saturation Recently, we have developed a “powder magnetic core flux density compared to ferrite and have a good high cur- material" obtained by press compacting magnetic metal rent handling capacity (DC bias characteristic), thus facili- powder, which is used used as a soft magnetic material in tating a compact device. However, compared to ferrite, they the iron core of coil components.(1),(2) Using the developed have a number of problems, including heat generation and technoogy to the problems of power devices described low power efficiency due to large iron loss. In addition to above, we have improved the high-frequency response in the above, there is also amorphous ribbon that realizes the several-hundred-kHz band, which is the target fre- high flux density and conversion efficiency, but because it quency band wherein the material characteristics are to uses a laminated structure that requires a rolling process be applied, and as a result, we have successfully achieved to manufacture, it is difficult to form the core into complex SEI TECHNICAL REVIEW · NUMBER 72 · APRIL 2011 · 117 Table 1. Soft magnetic materials used in power coils for high-frequency applications, and their power Magnetic material Material Schematic View Downsizing Power capacity Power efficiency Shape complexity content Controlling factors in Magnetic flux den- Magnetic flux den- Iron loss Electrical Complex shape − soft magnetic material sity Packing density sity Packing density resistance formability Ferrite 100vol% (Spinel type) △ △ ◎ ○ General-use dust 50 ~ 70 (Fe-Si-Al, Fe-Si-B） ○ ○ △ ○ Amorphous foil 80 ~ 90 (Fe-Si-B) ◎ ◎ ○ × Conventional sintered soft magnetic material 90 ~ 95 ◎ ◎ × ◎ (pure iron, Fe-3Si) Note: ◎: Excellent ○: Good △: Fair ×: Poor Powder magnetic (i.e., tens to hundreds of kHz) of switching power supplies, core materials 1.5 the material shows extremely large iron loss, thus resulting Motors, actuators in heat generation greater than that in general-use dust lamineted steel cores and lower conversion efficiency. Therefore, we set Low-Iron-Loss Powder magnetic core material for high-freqquecy application 1.0 out to develop a new soft magnetic material superior to High-capacity switching general-use dust cores and ferrite through optimization of power supplies Low-capacity switching power supplies the characteristics of our previous powder magnetic core General-use dust cores & inductors for signal applications materials to the several-hundred-kHz frequency range. 0.5 Figure 2 summarizes the controlling factors that affect iron loss in powder magnetic core materials.(2) Iron loss is Ferrite the sum of hysteresis loss (Wh) and eddy-current loss (We). A low coercive force is required to reduce the Wh, which downsizing and electric power capacity Operating flux density (T): contributor to 0 1k 10k 100k 1M 10M is the external energy required to change the magnetic Switching frequency (Hz): contributor to conversion efficiency field direction. Reducing the amount of impurities in the material and using a magnetic alloy with a small magnetic Fig. 1. Applications and performance ranges of typical soft magnetic ma- anisotropy or magnetostriction constant are effective, ad- terials and their development objectives ditionally, eliminating the strain generated during powder compacting through heat treatment is an important tech- nology, particularly in powder magnetic core materials. On the other hand, the Joule heat from the eddy-current gen- shapes. Further, the production cost is much higher than that of powder metallurgical processes, and the electrical resistance is low, which greatly reduces the conversion ef- ficiency in the high-frequency range. For these and other reasons, amorphous ribbon has limited application. As ex- Iron Loss <Controlling factors> <Countermeasure> [Composition factors] plained above, existing soft magnetic materials all present Hysteresis loss ①Achieving Magnetic anisotropy (BH hysteresis loop area) low coercive force obstacles to realizing the kind of compact devices with high Magnetstriction 䚽 B Find new alloy electorical power capacity that will be in demand in the Small loss [Constitution factors] 䚽 High purity Large loss 䚽 near future, so a new kind of soft magnetic material with Inpurity Thin and uniform coating 䚽 superior characteristics is desired. H Particle Boundary High-temp anneal Hc (Insulation Layer) (heat-resistant insulation coating) Dislocation 䚽 Strain-free, Reduced coercive high-density forming Reduced hysteresis loss = force Hc, Improved Grain Boundary permeability µ interparticle ②Subdivision of 3. Various Characteristics of Developed Material eddy-current Eddy Interarticles Fe Fe Insulation generating regions Current eddy-current interpaticles 䚽 loss loss High-resistance 3-1 Approach and Experimental Method of Development Fe Fe insulation coating We previously developed a powder magnetic core ma- ③Achieving high Intraparticle intraparticle electrical resistance terial with superior soft magnetic properties for use in de- eddy-current generating regions vices such as motors and actuators that operate in a loss Electrical resistance 䚽 of core material Addition elements relatively low frequency range (up to several kHz), whereby that raise electrical resistance technology that greatly improved the saturation flux den- sity and loss characteristic of the powder magnetic