電子機器の高速化や小型化等の手段として，駆動周波数や使用電波等の高周波化が進展している。これに対応するべく新規高周波磁性材料も続々と開発されており，高周波磁気／マイクロ磁気は，今後も重要な学問・技術である。最新動向としては，電磁ノイズ抑制分野，スイッチング電

This paper explains and introduces the magnetic materials for high-frequency applications, e.g., ferrites, composites, and films. High resistivity and downsizing are necessary for a reduction of Joule losses (skin effects), and furthermore, high resonance frequency of the complex permeability is also demanded for a decrease of the magnetic losses.

Random numbers are widely used in information technologies such as security, simulation, and machine learning. With the development of information technology, there is a need for technology to provide high-quality random numbers at higher speeds and at lower cost. Magnetic domain patterns of magnetic garnets are promising noise source for random number generators because they are unpredictable in terms of both chaos and quantum theory, and can be digitized at high speed by image sensors. Random numbers were generated based on the magnetic domain images and were passed randomness test (NIST SP800-22).

Two types of Fe-based nanocrystalline powder composite magnetic core for tens of MHz were fabricated and applied to the planar power inductor. One was Fe-Si-B-Nb-Cu-C spherical fine powder composite magnetic core for a closed magnetic circuit in the planar power inductor, which exhibited a relative permeability of 10 and loss factor tand of 0.004 at 10MHz. The other one was Fe-Si-B-Nb-Cu flake powder composite magnetic core for shielding the leakage magnetic flux from low permeability closed magnetic circuit, which exhibited a relative permeability of 150 and loss factor of 0.19 at 10MHz. The planar power inductor, with Fe-based nanocrystalline spherical fine powder composite magnetic core and flake powder composite magnetic core, had 4.8×3.2×1.84 mm in size, inductance of about 240nH, Q-factor of about 40 at 10MHz. The fabricated planar power inductor was implemented into a 24V input, 12MHz switching GaN-FET two-phase buck converter, and the converter exhibited efficiencies of 81% when 5V-3A output and 92% when 12V-2.5A output.

Novel power semiconductor devices with high withstand voltage and high-speed operation use digital isolators for isolated transmission of gate drive signals. The isolators are required to have the similar level of withstand voltage and operating speed as the power semiconductor devices. In this work, thin-film single-layer coreless micro-transformers fabricated on silicon chips are proposed for digital isolators in power switches operating at high voltage and high speed. This paper describes the dimensional dependence of the micro-transformers. Guidelines for dimensional optimization are discussed based on theoretical analysis of the relationship between transmission characteristics and the occupied area, the number of turns, and the conductor width for the micro-transformers.

This paper introduces an extended Cauer-equivalent circuit model of magnetic components that can represent both eddy-current effects and capacitive properties. As this model considers the skin and proximity effects of the winding as well as the parasitic capacitance, it is accurate from DC to more than 500 MHz, even with multiple resonances. The resonant characteristics are expressed by RLC series circuits in parallel with a Cauer circuit representing the inductive part. The proposed circuit is applied to two test inductors, and then the effectiveness of this method is verified.

The electrical and magnetic properties of the core composed of Electrolytic iron particles with different shapes (needle shape, flakey shape, and spherical shape) were investigated. In the frequency range below several tens kHz which the hysteresis loss is mainly dominant, the core loss became smaller in the order of the flakey particles, needle particles, and spherical particles. On the other hand, in the frequency range between several hundred kHz to several MHz which the eddy current loss is mainly dominant, the core loss of the flakey particles was the smallest of all particles. These results demonstrate that cross-sectional shape and area of particle in core play one of the important roles in the reduction of core loss.

In order to use pure water as an alternative to insulating oil, the conductivity of pure water must be at least four orders of magnitude lower. It is important to investigate the frequency characteristics of relative dielectric constant and conductivity of pure water to investigate the charge carriers that determine the conductivity. It is necessary to use metal electrodes to determine the relative dielectric constant and conductivity. However, since electrodes can affect relative dielectric constant and conductivity, true relative dielectric constant and conductivity are not yet clarified. In this paper, we investigated the frequency characteristics of the relative dielectric constant and conductivity of pure water dissolved in carbon dioxide using high purity carbon electrodes. As a result, it was clarified that the conductivity of pure water in which carbon dioxide was dissolved did not change much in low frequency region, but relative dielectric constant showed huge value as the frequency decreased.

For the self-inductance of single-layer regular polygonal coils, exact formulas applicable to coils with an arbitrary side number have been obtained, and their validity has been verified by experiment. In this letter, we attempted a quantitative evaluation of two previously proposed approximate calculation methods using the exact formulas. As a result, we got new knowledge about error characteristics and application conditions of both approximation methods.