Nobuo Satoh

Verification of charge and discharge system composed of multiple lithium ion batteries

We constructed a charge and discharge system for effective utilization of an electric storage device composed of batteries. We verified the system operation by simulation analysis and actual measurement. The simulation analysis showed that it is useful to alternately use two secondary batteries as the lithium ion battery (LIB). From the measurement of 60 cycles, the use of secondary batteries under the condition of the total energy amount has proven to be recommended for alternating use with switching control.

A flyback converter using power-MOSFETs to achieve high-frequency operation beyond 10MHz

In this study, we demonstrated an isolated-type flyback converter circuit for high-frequency operations. We estimated the high-frequency characteristics of passive devices in various conditions of a DC-DC converter circuit. We experimentally developed the isolated-type flyback converter circuit and evaluated the frequency response properties. In order to verify the fast-switching operation, the input voltages, output voltages, and currents of the converter were measured for both continuous and discontinuous operations.

Surface potential measurement of n-type organic semiconductor thin films by mist deposition via Kelvin probe microscopy

We partially deposited fullerene (C60) and phenyl-C61-butyric acid methyl ester thin films that are typical n-type semiconductor materials on indium–tin oxide by mist deposition at various substrate temperatures. The topographic and surface potential images were observed via dynamic force microscopy/Kelvin probe force microscopy with the frequency modulation detection method. We proved that the area where a thin film is deposited depends on the substrate temperature during deposition from the topographic images. It was also found that the surface potential depends on the substrate temperature from the surface potential images.

Observation of silicon carbide Schottky barrier diode under applied reverse bias using atomic force microscopy/Kelvin probe force microscopy/scanning capacitance force microscopy

We have observed a commercial silicon-carbide Schottky barrier diode (SiC-SBD) using our novel analysis system, in which atomic force microscopy (AFM) is combined with both Kelvin probe force microscopy (KFM; for surface-potential measurement) and scanning capacitance force microscopy (SCFM; for differential-capacitance measurement). The results obtained for the SiC-SBD under an applied reverse bias indicate both the scan area in the sample and a peak value of the SCFM signal in the region where the existence of trapped electrons is deduced from the KFM analysis. Thus, our measurement system can be used to examine commercial power devices; however, novel polishing procedures are required in order to investigate the Schottky contact region.

Evaluation of carrier concentration reduction in GaN-on-GaN wafers by Raman spectroscopy and Kelvin force microscopy

The carrier concentration in a gallium nitride (GaN) substrate of a GaN-on-GaN wafer grown by hydride vapor phase epitaxy (HVPE) was evaluated by Raman spectroscopy and Kelvin probe force microscopy (KFM). On the basis of the longitudinal optical phonon–plasmon coupled (LOPC) mode of Raman spectra and surface potential measurements by KFM, the carrier concentration at the periphery of the HVPE-GaN substrate was found to be about one order of magnitude lower than that at the center. The decrease in carrier concentration is considered to be due to the out-diffusion of dopants during the metal organic chemical vapor deposition (MOCVD) of the epitaxial layer. In silicon (Si) epitaxial wafers, the autodoping of out-diffused dopants introduces nonuniform device characteristics. This undesirable effect needs to be suppressed to successfully move from prototype GaN-on-GaN power devices to commercial products.

Nanoscale observation of organic thin film by atomic force microscopy

Organic photovoltaics (OPVs) fabricated using organic semiconductors and hybrid solar cells (HSCs) based on organic semiconductors/quantum dots (QDs) have been attracting significant attention owing to their potential use in low-cost solar energy-harvesting applications and flexible, light-weight, colorful, large-area devices. In this study, we observed and evaluated the surface of a photoelectric conversion layer (active layer) of the OPVs and HSCs based on phenyl-C61-butyric acid methyl ester (PCBM), poly(3-hexylthiophene) (P3HT), and zinc oxide (ZnO) nanoparticles. The experiment was performed using atomic force microscopy (AFM) combined with a frequency modulation detector (FM detector) and a contact potential difference (CPD) detection circuit. We experimentally confirmed the changes in film thickness and surface potential, as affected by the ZnO nanoparticle concentration. From the experimental results, we confirmed that ZnO nanoparticles possibly affect the structures of PCBM and P3HT. Also, we prepared an energy band diagram on the basis of the observation results, and analyzed the energy distribution inside the active layer.

A flyback converter using power MOSFET to achieve high frequency operation beyond 13.56 MHz

This paper discusses the development of 5-watts-class flyback converter which can operate at switching frequencies from 1 to 16 MHz. In order to analyze fast switching operations, the input and output voltages with currents are measured for estimating the converter. The self-induced soft switching operation is experimentally investigated at a ringing frequency caused by parasitic components, including junction capacitances in SiC MOSFET.