• Journal of Power Electronics
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• v.9 no.1
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• pp.36-42
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• 2009

This paper presents a review of an improved high power-high frequency III-V wide bandgap (WBG) semiconductor device, Gallium Nitride (GaN). The device offers better efficiency and thermal management with higher switching frequency. By having higher blocking voltage, GaN can be used for high voltage applications. In addition, the weight and size of passive components on the printed circuit board can be reduced substantially when operating at high frequency. With proper management of thermal and gate drive design, the GaN power converter is expected to generate higher power density with lower stress compared to its counterparts, Silicon (Si) devices. The main contribution of this work is to provide additional information to young researchers in exploring new approaches based on the device's capability and characteristics in applications using the GaN power converter design.

• Journal of the Korea Institute of Military Science and Technology
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• v.15 no.4
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• pp.450-457
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• 2012

In this paper, we designed low phase noise frequency synthesizer with compact size for High frequency band (Ku-band). The paper addresses merits and demerits of single loop and dual loop frequency synthesizer. The phase noise characteristics of the phase-locked loop frequency synthesizer were predicted based on the analysis for phase noise contribution of noise sources. The proposed model in this paper more accurately predicts the low phase noise frequency synthesizer with compact size for high frequency band.

• Journal of the Korea Institute of Military Science and Technology
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• v.22 no.5
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• pp.716-723
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• 2019

It is crucial to understand the characteristics of cloud occurrence frequency for development of high precision guided missile using infrared imaging sensor. In this paper, we investigated the vertical structure of cloud for altitude using upper-air observation data. We find that cloud occurrence frequency is high at altitudes of 1.3 km and 9.5 km. Theses features have seasonal and temporal dependency. In the summer, cloud often occur more than average regardless of altitude. In the winter, low clouds occur frequently, and high clouds do not occur well. In temporal characteristics, clouds occur more frequently in daytime than in nighttime regardless of altitude. Many of clouds exist in single layer or double layers in the air. We also find that the 40 % of cloud occurrence frequency at high altitude when low clouds under altitude of 2 km cover entire sky.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2000.05a
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• pp.1021-1026
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• 2000

The thermal characteristics of high-speed air spindle system with built-in motor are studied. Experiment and finite element method analysis obtain temperature rise and temperature distribution of housing. For the analysis three-dimensional model is built and temperature rise and distribution in thermal steady state are computed for each rotational frequency. Generally. It is said that the heat generation of air bearing is negligible. But it is certain that the heat generation of air bearing can not be negligible especially in high-speed conditions Frequency response test for air spindle system is executed. In case that the heat generation of air spindle system is high, natural frequency of the system becomes lower when it reaches thermal steady-state and it means that the stiffness of air hearing becomes smaller due to the change of bearing clearance. It is shown that the temperature rise of all spindle system causes thermal expansion md induces the variation of hearing clearance. In consequence the st illness of air bearing becomes smaller.

• Proceedings of the Korean Magnestics Society Conference
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• 2002.12a
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• pp.214-215
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• 2002

High saturation magnetization (B$\_$s/) and excellent high frequency characteristics of magnetic thin films has been recognized as the most important requirement for further miniaturization and higher frequency operation in magnetic devices, such as magnetic heads and magnetic sensors. Up to now, Fe-X-N (X =Ti, Hf, and Al etc.) films with B$\_$s/ of 15 ∼ 19 kG and coercivity (H$\_$c/) of 0.5 ∼ 5.0 Oe have been successfully fabricated and proven to satisfy various requirements as a potential candidate for thin film head materials. (omitted)

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.26 no.12
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• pp.87-92
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• 2012

In this paper, EHD(electrohydrodynamics) characteristics of AC corona discharge for the various frequency was investigated. Ionic wind velocity is controlled by the frequency of applied ac high voltage, and maximum velocity of the ionic wind is obtained at 1.2kHz. Maximum velocity are 1.90 m/s by metal corona electrode and 2.72m/s by wet porous corona electrode, These attain 91~99% of the maximum velocity in the DC corona discharge by adjusting the frequency through the experiments. In this paper, wet porous corona electrode has high possibility of cooling methode because a AC corona discharge using wet porous corona electrode is able to eject more water droplets than DC corona discharge.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.65-71
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• 2001

Absorbers such as porous materials and panels have limited absorption characteristics to some frequency bands. There is a need for absorbers with high absorption coefficients in a wide frequency ranges to make good response of room acoustics. This is almost impossible for a single absorption material. Composite absorption structure with cover, porous material. and air gap is known to have those wide frequency characteristics. In this basis. various composite absorption structures are measured and investigated as wide range absorption structures. Measurements are performed according to an international standard, ISO 354. Various surface types such as wooden slits, wood/steel perforated panels are selected as surface covers, and also various porous materials such as polyurethanes, polyesters, and glasswools are used inside the covers. Result shows that the area of void parts of surface materials is critical to high frequency absorptions, and thickness of air gaps are critical factor of the peak absorptions of low frequency bands.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1997.04a
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• pp.551-556
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• 1997

Based on the experiment for the frequency characteristics and the feedback theory of the impinging-tones, the unstable characteristics of the symmetric mode is analyzed among the various unstable modes of circular impinging jets. There are two different symmetric modes; one is the low-frequency mode S1 due to the vortex at the outside of the jet and the high-frequency mode S2 due to the inside vortex. Each mode has its own characterictics of convection speed decreasing with frequency.

• Journal of information and communication convergence engineering
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• v.6 no.3
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• pp.301-304
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• 2008

This paper presents a lossless inductor snubber-assisted series resonant high frequency inverter using the trench gate IGBTs for the electromagnetic induction eddy current-heated fixing roller in copy and printing machines in a new generation. This soft switching high-frequency inverter can completely achieve zero current soft switching (ZCS) commutation mode transitions and wide power regulation under its constant frequency pulse density modulation (PDM) scheme. Its transient and steady state operating principle is originally presented for a constant frequency PDM strategy under a ZCS operation, together with its output power regulation characteristics-based on the pulse density modulation. The effectiveness of this soft switching high-frequency inverter is confirmed for the fixing equipment with the induction-heated fixing roller.

• Proceedings of the Acoustical Society of Korea Conference
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• 1994.06a
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• pp.824-829
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• 1994

High-frequency and complex vibration ultrasonic wire bonding systems are propsed and their welding characteristic are studied. Ultrasonic wire bonding is used widely for joining thin connecting wire of various electronic devices including IC or LSI. Conventional bonding systems use vibration frequency of 40 or 60 kHz and linear vibration welding tips. Complex vibration welding tip which vibrates in elliptical to circular or rectangular to square in the same or different frequency is effective to join welding specimens in shorter welding time and under smaller vibration amplitude, and furthermore high-frequency systems such as 90, 120, 190 kHz are also significantly effective. High-frequency and complex vibration welding system of 90, 120 and 190 kHz are designed. Welding characteristics of these systems are found very superior than a conventional system. Welding specimens of aluminum wire of 0.1mm diameter are successfully.