• Journal of Power Electronics
• /
• v.14 no.6
• /
• pp.1224-1232
• /
• 2014

A full-bridge secondary dual-resonant DC-DC converter using the asymmetrical pulse-width modulated (APWM) strategy is proposed in this paper. The proposed converter achieves zero-voltage switching for the power switches and zero-current switching for the rectifier diodes in the whole load range without the help of any auxiliary circuit. Given the use of the APWM strategy, a circulating current that exists in a traditional phase-shift full-bridge converter is eliminated. The voltage stress of secondary rectifier diodes in the proposed converter is also clamped to the output voltage. Thus, the existing voltage oscillation of diodes in traditional PSFB converters is eliminated. This paper presents the circuit configuration of the proposed converter and analyzes its operating principle. Experimental results of a 1 kW 385 V/48 V prototype are presented to verify the analysis results of the proposed converter.

• Journal of The Korean Association For Science Education
• /
• v.21 no.5
• /
• pp.944-967
• /
• 2001

The purpose of this study is to compare and analyze secondary school chemistry education in Korea and China in order to identify implications for the improvement of Korean chemistry education. The school systems, curricula, and teacher education related to secondary chemistry education of both countries were compared and analyzed. The 6-3-3-4 school system is used in both countries, and national school curricula are formulated by the Ministries of Education in both countries. The 1996 chemistry curricular standard for advanced middle schools in China, and 1997 chemistry curriculum for Korean high schools were compared in several aspects, followed by comparisons of chemistry teacher education in both countries. Based on the comparative analysis of chemistry education, some ideas and issues which provide implications for improving Korean high school chemistry education were identified. Chemistry teaching in the junior secondary schools, tentative implementation of curricula, required course work in chemistry, structure of curricula, oral assessments, probation of teachers and other issues are identified and discussed in this study.

• Journal of information and communication convergence engineering
• /
• v.12 no.2
• /
• pp.97-103
• /
• 2014

In spectrum sensing, there is a tradeoff between the probability of missed detection and the probability of a false alarm according to the value of the sensing threshold. Therefore, it is important to determine the sensing threshold suitable to the environment of cognitive radio networks. In this study, we consider a cognitive radio-based ad hoc network where secondary users directly communicate by using the same frequency band as the primary system and control their transmit power on the basis of the distance between them. First, we investigate a condition in which the primary and the secondary users can share the same frequency band without harmful interference from each other, and then, propose an algorithm that controls the sensing threshold dynamically on the basis of the transmit power of the secondary user. The analysis and simulation results show that the proposed sensing threshold control algorithm has low probabilities of both missed detection and a false alarm and thus, enables optimized spectrum sharing between the primary and the secondary systems.

• 한국전산유체공학회:학술대회논문집
• /
• 1998.11a
• /
• pp.114-120
• /
• 1998

The present numerical study investigates flow characters and heat transfer enhancement by the viscoelastic-driven secondary flow and buoyancy effect in a 2:1 rectangular duct. Three versions of thermal boundary conditions involving difference combination of heated walls and adiabatic walls are analyzed in this study. The Reiner-Rivlin model is adopted as a viscoelastic fluid model to simulate the secondary flow and temperature-dependent viscosity model is used. Calculated Nusselt numbers are very good agreement with experimental results for reported viscoelastic fluids. It is found that the heat transfer enhancement is mainly caused by the viscoelastic-driven secondary flow and buoyancy-induced secondary flow play a role of promoting this effect.

• Proceedings of the KIEE Conference
• /
• 2002.07c
• /
• pp.1745-1747
• /
• 2002

This paper describes the effect of secondary surges entering low voltage side on the primary winding of pole transformers. After having connected the secondary winding by 4 different methods, surge voltages were applied to the low voltage side, and voltage waveforms were measured at a surge applied point and a high voltage bushing terminal. The measured voltages were compared by the waveform, magnitude and damping characteristics with each connection. As a result, the voltage waveforms induced by the secondary surges were different one another with each connection, especially, the conventional connecting method for 2 voltage sources was far different from the present method supplying only 1 voltage source in shape of the voltage waveform.

• Journal of Korea Technical Association of The Pulp and Paper Industry
• /
• v.46 no.3
• /
• pp.37-43
• /
• 2014

Kinetics of holocellulose hydrolysis in concentrated sulfuric acid was analyzed using $^1H$-NMR spectroscopy with different reaction time, temperature and acid concentration in secondary hydrolysis. In this work, reaction condition of secondary hydrolysis was similar to concentrated sulfuric acid process with electrodialysis or simulated moving bed chromatography process for sulfuric acid recycling. By $^1H$-NMR spectroscopy, acid hydrolyzates from higher secondary acid hydrolysis (25-35% acid concentration) was successfully analyzed without any difficulties in neutralization or adsorption of acid hydrolyzate to solid salt. Higher acid concentration, higher temperature and longer reaction time led to more cellulose for glucose conversion but accompanied with glucose to galactose isomerization, glucose to unknown compounds and degradation of glucose to organic acid via furans.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2004.05b
• /
• pp.381-385
• /
• 2004

본 인구에서는 레이놀즈응력모형을 이용하여 직사각형 개수로 흐름을 수치모의 하고 이차흐름의 생성 메커니즘을 제시하였다. 수치모의 결과 자유수면과 측벽의 접합부 근처에서 inner secondary flow가 발생하였다. 이는 최근 Grega 등(1995)과 Hsu 등(2000)에 의해 밝혀진 새로운 이차흐름이다. 또한 측벽에서의 전단력 분포를 계산한 결과 inner secondary flow에 의하여 수면 근처에서의 전단력 값이 증가하는 것으로 나타났다. 계산된 결과를 이용하여 와도 방정식에서 각 항의 크기를 비교하여 이차 흐름의 생성 메커니즘을 살펴보았다. 그 결과 벽 및 측벽 경계 부근에서는 난류의 비등방성에 의한 와도 생성항에 의해 이차 흐름이 생성되고, 경계와 멀리 떨어진 영역에서는 레이놀즈응력에 의한 와도 생성항이 이차흐름을 생성시키는데 중요한 역할을 하는 것으로 나타났다.

• Korean Journal of Ecology and Environment
• /
• v.43 no.1
• /
• pp.19-23
• /
• 2010

Life cycle and secondary production of Nymphs of Nemoura gemma Ham and Lee were estimated by using specimens collected from a stream in Mt. Jumbong in the central Korean peninsula. N. gemma in the study stream was univoltine. Youngest nymphs were collected in April. They appeared to grow continuously until the emergence in early spring next year. The cohort production interval for the species was estimated as 399 days. The annual secondary production (ash free dry weight) estimated by removal-summation and the size-frequency methods were 582 and $786\;mg\;m^{-2}\;yr^{-1}$, respectively. Gut content analysis showed that N. gemma was a shredder.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.59 no.7
• /
• pp.1276-1281
• /
• 2010

To analyze the gas discharge phenomena in parallel-plane electrodes, the fully coupled finite element method (FEM) considering secondary electron emission effects in discharge column was adopted in this paper. Two coupled equations of the hydrodynamic diffusion-drift equations for three carriers and the Poisson's equation for electric scalar potential should be solved as a system equation. The proposed method including two secondary electron processes of the photoemission and background ionization has been successfully applied to evaluating the breakdown voltage in parallel-plane electrodes and is verified by comparing its numerical results with the experimental ones. From the obtained results, it is inferred that the proposed numerical scheme will be useful for predicting and understanding streamer transient phenomena.

• Proceedings of the Korean Institute of Building Construction Conference
• /
• 2019.05a
• /
• pp.74-75
• /
• 2019

There are certain types of construction that produces many defects during the construction of apartment buildings. It may be due to construction errors in the type itself, but more often, it is due to defects derived from other types of construction. Thus, to reduce such secondary defects and prevent fundamental defects in particular types of construction, academic research on the types of construction causing secondary defects should be actively conducted. In addition, as claimed in this study, the industries should work harder to improve the quality of the type of construction that causes secondary defects, rather than just trying to improve the ostensible problems. If the efforts of each sector are backed up, the disputes over defect repair in apartment building construction will reduce in general, which in turn will lead to a reduction in national loss within the construction industry.