• Journal of Power Electronics
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• v.14 no.2
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• pp.282-291
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• 2014

This paper presents a detailed theoretical analysis and performance assessment of the capacitor voltage balancing strategies for staircase modulated modular multilevel converters (MMC) in terms of the algorithm structures, voltage balancing effect, and switching frequency. A constant-frequency redundancy selection (CFRS) method with minimal switching loss is proposed and the function realization of specific modules of the algorithm is given. This method is simple and efficient in both switching frequency and regulation capacity. Laboratory results show very good agreement with the theoretical analysis and numerical simulations.

• Journal of Power Electronics
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• v.14 no.6
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• pp.1130-1138
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• 2014

This study analyzes a ladder multilevel converter (double ladder topology) with the use of a new averaging modeling technique. This technique introduces an analytical method to compute for the switching losses and is used to conduct an in-depth analysis of the influence of the switching frequency and parasitic resistance of components on converter efficiency. The obtained results enable the selection of switches and switching frequency to minimize losses. Moreover, simulation results and experimental measurements validate the analytical calculations.

• Journal of Power Electronics
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• v.10 no.3
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• pp.277-284
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• 2010

This paper presents a novel hybrid carrier based space vector modulation for cascaded multilevel inverters. The proposed technique inherits the properties of carrier based space vector modulation and the fundamental frequency modulation strategy. The main characteristic of this modulation are the reduction of power loss, and improved harmonic performance. The carrier based space vector modulation algorithm is implemented with a TMS320F2407 digital signal processor. A Xilinx Complex Programmable Logic Device is used to develop the hybrid PWM control algorithm and it is integrated with a digital signal processor for hybrid carrier based space vector PWM generation. The inverter offers less weighted total harmonic distortion and it operates with equal electrostatic and electromagnetic stress among the power devices. The feasibility of the proposed technique is verified by spectral analysis, simulation, and experimental results.

• Korea Journal of Hospital Management
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• v.20 no.1
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• pp.53-64
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• 2015

Our objective was to evaluate differences in linear regression versus multilevel(cross-level interaction model) modeling for affecting factors lumbar discectomy. The data were used in 2011 patients with HIRA sample data. Total number of analysis is 3,641 patients and 248 hospitals. The results of research model showed that the type and location of the hospital-level factors were significant. However, all factors of patient-level were similar in the two models. Therefore, it requires the selection of an appropriate model for a more accurate analysis of the influencing factors in the daily medical charge.

• Coupled systems mechanics
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• v.1 no.3
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• pp.269-283
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• 2012

In the present paper, a multilevel approach for the local nanobuckling analysis of carbon nanotube (CNT) based composite materials is proposed and described. The approach comprises four levels, all of them at nanoscale. The first level aims to propose the potential that describes the interatomic forces between carbon atoms. In the second level, molecular dynamics simulations are performed to extract the elastic properties of the CNT. The third level aims to determine the stiffness of the material that surrounds the CNT (matrix), using the annular membrane analysis. In the fourth level, finite strip analysis of the CNT elastically restrained by the matrix is performed to calculate the critical strain at which the CNT buckles locally. In order to achieve accurate results and take the CNT-matrix interaction into account, the $3^{rd}$ and $4^{th}$ steps may be repeated iteratively until convergence is achieved. The proposed multilevel approach is applied to several CNTs embedded in a cylindrical representative volume element and illustrated in detail. It shows that (i) the interaction between the CNT and the matrix should be taken into account and (ii) the buckling at nanoscale is sensitive to several types of local buckling modes.

• Journal of Preventive Medicine and Public Health
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• v.55 no.5
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• pp.444-454
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• 2022

Objectives: We aimed to identify the factors related to depression and quality of life in patients with hypertension by using multilevel regression analysis. Methods: In 2019, 229 043 participants in the Korean Community Health Survey were selected as the study group. Individual factors were identified using data from the 2019 Community Health Survey. Regional factors were identified using data from the National Statistical Office of Korea. Multilevel regression analysis was conducted to find individual and local factors affecting depression and quality of life in patients with hypertension and to determine any associated interactions. Results: As individual factors in patients with hypertension, women, those with lower education-levels, recipients of basic livelihood benefits, and those with poor dietary conditions showed stronger associations with depression and quality of life. As regional factors and individual-level variables in patients with hypertension, lower gross regional personal income, fewer doctors at medical institutions, and lower rates of participation in volunteer activities presented stronger associations with depression and quality of life. In addition, the associations of depression with gross regional personal income, the number of doctors at medical institutions, and dietary conditions were significantly stronger in patients with hypertension than in patients without hypertension. The associations of gender and employment status with quality of life were also significantly greater. Conclusions: Policy interventions are needed to adjust health behaviors, prevent depression, and improve quality of life for patients with hypertension, especially for those with the risk factors identified in this study.

• Journal of the korean academy of Pediatric Dentistry
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• v.51 no.1
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• pp.40-54
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• 2024

This study aims to identify the regional distribution in the prevalence of dental caries and related multidimensional factors among 12-year-old children in Korea. Data from the 2018 Child Oral Health Survey were used to calculate the average DMFT index of 12-year-old children in metropolitan cities, and a multi-level regression model was applied to explain the regional distribution of dental caries prevalence and related factors. Factors were divided into two levels by administrative structure. This study finds a significant regional difference in the prevalence of dental caries in 12-year-old Korean children across metropolitan cities. Multilevel analysis showed that district-level factors (average number of pit and fissure-sealed permanent teeth, dental treatment demand rate, preventive treatment rate, sex ratio, and number of dentists per 100,000 people) and metropolitan-level factors (intakes of cariogenic beverages and number of pediatric dental hospitals and clinics per 100,000 people) had a significant effect on dental caries prevalence (p < 0.05). Individual characteristics and local socio-environmental factors influence the prevalence of dental caries. Especially considering the strong dependence on preventive treatment and accessibility to dental care services, it is necessary to provide adequate preventive treatment and expand health care resources in high-risk areas of dental caries.

• Journal of Korean Academy of Nursing
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• v.38 no.2
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• pp.287-297
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• 2008

Purpose: The purpose of this study was to identify factors related to nurses' organizational citizenship behavior using multilevel analysis which included both nurse characteristics at individual levels and nursing unit characteristics at group levels. Methods: The sample was composed of 1,996 nurses who were selected from 182 nursing units in 28 hospitals in six metropolitan cities and seven provinces using cluster sampling. Data were collected using self-reported questionnaires from February to March 2006. Results: The results of the study indicated that individual level variables related to organizational citizenship behavior were religion, job position, clinical career, self efficacy, positive affectivity, and supervisor support. The group level variables related to organizational citizenship behavior were collective efficacy, number of nurses in a nursing unit, and the average salary level of a nursing unit. 30.9% of individual level variances of organizational citizenship behavior were explained by the nurses' individual level variables. The explanatory power of group level variables, which is related to group level variances of organizational citizenship behavior, was 75.5%. Conclusion: This research showed that it was necessary to develop appropriate strategies related to not only individual factors, but also higher-level organizational factors such as collective efficacy, to improve individual performances in the hospital.

• Korean Journal of Health Education and Promotion
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• v.24 no.3
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• pp.35-49
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• 2007

Objectives: This study examines the relationship between social capital and suicidal impulse for the Metropolitan Seoul residents, paying particular attention on individual and area level risk factors using multilevel analysis techniques. Methods: Data are obtained from the 2005 Seoul Health Indicators Survey and the Seoul Survey. At the individual level, we analyze demographic variables including sex, age, marital status, educational attainments, family income, field of economic activities, having close friends or relatives, and having participating groups or organization. To measure the area level social capital, happiness index, attitude toward minority, the percent of volunteer activity, the percent of having close friends or relatives, and the percent of participating groups or organization are considered. A multilevel logistic regression analysis is conducted utilizing the SAS 9.1. Result: Results show that non-aged, female, not currently married, unemployed individuals tend to have relatively high motivation of suicide. Controlling for individual level covariates, the relationship between social capital and suicidal impulse is neither significant nor substantial. Further, the consideration of area social capital does not explain the variances in suicidal impulse across 25 Kus. Conclusion: Suicidal impulse among Seoul residents is mainly attributable to the composition of individual characteristics but little to the contextual social capital.

• Research in Community and Public Health Nursing
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• v.29 no.4
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• pp.476-487
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• 2018

Purpose: The present study is focused on understanding weight perception related with individual- and school-level multifactorial origins, underestimated and overestimated respectively, in normal body weight adolescents. Methods: Using the 2017 Korea Youth Risk Behavior Web-based Survey data of 45,902 students from 799 secondary schools, a multilevel multinomial logistic regression analysis was performed where adolescents (level1) were nested within schools (level 2). Results: At the school level, the average school body mass index (BMI, $kg/m^2$) and physical education were associated with weight perception among both boys and girls. In boys, geographic areas were associated with weight underestimation. At the individual level, perceived economic status, weekly allowance, BMI ($kg/m^2$), smartphone usage time (hrs/day) and perceived stress were associated with weight perception among both boys and girls. Age, paternal education, academic achievement and alcohol use were associated with weight perception among girls, while part-time job and physical activity were associated with weight perception among boys. Conclusion: Our findings underscore the importance of individual- and school-level environments in developing correct weight perception and have implications for school health education to establish healthy lifestyle behaviors for all adolescents.