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Jungjoon Kim,Yeonjoo Lee,Dohun Kwon,Hwijun Kim,Hyunjoo Choi 한국자기학회 2020 Journal of Magnetics Vol.25 No.2
In this study, iron-based amorphous powders with two different sizes, which are the average sizes of 18.92 μm and 74.69 μm, were mixed to increase the powder packing fraction and resulting soft magnetic properties. By varying the mixing ratio, the powder packing fraction was experimentally measured and also estimated by the Desmond model and the computational simulation on the basis of the discrete element method (DEM). As a result, the DEM simulation exhibited higher validity compared to the Desmond model possibly because it accounts for the interaction between the powders, such as repulsion and aggregation, which are not considered in the Desmond equation. Finally, the maximum powder packing fraction of 73.86 % was achieved when the powders were mixed at the ratio of 5:3 (~25 μm: 45~63 μm). This ratio produced an increase of 32.5 % for coercivity and 17.8 % for saturated magnetization compared to the case of 100 % large powders with a 74.69 μm average diameter.
Optimization of Envelope Tracking Power Amplifier for Base-Station Applications
Jungjoon Kim,Jungwhan Son,Seunghoon Jee,Seokhyeon Kim,Bumman Kim IEEE 2013 IEEE transactions on microwave theory and techniqu Vol.61 No.4
<P>We have proposed two methods of enhancing efficiency of an envelope tracking power amplifier (ET PA) from an interlock operation. The first is the utilization of sinking current. The sinking current is a critical efficiency reduction factor since it is a wasted power. To reduce the sinking current, the gate bias of the power amplifier (PA) is increased so that the sinking current is delivered to the PA and is utilized for amplification. The other one is the RF input shaping method. The input signal of ET PAs is a modulated RF signal, and the signal does not guarantee fully saturated operation of the PA at all power levels due to <I>g m</I> nonlinearity of a device. To obtain the maximum efficiency for all of the envelope voltage, we have found the optimum RF input conditions and applied it to the input of the PA. To verify the methods, the proposed ET PA is implemented using a Cree CGH40045 GaN HEMT. For a long-term evolution 5-MHz signal with 6.5-dB peak-to-average power ratio, the PA delivers power-added efficiency of 58.76% with 40.14-dBm output power at 889 MHz.</P>
Jungjoon Ihm,Hyoseon Choi,Sangho Roh 한국의학교육학회 2017 Korean journal of medical education Vol.29 No.2
Purpose: This study explores how to design a flipped classroom for a predental science course and evaluate its course through student self-assessment in order to provide practical implications for flipped learning in an undergraduate level. Methods: Second- and third-year predental students in the Seoul National University School of Dentistry enrolled in Biodiversity and Global Environment, a 15-week, three-credit course based on a flipped learning model. At the end of the course, the students were asked to rate their self-directed learning, attitude toward social media, discussion skills, learning readiness, and class satisfaction. Out of the 82 predental students, 61 (74.3%) answered the survey. Pearson correlation and multivariate regression analyses were employed to examine the relationship between the self-rated measurements and the performance scores. Results: The majority of the students felt somewhat more prepared than the medium level before the class (mean score of 3.17 out of 5.00), whereas they expressed relatively low preference concerning social media use and attitude (mean score of 2.49). Thus, it was found that learning readiness was significantly associated with both discussion skills and class satisfaction. In particular, multivariate regression analysis confirmed that learning readiness had a significant influence on learning outcomes. Conclusion: This study offered insights into how to design a flipped learning course in terms of predental students’ preference and their learning readiness. Although learning success in a flipped classroom depends on the students’ self-perceived level of preparedness, much still remains to be achieved in order to apply social media benefits in a flipped learning context.