• ETRI Journal
• /
• v.37 no.4
• /
• pp.677-684
• /
• 2015

Recently, orthogonal frequency-division multiplexing (OFDM) was applied to VLC systems owing to its high rate capability. On the other hand, a real-valued unipolar OFDM signal for VLC significantly reduces bandwidth efficiency. For practical implementation, channel estimation is required for data demodulation, which causes a further decrease in spectral efficiency. In addition, the large fluctuation of an OFDM signal results in poor illumination quality, such as chromaticity changes. This paper proposes a spectrally efficient method based on a hidden-pilot-aided precoding technology for VLC with less flickering than a conventional OFDM-based method. This approach can obtain channel information without any loss of bandwidth efficiency while ensuring illumination quality by reducing the flickering effect of an OFDM-based VLC. The simulation results show that the proposed method provides a 6.4% gain in bandwidth efficiency with a 4% reduction in flicker compared to a conventional OFDM-based method.

• The Bulletin of The Korean Astronomical Society
• /
• v.37 no.2
• /
• pp.130.1-130.1
• /
• 2012

Canceling magnetic features (CMFs) are likely to be a result of magnetic reconnection in the lower atmosphere of the Sun. CMFs are related with chromospheric phenomena such as brightening or jets. In order to observe the fine-scale and highly dynamical structures in the chromospheres, Fast Imaging Solar Spectrograph (FISS) was developed and installed at 1.6 m New Solar Telescope at Big Bear Solar Observatory. Using this FISS data we have studied chromospheric brightenings associated with CMFs. As a result, the chromospheric brightenings related with CMFs have stronger shock waves than one of other regions such as internetwork regions or unipolar magnetic elements

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.4 no.1
• /
• pp.283-288
• /
• 2000

This paper proposes a novel Pulse Width Modulation (PWM) method for microprocessor implementation of full-bridge PWM inverters. In the method, the period of unipolar full-bridge inverters is divided equally into the frequency modulation ratio and the reference fundamental amplitude and the inverter output fundamental amplitude are compared. Using this method, the turn-on time computation of each interval is very favored for the microprocessor implementation. The harmonic analysis shows that the harmonics can be ignored when the modulation ratio is enough large. The simulation and experimental results show the effectiveness of the proposed algorithms.

• The Transactions of the Korean Institute of Power Electronics
• /
• v.19 no.3
• /
• pp.227-232
• /
• 2014

A highly efficient MOSFET inverter for single-phase grid-connected photovoltaic power generation systems is presented in this paper. It is a full-MOSFET version of the conventional transformerless full-bridge inverter with dual L-C filters using unipolar PWM. The key idea lies on smart pre switching(SPS), which can make the large switching loss due to a poor reverse recovery of the MOSFET's body diode reduced dramatically. The validity of the proposed inverter is verified by experiment.

• Journal of the Korean institute of surface engineering
• /
• v.50 no.6
• /
• pp.432-438
• /
• 2017

AZ91D casting alloy requires an advanced plasma anodizing processing because large amount of defects are liable to generate during anodization. In this study, plasma electrolytic oxidation (PEO) of AZ91D Mg alloy was conducted by the application of either constant voltage or current using a pulse mode and its effects on pore formation, surface roughness and corrosion resistance were investigated. The PEO films showed a three-layer structure. The PEO film thickness was found to increase linearly with voltage. The surface roughness, Ra, ranged between $0.2{\mu}m$ and $0.3{\mu}m$. The corrosion resistance increased from RN 3.5 to 9.5 by the PEO treatment when evaluated according to the 72 hour salt spray test. The PEO-treated surface exhibited higher pitting potential than the raw material.

• Proceedings of the KIEE Conference
• /
• 2006.07c
• /
• pp.1560-1561
• /
• 2006

A high rate deposition sputtering process of magnesium oxide thin film in oxide mode has been developed using a 20 kW unipolar pulsed power supply. The power supply was operated at a maximum constant voltage of 500 V and a constant current of 40 A. The pulse repetition rate and the duty were changed in the ranges of $10{\sim}50\;kHz$ and $10{\sim}60%$, respectively. The deposition rate increased with rising incident power to the target. Maximum incident power to the magnesium target was obtained by the control of frequency, duty and current. The deposition rate of a moving state was 9 nm m/min at the average power of 1.5 kW.

• Proceedings of the KIEE Conference
• /
• 2003.07c
• /
• pp.1732-1734
• /
• 2003

An MgO thin film sputtering system for the PDP (Plasma Display Panel) applications has been developed. This system was manufactured with a vertical In-Line type of 42 inch, which has the length of 520 mm and the width of 900 mm. A reactive magnetron discharge for this sputtering was generated using an unipolar pulsed power supply which has functions of constant voltage (Max. 500 V) and current (Max. 15 A) control, frequency of $10{\sim}100$ kHz and duty ratio of $10{\sim}60$ %. The experiment was conducted under various conditions : $3{\sim}10$ mTorr of pressure, the ratio of $O_2$/Ar = $0.1{\sim}0.5$, 50 % of duty and power of $0.5{\sim}1.7$ kW. From the experiment, the deposition rate of a static state and a moving state were measured to be about 45 nm/min and 6 nm m/min at the distance of 50 mm between the target and the substrate, respectively.

• Proceedings of the KIEE Conference
• /
• 2003.07c
• /
• pp.1424-1426
• /
• 2003

PMN-PZT 적충형 세라믹 액츄에이터 소자를 제작한후 그 열화 특성을 조사하였다. 액츄에이터는 tape casting 방법으로 green sheet를 제작한후, screen printing방법을 이용하여 전극형상을 만들고, lamination과 cutting공정을 통하여 소자를 제작하였다. 작된 소자의 구조적인 특성을 분석하기 위하여 X-ray diffractometer를 사용하였으며, Doppler effect를 사용하는 laser vibrometer를 이용하여 전압에 따른 변위량을 측정하였다. 제작된 소자의 열화 특성을 알아보기 위하여 60 Hz의 triangular 교류 파형을 적충형 세라믹 액츄에이터에 인가하여 열화전과 후의 P-E hysteresis loop의 변화를 살펴보았으며, unipolar AC 전압을 지속적으로 인가하여 소자를 depling 시킴으로써 열화현상을 관찰하였다. 파단면에 대한 SEM 분석을 통하여 소자의 파괴 메카니즘을 알아 보도록 하였다. 이로부터 전기적, 기계적 열화가 소자의 동작에 미치는 영향에 대해서 알아 보았다.

• Proceedings of the KIPE Conference
• /
• 2001.07a
• /
• pp.25-29
• /
• 2001

The BLDCM (Brushless DC Motor) characterized by linear torque to current, and speed to voltage has low acoustic noise, and fast dynamic response. Moreover, it has high power density with high proportion of torque to inertia in spite of small size drive. But, it produce torque ripple due to the motor inductance components in stator windings and back-EMF, when armature current is commutated. Therefore, it is difficult to apply the BLDCM to a precision servo drive system. In this paper is proposed to a new current control algorithm with using fourier series coefficients can minimize torque ripple due to the phase current commutation of BLDCM. Simulation and Experimental results prove the effectiveness the proposed algorithm through comparison with the conventional used unipolar PWM method.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.66 no.1
• /
• pp.40-47
• /
• 2017

In this paper, a small signal model for grid-connected inverter with unipolar pulse width modulation method is presented. Small-signal analysis allows to predict the stability and dynamics of the inverter. To regulate output voltage and to achieve power factor correction, inverter has two control loops. Loop gains are useful to identify the stability for multi-loop controlled system. Based on small-signal model, controllers are designed to improve audio susceptibility and output impedance characteristics. Proposed small-signal model and controllers are verified by PSIM simulation and experiments.