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Hyunseok Choi,Yumi Lee,Songcheol Hong Professional Technical Group on Microwace Theory a 2014 IEEE Transactions on Microwave Theory and Techniqu Vol. No.
<P>A digitally modulated CMOS power amplifier (DPA) for a polar transmitter with a high dynamic range is presented. To improve local oscillator (LO) leakage, which limits the minimum output power of the DPA, the unit amplifier cell of the DPA employs a balanced mixer-type LO canceller at the power stage and a virtual ground is introduced in the layout of the output-power combining networks. A high dynamic range of the output power is simply achieved by adjusting gate-bias voltage of a cascode transistor in each power amplifier (PA) cell using a digitally controlled bias generator. This architecture allows a few-mA drain current at a low Tx power level without degrading the linearity. An array of unit PA cells is segmented to have a 10-bit amplitude resolution, and the bias generator is designed to have 8-bit control of the average output power. The peak output power is 24.4 dBm with an overall efficiency of 43% at 800 MHz. The total output dynamic range is 102.8 dB. A simple static pre-distortion helps the DPA achieve an average efficiency of 35% with a root mean square error vector magnitude of 3.59% while delivering linear output power of 22 dBm for WCDMA. The range of the digital transmit-power control for the WCDMA signal is 49.7 dB. This chip is fabricated in a 65-nm RF CMOS process.</P>
Hyunseok Choi(최현석),Tao Jiang(강타오),Hong Seok Kim(김홍석) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4
Titanium alloys are widely used in orthopedic devices because of their low density, high tensile strength, good corrosion resistance, and biocompatibility. However, since titanium and its alloys have inferior surface properties in terms of friction and wear resistance, there are still limitations in their application to the parts where sliding contact occurs. In this study, SiC nanoparticles were laser clad on the Ti-6Al-4V surface to simultaneously improve the friction and wear properties of the titanium alloys. Laser cladding was performed by changing the ambient conditions and laser parameters. The effect of process variables on the microstructure, hardness, and friction coefficient of the coating layer was analyzed. SiC nanoparticles reacted with the Ti element in the molten pool to create the reinforcement phases such as TiC and Ti<sub>5</sub>Si<sub>3</sub>. Oxygen supplied in the laser cladding process contributed to significantly reducing the coefficient of friction of the Ti-6Al-4V surface by creating the oxide phases such as TiO<SUB>2</SUB> and SiO<SUB>2</SUB>. Consequently, after the laser surface treatment of SiC nanoparticles in an oxygen environment, the hardness of Ti-6Al-4V improved by about three times and the coefficient of friction was reduced by more than 80%.
Dynamic driveline torque estimation during whole gear shift for an automatic transmission
Choi, Hyunseok,Hwang, Jaewoong,Choi, Seibum Elsevier 2018 Mechanism and machine theory Vol.130 No.-
<P><B>Abstract</B></P> <P>This study proposes a novel method for estimation of driveline torque of wet type AT (Automatic transmission). The previous methods of torque estimation in the clutch-to-clutch shift mechanism is mainly applicable to the single gear shift. Thus, they require specific definitions of each gear shift and signal processing corresponding to the multi-gear shift. This study distinctly develops driveline torque estimator based on the whole transmission model, which is capable in applying on any conditional gear shift. This paper, also, incomparably investigates brake/clutch model including centrifugal force, describing physical behavior of real parts. In addition, the development of the method includes adaptive brake and clutch torque estimator in order to compensate variation factor during the shift. The estimation performance of the proposed estimator is evaluated both in simulations and experiments. The results demonstrate that the proposed methods can estimate the driveline torques effectively during the whole gear shift and well describes the typical clutch-to-clutch phenomena such as inter-locking.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A whole gear shift model without signal processing for a vehicle with an AT is developed with practical aspects. </LI> <LI> Wet typed brake and clutch are modeled describing physical behavior of real parts. </LI> <LI> A torque estimator with adaptation is designed to estimate torque through both brakes or clutches and the output shaft in an AT. </LI> <LI> The effectiveness of the developed torque estimator is demonstrated by simulations and experiments. </LI> <LI> The proposed methods well describe the typical clutch-to-clutch phenomena such as inter-locking. </LI> </UL> </P>
A Triple-Power-Mode Digital Polar CMOS RF Power Amplifier With LO Duty Cycle Control
Choi, Hyunseok,Lee, Dong-Ho,Hong, Songcheol IEEE 2016 IEEE microwave and wireless components letters Vol.26 No.9
<P>A triple-power-mode digital polar CMOS power amplifier (DPA) with a high dynamic range is presented. The triple-power-mode to enhance the efficiency when operating under back-off power is facilitated by the combination of a switched output transformer to change the load impedance and a reduced duty cycle of the LO input signal of the DPA for additional back-off power. This static power-mode method for the low-power region can be used with a DPA which has a high dynamic range in conjunction with a digitally controlled bias generator. An improvement in the PAE from 14.5% to 28.7% is achieved at a back-off power of 6.5 dB from a peak output power of 21.54 dBm at 1.7 GHz. A digital transmit power control (TPC) range of 68.2 dB for WCDMA is realized without any external components.</P>