Browsing by Author "Wu, Chih-Chiang"

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A novel no-dead-time sinusoidal modulation method for three phase inverter
(2017-01-01)
Jeng, Shyr-Long
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Wu, Chih-Chiang
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機械工程學系
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Department of Mechanical Engineering
A lthough the dead time in pulse-width modulation (PWM) inverters prevents short circuiting of the dc source, it also causes serious problems such as waveform distortion and voltage decreases. In this paper, a novel modulation method that combines the concepts of a six-step square wave and space vector modulation (SVM) is proposed. Compared with the conventional SVM methods in which the dead time should be set to avoid short circuiting problems, the proposed novel no-dead-time modulation method not only increases the switching frequency potential by enabling one transistor at a time in each phase leg without considering dead time setting but also reduces the switching loss with only three transistors operating in every section in one cycle of rotation. The PWM duty ratio formula of the specified section for each power transistor is derived and verified using software for power electronics simulation (PSIM). The simulation model of this novel modulation method is constructed and tested by using a three-phase inductance-resistance load. The simulation results are presented to confirm the validity of the proposed modulation method.
Comparison of Parasitic Capacitances of Packaged Cascode Gallium Nitride Field-efect Transistors
(2018-01-01)
Wu, Chih-Chiang
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Jeng, Shyr-Long
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機械工程學系
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Department of Mechanical Engineering
In this work, we examined the electrical characteristics of laboratory-fabricated cascode gallium nitride field-effect transistors (GaN FETs) and analyzed their parasitic capacitances. The calculated results were in good agreement with the experimental results and showed that commercial GaN FETs have superior switching performance, whereas laboratory-fabricated GaN FETs require further improvement.
Forecasting Time-Varying Covariance with a Robust Bayesian Threshold Model
(2011-08-01)
Wu, Chih-Chiang
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Lee, Jack C.
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資訊管理與財務金融系註：原資管所+財金所
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Department of Information Management and Finance
This paper proposes a robust multivariate threshold vector autoregressive model with generalized autoregressive conditional heteroskedasticities and dynamic conditional correlations to describe conditional mean, volatility and correlation asymmetries in financial markets. In addition, the threshold variable for regime switching is formulated as a weighted average of endogenous variables to eliminate excessively subjective belief in the threshold variable decision and to serve as the proxy in deciding which market should be the price leader. The estimation is performed using Markov chain Monte Carlo methods. Furthermore, several meaningful criteria are introduced to assess the forecasting performance in the conditional covariance matrix. The proposed methodology is illustrated using daily S&P500 futures and spot prices. Copyright (C) 2010 John Wiley & Sons, Ltd.
Gallium Nitride Electrical Characteristics Extraction and Uniformity Sorting
(2015-01-01)
Jeng, Shyr-Long
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Wu, Chih-Chiang
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Chieng, Wei-Hua
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機械工程學系
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Department of Mechanical Engineering
This study examined the output electrical characteristics-current-voltage (I-V) output, threshold voltage, and parasitic capacitance-of novel gallium nitride (GaN) power transistors. Experimental measurements revealed that both enhanced-and depletion-mode GaN field-effect transistors (FETs) containing different components of identical specifications yielded varied turn-off impedance; hence, the FET quality was inconsistent. Establishing standardized electrical measurements can provide necessary information for designers, and measuring transistor electrical characteristics establishes its equivalent-circuit model for circuit simulations. Moreover, high power output requires multiple parallel power transistors, and sorting the difference between similar electrical characteristics is critical in a power system. An isolated gate driver detection method is proposed for sorting the uniformity from the option of the turn-off characteristic. In addition, an equivalent-circuit model for GaN FETs is established on the basis of the measured electrical characteristics and verified experimentally.
HIGH-PERFORMANCE SINGLE-PHASE FULL-BRIDGE INVERTER USING GALLIUM NITRIDE FIELD EFFECT TRANSISTORS
(2017-01-01)
Wu, Chih-Chiang
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Jeng, Shyr-Long
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機械工程學系
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Department of Mechanical Engineering
This paper presents the perfonnance of a single-phase full-bridge inverter based on wide-bandgap devices. The control strategy for the full-bridge inverter applies unipolar sinusoidal pulse width modulation. The experimental results demonstrated that a smaller figure of merit is preferred for a more efficient design; specifically, the full-bridge inverter using gallium nitride field effect transistors inside could easily reach 96% efficiency or more within a 100- to 1000-W range.
Investor sentiment and evaporating liquidity during the financial crisis
(2018-05-01)
Chiu, Junmao
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Chung, Huimin
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Ho, Keng-Yu
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Wu, Chih-Chiang
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資訊管理與財務金融系註：原資管所+財金所
This study examines how investor sentiment affected equity liquidity and trading behavior during the financial crisis of 2007-2008. Using intraday data on equity index and financial ETFs, we show significant asymmetric response to investor sentiment on quoted spread, market depth, asymmetric depth, and net buying pressure. We also document that funding constraints can further increase the asymmetric impact of investor sentiment on liquidity and trading behavior. Our results can be explained by the psychological bias of negativity and help investors and risk management practitioners comprehensively understand why and how the evaporation of liquidity accelerates during the financial crisis.
Simulation Model Development for Packaged Cascode Gallium Nitride Field-Effect Transistors
(2017-08-01)
Wu, Chih-Chiang
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Jeng, Shyr-Long
;
機械工程學系
;
Department of Mechanical Engineering
This paper presents a simple behavioral model with experimentally extracted parameters for packaged cascode gallium nitride (GaN) field-effect transistors (FETs). This study combined a level-1 metal-oxide-semiconductor field-effect transistor (MOSFET), a junction field-effect transistor (JFET), and a diode model to simulate a cascode GaN FET, in which a JFET was used to simulate a metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT). Using the JFET to simulate the MIS-HEMT not only ensures that the curve fits an S-shape transfer characteristic but also enables the pinch-off voltages extracted from the threshold voltage of the MIS-HEMT to be used as a watershed to distinguish where the drop in parasitic capacitance occurs. Parameter extraction was based on static and dynamic characteristics, which involved simulating the behavior of the created GaN FET model and comparing the extracted parameters with experimental measurements to demonstrate the accuracy of the simulation program with an integrated circuit emphasis (SPICE) model. Cascode capacitance was analyzed and verified through experimental measurements and SPICE simulations. The analysis revealed that the capacitance of low-voltage MOSFETs plays a critical role in increasing the overall capacitance of cascode GaN FETs. The turn-off resistance mechanism effectively described the leakage current, and a double-pulse tester was used to evaluate the switching performance of the fabricated cascode GaN FET. LTspice simulation software was adopted to compare the experimental switching results. Overall, the simulation results were strongly in agreement with the experimental results.
Switching performance of GaN FETs in terms of turn-off resistance characteristics: an experimental study
(2016-01-01)
Wu, Chih-Chiang
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Jeng, Shyr-Long
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機械工程學系
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Department of Mechanical Engineering
This study evaluated the AC parasitic capacitances and turn-off resistance of GaN FETs. Moreover, the study proposes an easily adjustable gate drive circuit appropriate for driving E-mode, cascode, and D-mode GaN FETs in the same driver topology. The proposed gate drive circuit involving only positive supply was established to provide not only a positive gate drive voltage to E-mode and cascode GaN FETs, but also a negative gate drive voltage to D-mode GaN FETs. This study also presents a simple circuit for shifting gate drive voltage levels to meet the driving voltage required for fully turning on or turning off GaN FETs. In cascode GaN FETs, the gate drive circuit incorporates additional resistor-capacitor-Zener diode (RCD) circuits applied to achieve a faster discharge rate, thereby increasing the switching speed by offering a negative turn-off voltage when the transistor is engaging in a turn-off process. In D-mode GaN FETs, the additional RCD circuits increase the drain current to enable the gate drive voltage to increase, resulting in the power transistor being turned on, thereby achieving a full conduction state to reduce on-state resistance. This paper presents comparisons of the switching performance of the different GaN FETs through the proposed gate drive circuit.
The economic value of co-movement between oil price and exchange rate using copula-based GARCH models
(2012-01-01)
Wu, Chih-Chiang
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Chung, Huimin
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Chang, Yu-Hsien
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資訊管理與財務金融系註：原資管所+財金所
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Department of Information Management and Finance
The US dollar is used as the primary currency of international crude oil trading; as such, the recent substantial depreciation in the US dollar has resulted in a corresponding increase in crude oil prices. In addition, oil price and exchange-rate returns have been shown to be skewed and leptokurtic, and to exhibit an asymmetric or tail dependence structure. Therefore, this study proposes dynamic copula-based GARCH models to explore the dependence structure between the oil price and the US dollar exchange rate. More importantly, an asset-allocation strategy is implemented to evaluate economic value and confirm the efficiency of the copula-based GARCH models. In terms of out-of-sample forecasting performance, a dynamic strategy based on the CGARCH model with the Student-t copula exhibits greater economic benefits than static and other dynamic strategies. In addition, the positive feedback trading activities are statistically significant within the oil market, but this information does not enhance the economic benefits from the perspective of an asset-allocation decision. Finally, a more risk-averse investor generates a higher fee for switching from a static strategy to a dynamic strategy based on copula-based GARCH models. (C) 2011 Elsevier B.V. All rights reserved.