S.Y.R. Hui

The University of Hong Kong, Hong Kong, Hong Kong

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Publications (259)444.95 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: Visualizing the magnetic flux paths for wireless power transfer systems enables researchers and engineers understand the operations and design the geometrical dimensions of the practical systems. However, time-domain transient simulations of 3-D electromagnetic fields of complex wireless power transfer systems with multiple coil-resonators are extremely time-consuming. This paper describes a fast hybrid method that combines the time-domain coupled circuit modeling and the magnetostatic analysis to form a fast time-domain analytical tool for studying complex wireless power transfer systems. The proposed methodology has been successfully applied to several wireless domino-resonator systems. For the first time, the time-varying magnetic flux variations of wireless power dominoresonator systems can be visualized in computer simulations.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: Hooke's law for mechanical springs was developed in the 17th century. Recently, new power electronics devices named “electric springs” have been developed for providing voltage regulation for distribution networks and allowing the load demand to follow power generation. This paper summarizes recent R&D on electric springs and their potential functions for future smart grid. Electric springs can be associated with electric appliances, forming a new generation of smart loads which can adapt according to the availability of power from renewable energy sources. When massively distributed over the power grid, they could provide highly distributed and robust support for the smart grid, similar to the arrays of mechanical springs supporting a mattress. Thus, the 3-century old Hooke's law in fact provides a powerful solution to solving some key Smart Grid problems in the 21st Century.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: This paper describes the droop control method for parallel operation of distributed electric springs for stabilizing ac power grid. It provides a methodology that has the potential of allowing reactive power controllers to work in different locations of the distribution lines of an ac power supply and for these reactive power controllers to support and stabilize the ac mains voltage levels at their respective locations on the distribution lines. The control scheme allows these reactive power controllers to have automatically adjustable voltage references according to the mains voltage levels at the locations of the distribution network. The control method can be applied to reactive power controllers embedded in smart electric loads distributed across the power grid for stabilizing and supporting the ac power supply along the distribution network. The proposed distributed deployment of electric springs is envisaged to become an emerging technology potentially useful for stabilizing power grids with substantial penetration of distributed and intermittent renewable power sources or weakly regulated ac power grid.
    IEEE Transactions on Smart Grid 01/2013; 4(3):1558-1566.
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    ABSTRACT: This paper presents a review on existing ballasts for light-emitting diodes (LED) with considerations to their compliance to regulations, technological challenges, and on meeting various application requirements. All existing LED ballasts, including those proposed in recent literature, have been appropriately classified and systematically organized for the discussion. The dissemination of this information and its understanding is helpful for future R&D pursuits in this area.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: For mid-range wireless power applications, the load is normally far away from the power source. In this project, a new method is proposed to determine the load impedance and load power without using any direct output feedback. Based only on the information of the input voltage and current, the load impedance and load power can be monitored and controlled without using any wired or wireless feedback from the load. This new method can therefore eliminate the need for any directly measured output feedback, which was previously thought to be essential. It also makes the power control of a wireless power transfer very simple. The concept is verified by the comparison between the computed results and practical results of an 8-ring domino wireless power transfer system. A good degree of accuracy has been achieved in the verification.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: This paper describes a patent-pending method that can be used to estimate the speed and position of surface-mounted permanent magnet synchronous motors (SMPMSM) with cylindrical rotor structure and sinusoidal back emf. The concept involves the use of the power inverter's switching actions and switching current to provide instantaneous real-time information for extracting the speed information. The theory and algorithm are included to explain the proposed concept, which has been verified by simulations on a SMPMSM drive under the control of a sensorless speed control scheme.
    ECCE Asia Downunder (ECCE Asia), 2013 IEEE; 01/2013
  • Xuehui Tao, S.Y.R. Hui
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    ABSTRACT: This paper presents a dynamic photoelectrothermal theory for light-emitting diode (LED) systems. In addition to photometric, electrical, and thermal aspects, this theory incorporates the time domain into the generalized equations. A dynamic model for a general LED system is developed for system analysis. This theory highlights the fact that the luminous output of an LED system will decrease with time from the initial operation to the steady state due to the rising temperature of the heat sink and the LED devices. The essential thermal time constants involved in the LED systems are explained. The time factor is critical in understanding how much the luminous output will decrease with time and is essential to the optimal designs of the LED systems that are operated continuously (e.g., general lighting) or momentarily (e.g., traffic lights). Experiments on several LED systems at different time frames have been conducted, and the practical measurements confirm the validity of this theory.
    IEEE Transactions on Industrial Electronics 05/2012; · 5.17 Impact Factor
  • W.M. Ng, D.Y. Lin, S.Y. Hui
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    ABSTRACT: A patent-pending single design of an ultralow-loss (ULL) magnetic ballast for T5 high-efficient (T5-HE) fluorescent lamps rated from 14 to 35 W is presented. Based on the use of a nonlinear physical low-pressure discharge lamp model, it is discovered that the same set of ballast parameters can be chosen for operating T5-HE 14-, 21-, 28-, and 35-W lamps at their respective rated power at a mains voltage in the range of 220-240 V. With energy efficiency higher than and lifetime much longer than their electronic counterparts and being recyclable, the ULL ballasts offer a more environmentally friendly solution to T5-HE lamps than electronic ballasts. This single-ballast design offers great convenience to both ballast manufacturers and users, because only one product design can cover a range of the most popular T5-HE lamps. Both theoretical analysis and experimental results are included to confirm the validity of the proposal.
    IEEE Transactions on Industrial Electronics 05/2012; · 5.17 Impact Factor
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    ABSTRACT: Synchronous rectification has previously been adop- ted in switched-mode circuits for reducing the conduction losses particularly in high-frequency, low-voltage, and high-current ap- plications. This paper presents a generalized "self-driven" ac-dc synchronous rectification technique that can be used even at mains frequency to develop an ac-dc synchronous rectifier that behaves like a diode bridge but with much reduced conduction losses and without control integrated circuits. This generalized concept can be extended from single-phase to multiphase systems. Experiments based on 1- and 2-kW single-phase systems have been successfully conducted for capacitive, inductive, and resistive loads. Very significant power loss reduction (over 50%) has been achieved in the rectification stage at both 110- and 220-V ac mains operations. This patent-pending circuit can be regarded as a direct replacement of a general-purpose diode rectifier. Due to the reduction of power loss, further reduction in the size and cost of the heat sink or thermal management for the power circuit becomes possible. Index Terms—Energy saving, mains-frequency synchronous rectifiers (SRs), self-driven SRs (SDSRs).
    IEEE Transactions on Industrial Electronics 01/2012; 59(1):392-401. · 5.17 Impact Factor
  • Chi Kwan Lee, W. X. Zhong, S. Y. R. Hui
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    ABSTRACT: In this paper, the effects of the magnetic coupling of nonadjacent resonators on the optimal frequency of wireless power transfer are addressed. A power analysis has been carried out to identify the adjacent and nonadjacent power flow components. It is found that such cross-coupling effects of nonadjacent resonators would cause the maximum efficiency operation to slightly shift away from the resonance frequency of the resonators. Theoretical reasons for such phenomena are provided and experimentally confirmed with practical measurements in a wireless power transfer system comprising several magnetically coupled resonators arranged in a straight domino form.
    IEEE Transactions on Power Electronics 01/2012; 27(4):1905-1916. · 4.08 Impact Factor
  • D. Lin, W. Yan, G. Zissis, S.Y.R. Hui
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    ABSTRACT: In this study, the procedure for developing a low-pressure discharge lamp model incorporating novel features of the electron density variation and the ambipolar diffusion process is introduced. The model can accurately predict not only the lamp electrical terminal characteristic but also the electron density and electron temperature inside the fluorescent lamp. The results are comparable with the outcome from the complicated plasma models. The lamp model can accurately predict the lamp characteristics at both high- and low-operating frequencies, and the voltage spikes caused by arc re-ignition at each half-cycle when the lamp is operated at mains frequency. This model can be easily implemented into MATLAB/Simulink as a simple, accurate and user-friendly CAD lamp model. The detailed genetic algorithm schemes used to determine the model parameters are included in this study.
    IET Science Measurement ? Technology 01/2012; 6(4):229-237. · 1.00 Impact Factor
  • S. Y. R. Hui, Huanting Chen, Xuehui Tao
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    ABSTRACT: LED technology is a multidisciplinary subject that involves semiconductor physics, photometry, electric power, heat, and chromaticity. It has been demonstrated that operating the LED load at its rated power does not necessarily guarantee optimal luminous performance unless the LED system is properly designed. This paper presents a tutorial of LED system theory that links the device characteristics to optimal system designs. Based on recent works on the photoelectrothermal theory and its extensions, this paper aims at providing a comprehensive LED system theory with physical explanations for electronics engineers and researchers working in LED system designs, with the emphasis on general and public lighting applications. The physical meanings of essential parameters are explained. Practical test procedures for extracting parameters not readily available in data sheets are included. It is envisaged that this LED system theory will form the basic design guidelines for future LED system designs. This tutorial paper is written not only for educational purpose, but it also highlights important parameters that LED device manufacturers should include in LED data sheets.
    IEEE Transactions on Power Electronics 01/2012; 27(11):4571-4583. · 4.08 Impact Factor
  • Si Nan Li, W. X. Zhong, Wu Chen, S. Y. R. Hui
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    ABSTRACT: Traditional current-mirror methods require one fixed current reference for controlling other current source or sources. In this paper, a new self-configurable current-mirror method that can dynamically determine the best current branch as the current reference in order to ensure good balance of all parallel current sources is proposed. The operating principle involves a dynamic and self-configurable transistor-based current-balancing circuit that can be operated in saturation or linear mode. In either operating mode, good current balance or sharing among all parallel-connected current sources can be guaranteed. The novel current-balancing circuit does not require a separate power supply for powering their control circuits. The proposal is a modular one that can be expanded to any number of parallel current sources. Its principle has been successfully applied to current balancing of parallel LED strings.
    IEEE Transactions on Power Electronics 01/2012; 27(4):2153-2162. · 4.08 Impact Factor
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    ABSTRACT: Although critical to the lifetime of LED, the junction temperature of LED cannot be measured easily. Based on the general photoelectrothermal theory for LED systems, the coefficient for the reduction of luminous efficacy with junction temperature is first related to the characteristic temperature of the LED. Then, a noncontact method for estimating the internal junction temperature Tj and junction-case thermal resistance Rjc of LED from the external power and luminous flux measurements is presented and verified practically. Since these external measurements can be obtained easily, the proposal provides a simple tool for checking Tj in new LED system designs without using expensive or sophisticated thermal monitoring equipment for the LED junctions. The proposed method has been checked with measurements on LED devices from three different brands with both constant and nonconstant Rjc. The theoretical predictions are found to be highly consistent with practical measurements.
    IEEE Transactions on Power Electronics 01/2012; 27(4):2184-2192. · 4.08 Impact Factor
  • H.T. Chen, X.H. Tao, S.Y.R. Hui
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    ABSTRACT: With the use of wall-plug efficiency, estimation techniques for the optical power and heat-dissipation coefficient of LEDs are introduced in this paper to enrich the photo-electro-thermal theory, which has provided a framework for analyzing LED systems. The estimation methods consist of simple procedures for optical and electrical power measurements, which are easy for LED device manufacturers and system designers to follow. The extended theory has been tested with several types of LED devices, with reasonably good agreements between theoretical and practical results.
    IEEE Transactions on Power Electronics 01/2012; 27(4):2176-2183. · 4.08 Impact Factor
  • C.K. Lee, W.X. Zhong, S.Y.R. Hui
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    ABSTRACT: This is a review paper describing recent progress of mid-range applications of wireless power transfer. Starting from Tesla's principles of wireless power transfer a century ago, it outlines magneto-inductive research activities in the last decade on wireless power transfer with the transmission distance in the order of or greater than the coil dimension. It covers the basic characteristics of 2-coil systems, 4-coil systems, systems with relay resonators and the wireless domino-resonator systems.
    Energy Conversion Congress and Exposition (ECCE), 2012 IEEE; 01/2012
  • W.X. Zhong, Xun Liu, S.Y.R. Hui
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    ABSTRACT: The planar contactless battery charging system is an emerging technology that can be applied to a wide range of portable consumer electronic products. Beginning with a brief historical background, this paper presents a new single-layer winding array and receiver coil structure with cylindrical ferrite cores for planar contactless battery charging systems. Complying with the “Qi” standard, this design enables multiple devices to be placed and charged simultaneously on the wireless charging pad in a free-positioning manner. The charging flux is totally localized within the covered area between the selected primary winding and the secondary winding inside the load. The electromagnetic characteristics of such winding design are studied in finite-element analysis and confirmed by practical implementation.
    IEEE Transactions on Industrial Electronics 10/2011; · 5.17 Impact Factor
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    ABSTRACT: This paper extends the single-phase self-driven synchronous rectification (SDSR) technique to multiphase ac-dc systems. Power MOSFETs with either voltage- or current-sensing self-driven gate drives are used to replace the diodes in the rectifier circuits. The generalized methodology allows multiphase SDSRs to be designed to replace the multiphase diode rectifiers. Unlike the traditional SR that is designed for high-frequency power converters, the SDSR proposed here can be a direct replacement of the power diode bridges for both low- and high-frequency operations. The SDSR utilizes its output dc voltage to supply power to its control circuit. No start-up control is needed because the body diodes of the power MOSFETs provide the diode rectifier for the initial start-up stage. The generalized method is demonstrated in 2-kW one-phase and three-phase SDSRs for inductive, capacitive, and resistive loads. Power loss reduction in the range of 50%-69% has been achieved for the resistive load.
    IEEE Transactions on Industrial Electronics 09/2011; · 5.17 Impact Factor
  • Wu Chen, S.Y.R. Hui
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    ABSTRACT: Current imbalance should be avoided when multiple LED strings are connected in parallel. In this paper, a dimmable LED driver with magnetic-amplifier postregulators for multistring applications is presented. Powered by a common master source, parallel LED strings are individually regulated by their corresponding adaptive slave sources for current balancing in this proposal. Without linear current regulators, the proposed driver offers relatively high efficiency. Its structure is simpler than multiconverter structures for red, blue, and green LED applications, and is particularly suitable for LEDs with wide parameter variations. The performance of the proposed driver is experimentally verified by a 16.5-W prototype with a load of three 5.5-W LED strings.
    IEEE Transactions on Power Electronics 07/2011; · 4.08 Impact Factor
  • S.Y. Hui, D.Y. Lin, W.M. Ng, Wei Yan
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    ABSTRACT: The high-voltage and low-current features of T5 lamps imply that the copper and core losses of the magnetic ballast can be greatly reduced. This paper shows that magnetic ballasts for high-voltage T5 lamps are not only feasible but their luminous and ballast-loss performance can be better than those of the electronic ballasts. Contrary to common belief, high frequency operation of T5 28 W lamps increases the luminous efficacy by an average of 3.6% only (less than 5%). Practical implementation of an ultra-low-loss magnetic ballast system for T5 28 W lamps is presented. Its total system power is less than the 32 W upper limit specified for Class A2 of energy-efficient electronic ballast for T5 28 W lamps. High-luminous efficacy of 75.63-77.66 lm/W can be achieved. This important breakthrough has the potential of reversing the existing trend of using electronic ballasts as the energy-saving technology in lighting industry. With a better luminous efficacy, lower product and maintenance costs, much longer lifetime, and the use of recyclable metallic materials over its electronic counterparts, this patent-pending proposal provides a truly sustainable lighting solution to the lighting industry.
    IEEE Transactions on Power Electronics 03/2011; · 4.08 Impact Factor

Publication Stats

2k Citations
444.95 Total Impact Points

Institutions

  • 1996–2012
    • The University of Hong Kong
      • Department of Electrical and Electronic Engineering
      Hong Kong, Hong Kong
  • 1997–2010
    • City University of Hong Kong
      • • Centre for Power Electronics
      • • Department of Electronic Engineering
      Chiu-lung, Kowloon City, Hong Kong
  • 2009
    • National University of Ireland, Galway
      Gaillimh, Connaught, Ireland
  • 2004–2005
    • South China University of Technology
      Shengcheng, Guangdong, China
    • Zhejiang University
      Hang-hsien, Zhejiang Sheng, China
  • 1992–2005
    • University of Sydney
      • School of Electrical and Information Engineering
      Sydney, New South Wales, Australia
  • 1988–2005
    • University of Nottingham
      • Department of Electrical and Electronic Engineering
      Nottingham, ENG, United Kingdom
  • 2003
    • Johnson & Johnson
      New Brunswick, New Jersey, United States
  • 1991–1999
    • University of Technology Sydney 
      Sydney, New South Wales, Australia