Siu-Chung Wong

Heavy metals in agricultural soils of the Pearl River Delta, South China.

There is a growing public concern over the potential accumulation of heavy metals in agricultural soils in China owing to rapid urban and industrial development and increasing reliance on agrochemicals in the last several decades. Excessive accumulation of heavy metals in agricultural soils may not only result in environmental contamination, but elevated heavy metal uptake by crops may also affect food quality and safety. The present study is aimed at studying heavy metal concentrations of crop, paddy and natural soils in the Pearl River Delta, one of the most developed regions in China. In addition, some selected soil samples were analyzed for chemical partitioning of Co, Cu, Pb and Zn. The Pb isotopic composition of the extracted solutions was also determined. The analytical results indicated that the crop, paddy and natural soils in many sampling sites were enriched with Cd and Pb. Furthermore, heavy metal enrichment was most significant in the crop soils, which might be attributed to the use of agrochemicals. Flooding of the paddy soils and subsequent dissolution of Mn oxides may cause the loss of Cd and Co through leaching and percolation, resulting in low Cd and Co concentrations of the paddy soils. The chemical partitioning patterns of Pb, Zn and Cu indicated that Pb was largely associated with the Fe-Mn oxide and residual fractions, while Zn was predominantly found in the residual phase. A significant percent fraction of Cu was bound in the organic/sulphide and residual phases. Based on the 206Pb/207Pb ratios of the five fractions, it was evident that some of the soils were enriched with anthropogenic Pb, such as industrial and automobile Pb. The strong associations between anthropogenic Pb and the Fe-Mn oxide and organic/sulphide phases suggested that anthropogenic Pb was relatively stable after deposition in soils.

Design for Efficiency Optimization and Voltage Controllability of Series–Series Compensated Inductive Power Transfer Systems

Inductive power transfer (IPT) is an emerging technology that may create new possibilities for wireless power charging and transfer applications. However, the rather complex control method and low efficiency remain the key obstructing factors for general deployment. In a regularly compensated IPT circuit, high efficiency and controllability of the voltage transfer function are always conflicting requirements under varying load conditions. In this paper, the relationships among compensation parameters, circuit efficiency, voltage transfer function, and conduction angle of the input current relative to the input voltage are studied. A design and optimization method is proposed to achieve a better overall efficiency as well as good output voltage controllability. An IPT system design procedure is illustrated with design curves to achieve a desirable voltage transfer ratio, optimizing between efficiency enhancement and current rating of the switches. The analysis is supported with experimental results.

Analysis and Comparison of Secondary Series- and Parallel-Compensated Inductive Power Transfer Systems Operating for Optimal Efficiency and Load-Independent Voltage-Transfer Ratio

Secondary series- and parallel-compensations are widely used in inductive power transfer (IPT) systems for various applications. These compensations are often studied under some isolated constraints of maximum power transfer, optimal efficiency at a particular loading condition, etc. These constraints constitute an insufficient set of requirements for engineers to select appropriate compensation techniques to be used as a voltage converter with optimal efficiency and loading conditions. This paper studies the characteristics of the IPT system at various frequencies of operation utilizing the two compensation techniques to work as a voltage converter. The frequencies that can provide maximum efficiency of operation and load-independent voltage-transfer ratio are analyzed. The optimal frequencies corresponding to the two compensation techniques are found and compared to facilitate the design of voltage converters with efficient power conversion and load-independent frequency of operation. The analysis is supported by experimental measurements.

Analysis, Design, and Control of a Transcutaneous Power Regulator for Artificial Hearts

Based on a generic transcutaneous transformer model, a remote power supply using a resonant topology for use in artificial hearts is analyzed and designed for easy controllability and high efficiency. The primary and secondary windings of the transcutaneous transformer are positioned outside and inside the human body, respectively. In such a transformer, the alignment and gap may change with external positioning. As a result, the coupling coefficient of the transcutaneous transformer is also varying, and so are the two large leakage inductances and the mutual inductance. Resonant-tank circuits with varying resonant-frequency are formed from the transformer inductors and external capacitors. For a given range of coupling coefficients, an operating frequency corresponding to a particular coupling coefficient can be found, for which the voltage transfer function is insensitive to load. Prior works have used frequency modulation to regulate the output voltage under varying load and transformer coupling. The use of frequency modulation may require a wide control frequency range which may extend well above the load insensitive frequency. In this paper, study of the input-to-output voltage transfer function is carried out, and a control method is proposed to lock the switching frequency at just above the load insensitive frequency for optimized efficiency at heavy loads. Specifically, operation at above resonant of the resonant circuits is maintained under varying coupling-coefficient. Using a digital-phase-lock-loop (PLL), zero-voltage switching is achieved in a full-bridge converter which is also programmed to provide output voltage regulation via pulsewidth modulation (PWM). A prototype transcutaneous power regulator is built and found to to perform excellently with high efficiency and tight regulation under variations of the alignment or gap of the transcutaneous transformer, load and input voltage.

Resonance-Assisted Buck Converter for Offline Driving of Power LED Replacement Lamps

LEDs are potential successors of incandescent lamps with high luminous efficacy and long lifetime. To improve the overall luminair efficacy and lifetime, the power efficiency and lifetime of LED ballasts become important factors. Efficiency gain in transformerless power converters appears attractive for applications without isolation. Driving solid-state LED bulbs in an existing lighting fixture such as PAR30 style housing from universal mains necessitates a high-voltage step-down ratio in order to produce an output voltage of about 10-20 V, which is very common in LED lighting applications. Traditional nonisolated step-down pulse width modulation buck converters may suffer from poor efficiency due to the long diode freewheeling time at small duty cycles. In this paper, we propose a resonance-assisted buck converter to achieve a high-voltage step-down ratio and high converter efficiency, whilst maintaining durability and compatibility with existing incandescent dimmers. The performance of the proposed LED driver is verified experimentally..

Noncascading Structure for Electronic Ballast Design for Multiple LED Lamps With Independent Brightness Control

LED light sources, which are more compact, capable to change color in real time, less dissipative, and more durable are finding more applications than conventional light bulbs in domestic, commercial, and industrial environments. However, requirements such as high-power factor, long lifetime, accurate current control, and high-efficiency pose challenges to the design of LED ballast circuits. This paper proposes an LED ballast with a dual noncascading structure. The first-stage noncascading structure is an isolated current-fed power factor correction (PFC) preregulator. In the proposed design, the short-lifetime high-voltage storage capacitor at the primary is replaced by a long-lifetime low-voltage capacitor at the secondary, thus extending the overall system lifetime. The PFC is programmed by the conventional averaged current-mode control for high-power-factor applications. Furthermore, the high-voltage stress on the main switch, which is typical in current-fed converters, is reduced substantially by appropriately exploiting the transformer leakage inductance. The design uses two secondary transformer windings and an LED current driver to form a second noncascading structure to improve efficiency. Multiple noncascading structures can be used for LED lamps for instant independent brightness control. Analysis, design example, and prototype verification are given for the LED ballast.

Lead contamination and isotope signatures in the urban environment of Hong Kong

The source and the extent of Pb pollution in the urban environment of Hong Kong were investigated at five different urban settings selected on the basis of their annual average daily traffic (AADT) varying from less than 100 to 61,700. In addition, a small distant island without any traffic was selected to establish the possible baseline values. The surface environmental samples studied consisted of street and tunnel dusts, gully sediments, and a limited number of roadside topsoils. The analytical results clearly indicated variable degrees of Pb contamination in these urban settings. However, the level of contamination varied significantly among different types of samples collected at the same location. Pb concentrations of roadside topsoils (79+/-22 micrograms/g) and gully sediments (278+/-88 micrograms/g) were lower than those of the corresponding road dusts (327+/-54 micrograms/g). The Pb isotope compositions in different urban settings varied considerably. The bedrock in the small island had the lowest Pb concentration (12 micrograms/g) but with the highest 206Pb/207Pb ratio (1.2206), whereas the tunnel ceiling dusts with the highest level of Pb (1410 micrograms/g) had the lowest 206Pb/207Pb ratio (1.1062). Despite the significant differences in vehicle types and traffic volumes, and the presence of several different petroleum retailers in Hong Kong, the Pb isotope ratios of road dusts (206Pb/207Pb: 1.1553+/-0.0043, 208Pb/207Pb: 2.4408+/-0.0084) varied within a relatively narrow range among all the five urban sampling sites. On the other hand, the Pb isotopic compositions of gully sediments (206Pb/207Pb: 1.1515+/-0.0145, 208Pb/207Pb: 2.4322+/-0.0198) varied noticeably within the same setting, but were reasonably comparable across the different study sites. In general, the 206Pb/207Pb ratios of road dusts can be used to estimate the direct contribution from automobile emissions, whereas those of gully sediments might reflect the effects of the mixing of different anthropogenic sources. The Pb isotope signatures in the urban environment of Hong Kong clearly suggested that anthropogenic Pb in the environment originated from Pb ore with a low 206Pb/207Pb ratio (such as the Australian Pb ore and similar sources in Southeast Asia) were significantly different from those of the anthropogenic Pb present in the neighboring Pearl River Delta (PRD) region.

A Unified Approach for the Derivation of Robust Control for Boost PFC Converters

In this paper, a generalized approach for the systematic generation of robust control rules for the boost power factor correction converter is presented. Based on a cascading multiloop control structure, a unified approach for deriving the control functions for the inner current loop and the outer voltage loop is proposed. The resulting control provides unity power factor and regulated average output voltage with fast transient response. The control performance is robust under all practical conditions as a result of the application of feedback linearization. This method eliminates the nonlinearity and the dependence of the error dynamics on the input disturbance. The control parameters can be designed according to the desired steady-state and transient response performances. The resulting control rules are readily implemented in analog circuitries. Experiments are conducted to evaluate the control performance.

Fast‐scale bifurcation in single‐stage PFC power supplies operating with DCM boost stage and CCM forward stage

This paper describes the fast-scale bifurcation phenomena of a single-stage single-switch power-factor-correction (PFC) regulator comprising a boost stage operating in discontinuous conduction mode (DCM) and a forward stage operating in continuous conduction mode (CCM). The two stages combine into a single stage by sharing one main switch and one control loop. Using ‘exact’ cycle-by-cycle computer simulations, the effects of various circuit parameters on fast-scale instabilities are studied. The results are qualitatively verified by experimental measurements. This work provides a clear picture of how the variation of certain practical parameters can render such a circuit fast-scale unstable. Copyright

Color Control System for RGB LED Light Sources Using Junction Temperature Measurement

The efficiency of LED lights is approaching that of fluorescent lamps. LED light sources are finding more applications than conventional light bulbs due to their compactness, lower heat dissipation, and most importantly, real-time color changing capability. Accurate control of colors for RGB LED lights is a challenging task, which includes optical color mixing, color light intensity control and color point maintenance due to LED junction temperature change and device aging. In this paper , we present a LED junction temperature measurement technique for a pulse width modulation (PWM) diode forward current controlled RGB LED lighting system. The technique can control the color effectively without the need for using expensive feedback systems involving light sensors. Performance of chromaticity and luminance stability for a temperature compensated RGB LED system will be presented.