Ruihong Zhang

Bio: Ruihong Zhang is an academic researcher from Harbin Institute of Technology. The author has contributed to research in topics: LED lamp & Capacitor. The author has an hindex of 10, co-authored 28 publications receiving 276 citations. Previous affiliations of Ruihong Zhang include City University of Hong Kong & Northwestern Polytechnical University.

A TRIAC-Dimmable LED Lamp Driver With Wide Dimming Range

Abstract: LED light bulbs are becoming increasingly popular as they consume much less power than traditional incandescent lamps. To enable full compatibility with incandescent lamps, apart from delivering the same light output and quality, they are expected to be operable with standard TRIAC-based light dimmers. However, the input current of the LED light bulbs at dimming could fall below the holding current of the TRIAC, resulting in limited dimming range and lamp flickering. This paper presents a TRIAC-dimmable LED lamp driver allowing wide dimming range. The concept is based on controlling the input reactive power, so that the input current is increased to a level higher than the holding current of the TRIAC, while the LED array power is regulated. The driver consists of two power conversion stages, including a four-quadrant ac-dc converter for shaping the input current and a resonant converter for regulating the output power to the LED array. The two converters share the same switching network. Modeling, analysis, and design of the driver will be presented. An LED prototype has been built and evaluated. Experimental results reveal that the true firing angle of the TRIAC can be adjusted down to 172°, and the lamp power can be dimmed from 7.2 to 0.3 W linearly.

Use of Daisy-Chained Transformers for Current-Balancing Multiple LED Strings

Abstract: A modular and scalable structure that can perform current-balancing multiple LED strings is presented. The current-balancing function is accomplished by connecting multiple transformers in a daisy chain-the primary and secondary windings of each transformer are connected to two different modules, and thus the two module currents are mutually coupled. The modules are shared with the same input, derived from the output of a switching network, and each module is used to drive an LED string. When there is a fault occurred in a string, such as open- or short-circuit of any LED, such string will be disconnected from the corresponding module. As the output voltage of the module will then increase under such situation, an energy-recycling circuit with each module having an OR-ing diode connected to its input will be activated to clamp the module voltage by recycling energy back to the source. Thus, the faulty string(s) will not affect the normal operation of the healthy strings. A prototype with seven 1 W LED strings has been built and evaluated. Experimental results show that the string current has less than ±1% variation under different dimming levels, string voltage variation of ±30%, and faulty operations in some LED string(s).

A TRIAC-dimmable LED lamp driver with wide dimming range

Abstract: A TRIAC-dimmable LED lamp driver allowing wide dimming range is proposed. The concept is based on introducing reactive power flow to increase the input current higher than the holding current of the TRIAC, while the input active power is regulated. A LED prototype has been built and tested. Experimental results reveal that the true firing angle of the TRIAC can be adjusted down to 172°, and the lamp power can be dimmed from 7.2W to 0.3W linearly.

Capacitor-Isolated Multistring LED Driver With Daisy-Chained Transformers

Abstract: By extending a prior-art transformer-isolated structure for driving multiple LED strings, a scalable and modular capacitor-isolated architecture is presented in this paper. The proposed idea is based on first utilizing a daisy-chained transformer structure to share equally the current delivered from a half-bridge switching circuit and supply the shared currents to multiple string networks and then using a capacitor network in each string network to derive the string current and separate the grounds of the input source and the strings. The capacitor networks give further advantage of balancing the string currents, as compared to the prior-art transformer-isolated structure. Dimming function is achieved by adjusting both the switching frequency and duty cycle of the switching devices in the switching circuit. A 21-W prototype with seven strings has been built and evaluated. The lamp can be dimmed to 10% of full output, and the LED string current can be regulated at less than ±1% error, even under significant variations in the string voltages and some LED failures.

A High Voltage Gain DC–DC Converter With Common Grounding for Fuel Cell Vehicle

Abstract: The essential features of a DC–DC converter for fuel cell vehicle are to ensure the higher voltage gain to meet out the higher DC link voltage demand, continuous input current to improve the life span of the fuel cell, presence of common grounding to avoid electromagnetic interference issue and lower voltage stress with reduced components. This paper presents a high voltage gain DC–DC converter by combining the switched capacitor and quasi switched boost network modules. The proposed high voltage gain converter provides the continuous input current, lower voltage stress, utilizes a fewer number of elements and common grounding feature. The operating characteristics, steady-state analysis both in continuous current mode (CCM) and discontinuous current mode (DCM), comparative analysis with contemporary converters are discussed. The theoretical claimed analysis is validated by the simulation and experimental study. The proposed converter is operated for 200 W output power rating and tested for providing the voltage gain in the range of 5–8 times the input voltage gain by varying the input voltage from 25 V–40 V. The efficiency of the proposed converter is also reported for different output power rating which is in the range 91.3%–93.7%.

Light-emitting diodes

Abstract: Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

A Survey, Classification, and Critical Review of Light-Emitting Diode Drivers

Abstract: Based on a survey on over 1400 commercial LED drivers and a literature review, a range of LED driver topologies are classified according to their applications, power ratings, performance and their energy storage and regulatory requirements. Both passive and active LED drivers are included in the review and their advantages and disadvantages are discussed. This paper also presents an overall view on the technical and cost aspects of the LED technology, which is useful to both researchers and engineers in the lighting industry. Some general guidelines for selecting driver topologies are included to aid design engineers to make appropriate choices.

DC Microgrid Protection: A Comprehensive Review

Abstract: DC microgrids have attracted significant attention over the last decade in both academia and industry. DC microgrids have demonstrated superiority over AC microgrids with respect to reliability, efficiency, control simplicity, integration of renewable energy sources, and connection of dc loads. Despite these numerous advantages, designing and implementing an appropriate protection system for dc microgrids remains a significant challenge. The challenge stems from the rapid rise of dc fault current which must be extinguished in the absence of naturally occurring zero crossings, potentially leading to sustained arcs. In this paper, the challenges of DC microgrid protection are investigated from various aspects including, dc fault current characteristics, ground systems, fault detection methods, protective devices, and fault location methods. In each part, a comprehensive review has been carried out. Finally, future trends in the protection of DC microgrids are briefly discussed.

Virtual power plant models and electricity markets - A review

Abstract: In recent years, the integration of distributed generation in power systems has been accompanied by new facility operations strategies. Thus, it has become increasingly important to enhance management capabilities regarding the aggregation of distributed electricity production and demand through different types of virtual power plants (VPPs). It is also important to exploit their ability to participate in electricity markets to maximize operating profits. This review article focuses on the classification and in-depth analysis of recent studies that propose VPP models including interactions with different types of energy markets. This classification is formulated according to the most important aspects to be considered for these VPPs. These include the formulation of the model, techniques for solving mathematical problems, participation in different types of markets, and the applicability of the proposed models to real case studies. From the analysis of the studies, it is concluded that the most recent models tend to be more complete and realistic in addition to featuring greater diversity in the types of electricity markets in which VPPs participate. The aim of this review is to identify the most profitable VPP scheme to be applied in each regulatory environment. It also highlights the challenges remaining in this field of study.

An Improved LCLC Current-Source-Output Multistring LED Driver With Capacitive Current Balancing

Abstract: Passive or active current balancing circuits are usually used to mitigate current imbalance in driving multiple light-emitting-diode (LED) strings. Passive current balancing schemes adopting capacitors with high reliability, small size and low cost are very popular in many applications. However, the high reactive power of the capacitive balancing scheme with variable frequency control will bring high power stress on the VA rating of the main switches which drive this passive current balancing circuit and decrease the overall efficiency. Fixed frequency control does not permit zero-voltage switching (ZVS) under load variations. Hence, this paper proposes a current-source-output LED driver based on LCLC resonant circuit to provide a constant output current regardless of variations in LED parameters. In the LCLC circuit, the number of additional capacitors is scalable with the number of LED strings for current balancing. Moreover, the input impedance of the improved LCLC circuit is designed to be resistive at the operating frequency to minimize reactive power. The conventional duty cycle control can easily incorporate ZVS. The analysis, implementation and verification are detailed in this paper.