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.

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Light-emitting diodes

This paper presents a dimmable light-emitting diode (LED) driver with adaptive feedback control for low-power lighting applications. An improved pulsewidth modulation dimming technique is studied for regulating the LED current and brightness. Under universal input voltage operation, high efficiency and high power factor can be achieved by a coupled inductor single-ended primary inductance converter power factor correction (PFC) converter with a simple commercial transition-mode PFC controller. The operation principles and design considerations of the studied LED driver are analyzed and discussed. A laboratory prototype is also designed and tested to verify the feasibility.

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A High-Efficiency Dimmable LED Driver for Low-Power Lighting Applications

The electrolytic capacitor is the key component that limits the operating lifetime of LED drivers. If an ac-dc LED driver with power factor correction (PFC) control is allowed to output a pulsating current for driving the LEDs, the electrolytic capacitor will no longer be required. However, this pulsating current will introduce light flicker that varies at twice the power line frequency. In this paper, a configuration of flicker-free electrolytic capacitor-less single-phase ac-dc driver for LED lighting is proposed. The configuration comprises an electrolytic capacitor-less PFC converter and a bidirectional converter, which serves to absorb the ac component of the pulsating current of the PFC converter, leaving only a dc component to drive the LEDs. The output filter capacitor of the bidirectional converter is intentionally designed to have a large voltage ripple, thus its capacitance can be greatly reduced. Consequently, film capacitors can be used instead of electrolytic capacitors, leading to the realization of a flicker-free ac-dc LED driver that has a long lifetime. The proposed solution is generally applicable to all single-phase PFC converters. A prototype with 48-V, 0.7-A output is constructed and tested. Experimental results are presented to verify the effectiveness of the flick-free electrolytic capacitor-less ac-dc LED driver.

A flicker-free electrolytic capacitor-less ac-dc LED driver

High power density is strongly preferable for the on-board battery charger of Plug-in Hybrid Electric Vehicle (PHEV). Wide band gap devices, such as Gallium Nitride HEMTs are being explored to push to higher switching frequency and reduce passive component size. In this case, the bulk DC link capacitor of AC-DC Power Factor Correction (PFC) stage, which is usually necessary to store ripple power of two times the line frequency in a DC current charging system, becomes a major barrier on power density. If low frequency ripple is allowed in the battery, the DC link capacitance can be significantly reduced. This paper focuses on the operation of a battery charging system, which is comprised of one Full Bridge (FB) AC-DC stage and one Dual Active Bridge (DAB) DC-DC stage, with charging current containing low frequency ripple at two times line frequency, designated as sinusoidal charging. DAB operation under sinusoidal charging is investigated. Two types of control schemes are proposed and implemented in an experimental prototype. It is proved that closed loop current control is the better. Full system test including both FB AC-DC stage and DAB DC-DC stage verified the concept of sinusoidal charging, which may lead to potentially very high power density battery charger for PHEV.

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Dual active bridge based battery charger for plug-in hybrid electric vehicle with charging current containing low frequency ripple

In this paper, an integrated double buck-boost (IDBB) converter is proposed as a high-power-factor offline power supply for power-LED lamps. The IDBB converter features just one controlled switch and two inductors and is able to supply a solid-state lamp from the mains, providing high power factor and good efficiency. In this paper, the IDBB converter is analyzed, and a design methodology is proposed. It is demonstrated that, with a careful design of the converter, the filter capacitances can be made small enough so that film capacitors may be used. In this way, the converter mean time between failures can be made as high as that of the solid-state lamp. A design example for a 70-W converter supplied from a 230 V/50 Hz mains for street lighting applications is shown. Finally, experimental results from a laboratory prototype are also presented.

Analysis and Design of the Integrated Double Buck–Boost Converter as a High-Power-Factor Driver for Power-LED Lamps

The photometric, electrical, and thermal features of LED systems are highly dependent on one another. By considering all these factors together, it is possible to optimize the design of LED systems. This paper presents a general theory that links the photometric, electrical, and thermal behaviors of an LED system together. The theory shows that the thermal design is an indispensable part of the electrical circuit design and will strongly influence the peak luminous output of LED systems. It can be used to explain why the optimal operating power, at which maximum luminous flux is generated, may not occur at the rated power of the LEDs. This theory can be used to determine the optimal operating point for an LED system so that the maximum luminous flux can be achieved for a given thermal design. The general theory has been verified favorably by experiments using high-brightness LEDs.

A General Photo-Electro-Thermal Theory for Light Emitting Diode (LED) Systems

An LED driver requiring no electrolytic capacitor in the whole power conversion process is presented It consists of two power conversion stages The first stage is a buck converter operating in discontinuous capacitor voltage mode, so that the input current is continuous It is used to deliver a regulated current for the second stage The second stage is a current-fed inverter, in which the semiconductor switches are operated at constant switching frequency and constant duty cycle of 05 The power supplying to the LED string is regulated by controlling the duty cycle of the main switch in the front-stage buck converter The two stages are interconnected by an LC filter, which is designed to attenuate harmonics at double of the line frequency Instead of using an electrolytic capacitor for the filter, a polyester capacitor of better lifetime expectancy is used An 18 W experimental prototype has been built and tested

Current source ballast for high power lighting emitting diodes without electrolytic capacitor

A simple method is proposed to measure the heat dissipation of LEDs and fluorescent lamps in an open system that allows light energy to escape. Based on this method, a comparative study on the thermal and luminous performance of high-brightness LEDs and fluorescent lamps is presented. At rated power, T5 and T8 fluorescent lamps generate about 73%-77% of their total power as heat, while three types of high-brightness LEDs dissipate about 87%-90% of input power as heat. Heat dissipation is an important factor particularly for air-conditioned buildings when overall energy efficiency is considered. T5 fluorescent lamps perform better than some existing LEDs in terms of luminous efficacy and heat generation in this study.

A Simple Method for Comparative Study on the Thermal Performance of LEDs and Fluorescent Lamps

An investigation into the effects of the structures of both LED devices and systems on the luminous performance is presented. Single-chip and multichip LED structures and concentrated and distributed LED systems are studied and compared. Practical tests on both of the device and system levels have confirmed the theoretical predictions that parallel LED structures can lead to much lower thermal resistance and thus higher luminous output. The results of this paper contribute to new information for both LED device manufacturers and system designers toward the optimization of the LED technology.

Comparative Study on the Structural Designs of LED Devices and Systems Based on the General Photo-Electro-Thermal Theory

The conventional control methods for the battery systems of photovoltaic (PV) battery systems in standalone dc microgrids are designed to stringently regulate the bus voltages at the maximum power points (MPP) of PV modules while the state of charge (SOC) of the battery packs is regulated within the tolerances. In this paper, a local hierarchical control (LHC) is proposed for the battery system to improve the energy efficiency of the entire PV-battery system at the MPP of PV modules while the SOC of the battery pack is still regulated within the tolerance. Specifically, by allowing the dc bus voltage to deviate within a preset allowable tolerance, the secondary control of the LHC is employed to compute real-time optimal references to its primary control, such that the energy conversion of the entire PV-battery system can be optimized. Simulation studies exhibit significant efficiency improvement of a 12-PV-battery system under both uniform and nonuniform insolation conditions on a cloudy day and a 600-kW PV-battery system on a sunny day using the proposed LHC. Experimental results validate that the energy efficiency of a single-PV-module-battery system controlled by the LHC can be enhanced using shortened sunny-day and cloudy-day irradiance profiles for various PV modules. The proposed control scheme can be easily implemented in digital controllers without additional hardware costs.

Y.X. Qin

Papers

A General Photo-Electro-Thermal Theory for Light Emitting Diode (LED) Systems

Current source ballast for high power lighting emitting diodes without electrolytic capacitor

A Simple Method for Comparative Study on the Thermal Performance of LEDs and Fluorescent Lamps

Comparative Study on the Structural Designs of LED Devices and Systems Based on the General Photo-Electro-Thermal Theory

Efficient Improvement of Photovoltaic-Battery Systems in Standalone DC Microgrids Using a Local Hierarchical Control for the Battery System