K. H. Loo

Bilevel Current Driving Technique for LEDs

The significant improvements recently achieved in LED technology in terms of lifetime, luminous efficacy, power rating, and color property render LED one of the most promising candidates to replace conventional light sources in various residential and industrial applications. The rapid advancement in the device characteristics has simultaneously stimulated interests in developing efficient LED drivers with optimized control circuitries. The two conventional techniques currently employed in most LED drivers, namely the amplitude-mode and pulsewidth modulation (PWM) mode driving techniques, suffer from the disadvantage that high luminous efficacy in the amplitude mode has to be traded for control flexibility in the PWM mode and vice versa . In this paper, a method is proposed to improve the luminous efficacy of conventional PWM-mode driving technique while retaining their control flexibility by introducing a dc-offset component into the PWM current. Two LEDs were used in the experimental verifications. Improvements of 17.6% and 18.1% on average were measured by maintaining a dc offset of 100 and 200 mA, respectively, in the LED current. Further improvement can be achieved by increasing the dc-offset current. The main tradeoff is the reduction of the dynamic range over which the average LED current can be controlled. For a given set of performance criteria, the proposed method offers designers of LED drivers the flexibility of balancing between luminous efficacy and dynamic range for control.

On Driving Techniques for LEDs: Toward a Generalized Methodology

LEDs must be externally driven by power sources to emit light. One problem associated with driving LEDs is its inherent nonlinear relation between the emission intensity and the forward current. Thus, the light output obtained from an LED is strongly dependent on the actual current waveforms employed to drive it. It is found that driving an LED with dc produces light output that surpasses all other techniques including the commonly used pulse-width modulation (PWM) technique. On the other hand, for dimming function, it is found that the PWM technique offers greater dimming flexibility in comparison to dc technique. In this paper, a generalized methodology for driving LEDs inheriting the features of both of these techniques is proposed. It employs a pulsating current switching between two discrete current levels, where the current levels and their durations can be concurrently varied for a more precise mapping of the driving conditions to the light output. The existing dc and PWM techniques can be viewed alternatively as being special cases of this more general approach.

On the Color Stability of Phosphor-Converted White LEDs Under DC, PWM, and Bilevel Drive

Most commercial white LEDs are made from nitride-based blue LEDs coated with yttrium aluminium garnet phosphor, which produce spectra that shift in opposite directions under the influences of drive current and junction temperature changes. This property gives rise to different emitted spectra, hence chromaticity properties, when the LED is driven/dimmed by different current waveforms. By using a commercial white LED sample, LUXEON K2, the effects of drive current and junction temperature on the changes of chromaticity coordinates are studied experimentally. The impact of dc, pulse width modulation (PWM), and bilevel current waveform is discussed through a graphical analysis, followed by experimental verification. It is proven that dc offers the best color stability over dimming due to the counteracting influences of drive current and junction temperature variations, whereas an LED constantly suffers from noneliminable chromaticity changes when driven by the PWM. Theoretical explanations are given to justify these cases, and it is found that, for the case of dc drive, an ideal heat sinks thermal resistance can be selected based on a simple equation to minimize the overall chromaticity change over dimming. This paper provides an in-depth discussion on the relations between the chromaticity properties of phosphor-converted (pc) white LEDs and the driving/dimming methods used.

A dynamic collisional-radiative model of a low-pressure mercury-argon discharge lamp: a physical approach to modeling fluorescent lamps for circuit simulations

A dynamic collisional-radiative model of a low-pressure mercury (Hg)-argon (Ar) discharge lamp for application in circuit simulations is presented. The model is implemented in MATLAB as a set of coupled rate equations that are solved simultaneously to give the electron density, the density of the 6/sup 3/P/sub 0,1,2/ states and the electron temperature. These parameters are used to compute the electrical parameters of the discharge positive column and hence the time-dependent voltage-current characteristics of the lamp. The parameters predicted by the model are compared with published experimental data and show good agreement. Calculated lamp voltage-current characteristics based on the model are shown to be in good agreement with direct measurements on commercial fluorescent lamps.

A generalized droop-control scheme for decentralized control of inverter-interfaced microgrids

In this paper, a generalized droop-control scheme for low-voltage microgrid is proposed. The control scheme includes three mutually interacting droop control loops for regulating the power flows between the primary energy sources and the common ac bus. The first droop control is adopted directly from the conventional P/f and Q/V droop control for enabling an automatic power sharing between parallel-connected inverters in the microgrid. The second droop control monitors the dc bus voltages condition and uses it to modify the settings for the first droop control, and hence, adjust the power sharing capacity of the inverter based on the power generating capacity of the primary energy sources. The third droop control resembles the first droop control, but acts on the dc-dc-converter interfaced primary energy sources for enabling an automatic power sharing between them. The proposed control scheme is useful for realizing a fully decentralized control of a microgrid with improved reliability since no critical communication is needed between the main components of the system. The design methodology of the control scheme is presented, and its effectiveness is verified by both simulation results.

Stationary and Adaptive Color-Shift Reduction Methods Based on the Bilevel Driving Technique for Phosphor-Converted White LEDs

The bilevel driving technique has realized a 2-D control of the luminosity and emitted color of white LEDs with duty cycle and forward current levels. Unfortunately, various combinations of these dimming control parameters can lead to significant changes in junction temperature, which further modify the luminosity and emitted color of LEDs. In this paper, the theoretical aspects of these complex interactions and the impact of bilevel drive on the color-shift properties of white LEDs are discussed in detail by using a mathematical color-shift model. Two color-shift reduction methods are proposed based on the insights obtained from this model. This study shows that a heat sinks thermal resistance that minimizes the overall color shift over dimming can be uniquely determined from the knowledge of some measurable LED parameters, and gives rise to a global minimum color shift. If such a thermal resistance cannot be realized due to practical limitations, the second method that utilizes an adaptive change of forward current levels over dimming can be adopted. Based on their nature, these methods are classified as stationary and adaptive methods, respectively. Their validity is supported by experimental measurements on a commercial white LED.

Color Control System for RGB LED With Application to Light Sources Suffering From Prolonged Aging

This paper presents a unified approach to controlling the white color point of the red/green/blue (RGB) light-emitting diode (LED) existing in all aging states. In contrast to conventional color control systems where the average driving currents of the primary-color LEDs can become saturated when the LEDs have undergone prolonged aging, which causes the resulting white color point to go out of regulation, the proposed method avoids this problem by adjusting the color set points when a predefined threshold current is reached by one or more of the primary-color LEDs. It is shown that the method can effectively maintain the white color point of RGB LED at the desired value when the primary-color LEDs are subjected to an accelerated aging through repetitive current stress cycles.

Quasi-Maximum Efficiency Point Tracking for Direct Methanol Fuel Cell in DMFC/Supercapacitor Hybrid Energy System

Direct methanol fuel cells (DMFC) have been widely researched for applications in portable electronics due to their use of liquid fuel for easy storage and transportation compared to gaseous hydrogen. However, DMFCs performance is strongly affected by methanol crossover that significantly degrades the energy efficiency at low output power, and is characterized by an increasing efficiency at increasing output power. The maximum efficiency point (MEP) is inherently difficult to track due to the commonly unknown methanol crossover rate, but since it is experimentally found to be located very close to the maximum power point (MPP), it is suggested that the MPP can be practically viewed as the quasi-MEP and an alternative tracking approach based on the MPP is proposed for the purpose of MEP tracking. In this paper, a fuel-cell-oriented MPP tracking algorithm based on resistance matching is developed, implemented, and tested in the context of a DMFC/supercapacitor hybrid energy system. To account for the generally slow fuel cell dynamics, the DMFC is constantly tracked at the MPP, while any surplus or deficit power is absorbed or delivered by the supercapacitor bank. The detailed formulation of the algorithm and the power flow design and realization are also discussed.

A dynamic conductance model of fluorescent lamp for electronic ballast design simulation

A Spice-compatible dynamic conductance model of a fluorescent lamp for use in electronic ballast simulation is presented. The time-dependent conductance of the fluorescent lamp is derived from a plasma ionization balance equation that uses simplified descriptions of the physical processes within the lamp as its basis. The model has been designed to enable user-defined lamp radius, length, buffer gas pressure and cold-spot temperature as input parameters thus representing a valuable tool for ballast simulations. Simulation results are compared to experimental measurements and satisfactory agreement is achieved.

Systematic Derivation of a Family of Output-Impedance Shaping Methods for Power Converters—A Case Study Using Fuel Cell-Battery-Powered Single-Phase Inverter System

For power converters used in renewable energy systems, output-impedance design has become an important design consideration for minimizing the impacts of low-frequency harmonic current on the lifetime of ripple-sensitive energy sources such as fuel cells and photovoltaic cells. In the literature, various methods are proposed to tackle this design issue but they are frequently treated in isolation from each other and specific to the systems being discussed. In this paper, a systematic derivation of four basic modes of output-impedance shaping method is presented. These basic modes can be directly inferred from the Masons gain formula and other methods are in essence derivatives or combinations of these basic modes. By using a fuel-cell-battery-powered single-phase inverter as an implementation example, their characteristics are discussed thoroughly and their performances in shaping converters output impedance are evaluated experimentally.