Huanting Chen

An Extended Photoelectrothermal Theory for LED Systems: A Tutorial From Device Characteristic to System Design for General Lighting

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.

Color Variation Reduction of GaN-Based White Light-Emitting Diodes Via Peak-Wavelength Stabilization

The color, electrical, and thermal properties of LED devices are highly dependent on one another. The peak wavelength of GaN-based white LED shifts in opposite directions under the influences of current and junction temperature change. This affects the correlated color temperature (CCT). Importantly, duty cycle control for LED dimming does not provide constant color (against conventional wisdom). An analysis model that links the peak wavelength, electrical, and thermal properties of LED devices is proposed. The color-shift trend of the LED with respect to the changes in its thermal and electrical operating conditions is described. The stabilized CCT performance of a dc or a bilevel-driven LED over a dimming range is found to be a result of the complex interactions between the selected current levels, duty cycle, thermal resistances of the heatsink and device, heat dissipation conversion ratio, and the physical parameters of the LED device. The predicted color variation is verified by experimental results, which demonstrate that the CCT stabilization of an LED with a dc drive requires less thermal energy than that with a bilevel drive. For a given thermal design, the reduction in CCT variation during light intensity change is possible via the combined adjustment of the current level and its duty cycle over the dimming operation.

Chromatic, Photometric and Thermal Modeling of LED Systems With Nonidentical LED Devices

With the emergence of new color-mixing LED systems based on LED devices of different color temperatures, the need for a new modeling technique for LED systems with nonidentical LED devices becomes imminent. This paper presents a modeling technique for LED systems with LED arrays comprising nonidentical LED devices that have nonidentical optical-thermal-electrical properties. Based on a general 3-D photo-electro-thermal LED node model, LED devices of different kinds can be arranged in various array forms according to their system construction and design. By linking the system matrix to the correlated-color-temperature prediction, the proposed modeling technique provides an accurate prediction of the temperature distribution, luminous flux, and correlated color temperature of the LED systems. The temperature distribution and light output of the LED systems have been measured using an infrared imaging system and a spectrophoto-colorimeter with an integrating sphere. The modeling technique has been successfully demonstrated and experimentally verified on several LED systems comprising nonidentical LED devices. It is particularly useful as a modeling tool to study new color-mixing LED systems based on different types of LED devices.

Precise Dimming and Color Control of LED Systems Based on Color Mixing

This paper proposes a closed-loop nonlinear scheme for precisely controlling the luminosity and correlated color temperature (CCT) of a bicolor adjustable light-emitting diode (LED) lamp. The main objective is to achieve a precise and fully independent dimming and CCT control of the light mixture emitted from a two-string LED lamp comprising warm-white and cool-white color LEDs, regardless of the operating conditions and throughout the long operating lifetime of the LED lamp. The proposed control method is formulated using the nonlinear empirical LED model of the bicolor white LED system. Experimental results show that with the proposed closed-loop nonlinear approach, both CCT and dimming control of the bicolor lamp is significantly more accurate and robust to ambient temperature variations, ambient light interference, and LED aging than the conventional linear approach used in existing products. The maximum error in luminous flux employing the proposed closed-loop nonlinear approach is 3%, compared with 20% using the closed-loop linear approach. The maximum deviation in CCT is only 1.78%, compared with 27.5% with its linear counterpart.

A New Noncontact Method for the Prediction of Both Internal Thermal Resistance and Junction Temperature of White Light-Emitting Diodes

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.

Nonlinear Dimming and Correlated Color Temperature Control of Bicolor White LED Systems

This paper proposes a nonlinear approach of controlling the luminous intensity and correlated color temperature (CCT) of white light-emitting diode (LED) systems with dual color temperatures. This LED system is made up of a warm color LED source (2700 K) and a cool color LED source (5000 K). The luminous intensity of each of these LED sources is individually controlled by pulsewidth modulation. The overall intensity of the LED system is due to the combined emitted flux of both LED sources. Its overall CCT is the mixed average CCT of both LED sources. This proposed method is based on the nonlinear empirical luminous and CCT models of the LEDs, which take into consideration the thermal effect of LEDs on its luminance and CCT properties. With reasonable approximation, the theoretical models are simplified into practical solutions, which are translatable into real-life applications. It is experimentally validated that the proposed approach is considerably more accurate than the existing linear approaches that do not consider color variations of LED sources. The idea is applicable to LED systems with multiple color temperatures and is not limited to white LEDs.

Characterization, Modeling, and Analysis of Organic Light-Emitting Diodes With Different Structures

This paper demonstrates that organic light-emitting diodes (OLEDs) of different structures can be characterized and modeled using a combination of the photo-electro-thermal (PET) theory and spectral power distribution modeling. The photometric, electrical, thermal, and chromatic properties of OLED devices are incorporated into a model framework so that the performance of the OLED of different structures can be compared. A concept of luminance uniformity over the OLED surfaces is also introduced for comparing OLED with large surface areas. Experimental results are included to verify the OLED models and compare the characteristics of two different OLED samples. Based on the same PET framework, some differences of OLEDs and inorganic LEDs are addressed and discussed.

Critical design issues of retrofit light-emitting diode (LED) light bulb

For retrofit applications, some high-brightness (HB) light-emitting diode (LED) products have the same form factor restrictions as existing light bulbs. Such form factor constraints may restrict the design and optimal performance of the LED technology. In this paper, some critical design issues for a commercial LED bulb designed for replacing an E27 incandescent lamp are quantitatively analyzed. The analysis involves a power audit on such densely packed LED system so that the amounts of power consumption in (1) the LED driver, (2) the LED wafer, (3) the phosphor coating, and (4) the bulb translucent cover are quantified. The outcomes of such an audit enable R&D engineers to identify the critical areas that need further improvements in a compact LED bulb design.

Power Flow Analysis and Critical Design Issues of Retrofit Light-Emitting Diode (LED) Light Bulb

For retrofit applications, some high-brightness light-emitting diode (LED) products have the same form factor restrictions as existing incandescent light bulbs. Such form factor constraints may restrict the design and optimal performance of the LED technology. In this paper, some critical design issues for commercial LED bulbs designed for replacing E27 incandescent lamps are quantitatively analyzed. The analysis involves power audits on such densely packed LED systems so that the amounts of power consumption in: 1) the LED wafer; 2) the phosphor coating; and 3) the lamp translucent cover are quantified. The outcomes of such audits enable R&D engineers to identify the critical areas that need further improvements in a compact LED bulb design. The strong dependence of the luminous output of the compact LED bulb on ambient temperature is also highlighted.

Precise Color Control of Red-Green-Blue Light-Emitting Diode Systems

The complex nature and differences of the luminous and thermal characteristics of red, green, and blue (RGB) light-emitting diodes (LEDs) make precise color control of RGB LED systems a great technological challenge. This paper presents a nonlinear model that includes coupling effects among LED devices for predicting color in RGB LED systems. A control method is included to demonstrate that this model can be used for precise color control. The proposed model and control method have been successfully evaluated in practical tests. The measurements agree well with model predictions. They form a new design tool for precise color control of RGB LED systems.