Characteristics of GaN-Based High-Voltage LEDs Compared to Traditional High Power LEDs
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In this letter, GaN-based high-voltage light-emitting diodes (HV-LEDs) arrays with 16 microchips connected in series are fabricated. The light output power-current-voltage (LOP-I-V) characteristic of HV-LEDs is measured. Under input power of 1.1 W, the LOP of HV-LEDs is enhanced by 11.9% compared to traditional high power light-emitting diodes (THP-LEDs) with the same chip size. Due to the reduced metal shadow effect and better current spread, the HV-LEDs exhibit higher light extraction efficiency under the same current density, which is simulated by a 3-D ray tracing method. As a result, the luminous efficiency of HV-LEDs is 21.6% higher than that of THP-LEDs under input power of 1.1 W. Furthermore, the efficiency droop of HV-LEDs is reduced to half of that of THP-LEDs.Keywords:
Wide-bandgap semiconductor
Commercially available light emitting diodes (LEDs) that have high efficiencies and long lifetime are offered in advanced packaging technologies. Many cooling systems were developed for current LED systems that enable a better removal of heat than counterpart devices offered earlier this decade. On the other hand, these lighting systems are still producing a considerable amount of heat that is still not effectively removed. Especially, p-n junctions of LEDs are the most critical regions where a significant amount of heating occurs, and it is crucial to determine the temperature of this active region to meet the lumen extraction, color, light quality and lifetime goals. In literature, there are some proposed junction temperature measurement methods such as Peak Wavelength Shift, Thermal (Infrared) Imaging and Forward Voltage Change methods mostly focused on blue LEDs. In this study, we are studying three common types of LEDs (Red, Green, and Blue) and comparing their forward voltage drop (Vf) behaviors. A set of theoretical, computational and experimental studies have been performed. It is found that optical power change with temperature in red LEDs are much higher than blue and green chips. The green LED chip experienced the largest slope having the largest change in forward voltage compared to other LED chips.
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Different high-end white power LEDs from different LED vendors were studied. The aim of the study was to find the optimal choice of LEDs and thermal management solutions for a street-lighting application. The primary concern was the (real) junction-to-heatsink thermal resistance of the LED or LED assembly and the real junction temperature and the actual light output of the individual LEDs under test. Since in many cases the junction-to-heatsink thermal resistance showed temperature dependence, like-with-like comparison in terms of light output characteristics was done as function of the real junction temperature instead of the reference temperature.
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High-power LEDs have many advantages for use in lighting systems. To benefit from these advantages we need to ensure proper heat management. In this paper forward voltage variation is studied as a function of ambient temperature variation at the same supply current. The aim of the study is to estimate the junction temperature as a function of ambient temperature and forward voltage. Two types of LEDs are analyzed (18W COB (chip on board) type and 1W) at different operating temperatures. The concept is not new but the temperature range for which measurements were performed includes temperatures lower than 25 ° C and even temperatures below zero; also one of the LEDs is COB. The results are presented and analyzed.
Atmospheric temperature range
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In this paper, an improved heat dissipating system for high power arrayed light emitted display (LED) was designed and validated. Heat sink was embedded to reduce the hot spots and improve the reliability of LED lighting system. As the power of LED increases lots of heat will be generated and, in turn raises the temperature. This will greatly deteriorate light efficiency and shift the wavelength. Heat dissipation under different heat sink has been carefully designed by finite element analysis. Various kinds of heat sinks are designed for LED chips and an extensive numerical investigation of the heat sink design performance is conducted by commercial finite element package ANSYS. Concerning a single chip LED and arrayed chips LED, Heat sinks with more fins lead to lower junction temperature because of larger surface. The effects of heat sink structure and material, adhesive and environment temperature are investigated. It can be seen that the higher the thermal conductivity of the adhesive, the thinner the thickness of the adhesive, the higher the thermal conductivity of the heat sink materials, the lower the junction temperature and junction-ambient thermal resistance are. As the ambient temperature increases, the junction temperature increases linearly while the junction ambient thermal resistance changes little.
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High junction temperature directly or indirectly affects the optical performance and reliability of high power LEDs in many ways. This paper is focused on junction temperature characterization of LEDs. High power LEDs (3W) were tested in temperature steps to reach a thermal equilibrium condition between the chamber and the LEDs. The LEDs were generated by pulsed currents with duty ratios (0.091% and 0.061%) in multiple steps from 0mA and 700mA. The diode forward voltages corresponding to the short pulsed currents were monitored to correlate junction temperatures with the forward voltage responses for calibration measurement. In junction temperature measurement, forward voltage responses at different current levels were used to estimate junction temperatures. Finally junction temperatures in multiple steps of currents were estimated in effectively controlled conditions for designing the reliability of LEDs.
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The paper presents an innovative method for High Power LEDs cooling in harsh conditions. To take full advantage of the benefits of LEDs, proper thermal management must be realized. This paper describes the limits of the cooling solution based on heat pipe, to maintain the junction temperature close to the nominal value, to avoid the color shift, recoverable light output reduction, voltage decreases. The most important, the reliability of any LED is a direct function of junction temperature, knowing that the higher the junction temperature, the shorter the lifetime of the LED.
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Heat at the junction of light-emitting diodes (LED) affects the overall performance of the LED in terms of light output, spectrum, and life. Usually it is difficult to measure junction temperature of a LED directly. There are several techniques for estimating LED junction temperature. One-dimensional heat transfer analysis is one of the most popular methods for estimating the junction temperature. However, this method requires accurate knowledge of the thermal resistance coefficient from the junction to the board or pin. An experimental study was conducted to investigate what factors affected the thermal resistance coefficient from the junction to the board of high-power LED. Results showed that the thermal resistance coefficient changed as a function of ambient temperature, power dissipation at the junction, the amount of heat sink attached to the LED, and the orientation of the LED with the heat sink. This creates a challenge for using onedimensional heat transfer analysis to estimate junction temperature of LEDs once incorporated into a lighting system. However, it was observed that junction temperature and board temperature maintains a linear relationship if the power dissipation at the junction is held constant.
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Summary form given only. The junction temperature of Light Emitting Diodes (LEDs) is a primary reliability parameter. Exceeding the maximum rated junction temperature could lead to accelerated light output degradation and sometimes even to catastrophic failures. Besides that junction temperature influences the desired LED properties in applications like light output efficiency, dominant wavelength and forward voltage. Therefore thermal management and proper thermal characterisation of high power LEDs is very important for a reliable product with good performance. By measuring the thermal resistance of a high power LED it has to take into account that the power applied to the device is converted into heat and light (-20-40% efficiency). This means that the thermal resistance of a LED can not be determined without knowing the energy flux emitted as light. Therefore in general the interpretation of a given thermal resistance of an optoelectronic device is not well defined. Establishing of a standard on how to do thermal resistance measurement for light emitting devices is necessary.
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The junction temperature of LED has directly influence upon light output efficiency,device life time,reliability,emitting wavelength of LED.With the invention of high-power LEDs,the requirement for drive current has increased significantly,thereby increasing power dissipation.But only approximately 5%-to-10% of the input electrical power in LEDs is dissipated optically while the other portion produces heat energy based on the current technology.Specially,the heat inside high-power LEDs rely mainly on natural cooling,but no active cooling was used because the active cooling will increase the cost,decrease the reliability,and shorten life time of the whole LED lighting system.Only if the cost of each lumen(for example,USD/lm) in the semiconductor lighting is lower than other lighting source,the semiconductor lighting could replace the conventional lighting sources.So,effective thermal management of high-power LEDs must be dealt with by way of low cost and natural cooling.Keeping the junction temperature of LED in the given range is a main research goal of chip fabricating,device packaging and application.Specially,it's also a key issue being dealt with during packaging and application of high-power LED.The influences of pn junction temperature on capabilities of LEDs are described in this paper,firstly.And then the relations of junction temperature and thermal resistance of high-power LEDs are analyzed.The conclusion that junction temperature and thermal resistance restrict farther developments of high-power LEDs has been deduced based on the thermal resistance analyses of high-power LEDs.Meanwhile,the following viewpoints is put forward:(a) How to increase light output efficiency is a basic sticking point in process of raising the power of LED devices.(b)There is no point in developing high-power LEDs which go beyond 5 W for engineering applications if the light output efficiency of LEDs has no improvement.
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Optical and electrical characteristics of power light-emitting diodes (LEDs) are strongly dependent on the diode junction temperature. However, direct junction temperature determination is not possible and alternative methods must be developed. Current-voltage characteristics of commercial high power LEDs have been measured at six different temperatures ranging between 295 and 400 K. Modeling these characteristics, including variation in the bandgap with temperature, revealed a linear temperature dependence of the forward voltage if the drive current is chosen within a rather limited current range. Theoretically, the voltage intercept can be deduced from the bulk semiconductor bandgap. However, accurate junction temperature determination is only possible if at least two calibration measurements at a particular drive current are performed. The method described in this paper can be applied to calculate the thermal resistance from the junction to any other reference point for any particular LED configuration.
Wide-bandgap semiconductor
Atmospheric temperature range
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