Browsing by Author "Muslu, Ahmet Mete"

Now showing 1 - 8 of 8

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A comparative study for the junction temperature of green light-emitting diodes
(IEEE, 2019-10) Özlük, Burak; Muslu, Ahmet Mete; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Özlük, Burak; Muslu, Ahmet Mete
Solid-state lighting devices offer a wide variety of color options applicable for general and automotive lighting, various display systems, and a number of niche applications. As they get smaller, generated heat fluxes become more intense and induce serious lifetime and performance issues. Although the light output from light-emitting diodes (LEDs) is the most efficient at a narrow optical spectrum compared with conventional lighting sources, they are still not adequate to satisfy consumer demands due to considerable amounts of lost energy and emerging thermal issues. On the other hand, it is possible to achieve effective thermal solutions if the junction temperature of LEDs is precisely determined. A number of techniques have been proposed for the junction temperature measurement of LEDs such as forward voltage change and infrared (IR) thermal imaging. In this study, green LEDs were studied to observe optothermal interactions using a number of proposed junction temperature measurement techniques. The effect of an LED lens on junction temperature and optical extraction was investigated by examining the change in the thermal and optical properties of an LED chip after the LED lens was removed. In addition, the results of the green LED were compared with a 450-nm blue LED and verified with numerical findings. As a result, it has been determined that the thermal behavior of LEDs is significantly affected by electrical conditions, since the junction temperature of green and blue LEDs has risen by around 45% after the operating current has been increased from 200 to 500 mA.
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Discrete phase analysis of self heating particles over an immersion liquid cooled high power blue light-emitting diode with suspended phosphor particles
(ASME, 2022-10) Cengiz, Ceren; Muslu, Ahmet Mete; Azarifar, Mohammad; Arık, Mehmet; Dogruoz, B.; Mechanical Engineering; ARIK, Mehmet; Cengiz, Ceren; Muslu, Ahmet Mete; Azarifar, Mohammad
In recent years, the interaction of unrestricted particles with dispersed multiphase flows has been linked to a number of important engineering applications. Among these applications, the novel idea of immersion-cooled phosphor particles, which has the potential of significantly increasing the thermal limits of phosphor converted white light-emitting diode (LEDs) (Pc-WLEDs), has yet to be thoroughly investigated. With this objective, this research utilizes the discrete phase modeling (DPM) technique for the characterization of phosphor location and movements within a buoyancy-driven flow, which is the determining factor in the optical behavior of the newly proposed Pc-WLED configuration. Two-phase flow analysis is conducted to characterize particle movement. Heat transfer, flow, and energy paths of self-heating phosphor particles are extracted, and the influence of particle sizes is analyzed in detail. The results show that with immersion liquid cooling, the highest phosphor particle temperature is recorded to be under 420 K, while larger size particles introduce higher heat transfer rates to the Pc-WLED package for the same number of particles. Moreover, depending on the particle size and position, individual phosphor particles can follow a different trajectory that can affect the probability of obtaining white light emission.
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Enhanced thermal performance of high flux led systems with two-phase immersion cooling
(IEEE, 2020) Cengiz, Ceren; Muslu, Ahmet Mete; Arık, Mehmet; Doğruöz, B.; Mechanical Engineering; ARIK, Mehmet; Cengiz, Ceren; Muslu, Ahmet Mete
Considering significant worldwide use of electricity for general lighting, the implementation of energy efficient technologies is highlighted in many platforms with the use of light emitting diodes (LEDs). Because of its environmentally friendly nature, LEDs offer a promising solution to minimize inefficient use of energy as the demanding operating conditions pose new challenges. Reduction of lumen output, shorter lifetime and degradation of light characteristics with increasing package temperatures are critical issues that need to be addressed with innovative solutions. Especially in high power LEDs exposed to raised heat fluxes, standard cooling methods fail to remove the dissipated heat effectively. Recently, immersion cooling of LEDs with suspended phosphor particles in a dielectric liquid has been offered as a viable option to dissipate a vast amount of heat, ensuring a uniform distribution of temperature, and removal of local hot spots in a high brightness LED package. Therefore, understanding the fluid flow and particle motion due to natural convection in the package is crucial to improve thermal and optical design of an LED system. Temperature distribution and light extraction of a package can be considerably affected by material characteristics, flow regime and flow direction. In this study, the impact of different heat generation rates of an LED package is investigated considering natural convection currents and corresponding phosphor particle trajectories inside a fluid domain. A discrete phase model of a high-power white LED package is created in order to keep track of individual particles interacting with the carrier fluid and heat flow in a closed LED system. Current findings provide good basis for smart control of phosphor particles to maximize thermal and optical performance of both RGB and white LED packages.
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Impact of electronics over localized hot spots in multi-chip white LED light engines
(IEEE, 2019) Muslu, Ahmet Mete; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Muslu, Ahmet Mete
In recent years, demand towards high power white LEDs has risen significantly covering a wide range of application areas. While many industries prefer more light extraction using less number of LED chips on their luminaires, the compact nature of the LED package poses some challenges for controlling junction temperatures of LEDs. Junction temperature measurement capabilities are limited to more simple measurements of board temperatures with added disadvantages of expensive measurement techniques. Although many studies focused on optothermal characterization of single LEDs, a few studies analyzed the junction temperatures of individual LEDs and their impacts on optical properties in multi-LED systems. Thus, this study introduces a new approach towards Forward Voltage Change Method and uses Integrating Sphere System to characterize thermal and optical traits of white multi-LED systems operating at different electrical conditions. Results show that additional electronic components in a multi-LED system can induce considerable thermal issues since it was determined that heat loads may reach up to total radiant power of LEDs and can decrease the conversion efficiency of a lighting unit by 6.1%. It was also shown that junction temperatures of LEDs can be affected by thermal conditions over the circuit and they need to be determined individually. Thus, a junction temperature measurement technique is introduced in this study for multi-chip LED systems enabling a high functionality in future studies for developing better cooling techniques and more lumen extraction.
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Impact of junction temperature over forward voltage drop for red, blue and green high power light emitting diode chips
(IEEE, 2017) Muslu, Ahmet Mete; Özlük, Burak; Tamdoğan, Enes; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Muslu, Ahmet Mete; Özlük, Burak
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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An investigation into the optothermal behavior of a high power red light emitting diode: impact of an optical path
(ASME, 2021-03) Muslu, Ahmet Mete; Özlük, Burak; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Muslu, Ahmet Mete; Özlük, Burak
Monochromatic light emitting diodes (LEDs) are getting more attention day by day for a very wide range of applications such as general lighting, signage, automotive lighting, display, water purification, and skin imaging. While they are taking place in a large number of applications, thermal challenges associated with the operation of LEDs have become a significant issue to address since their performance is greatly affected by thermal conditions. Thus, this study focuses on identifying thermal, optical, and electrical characteristics of an AlGaInP-based red LED considering the impact of the LED dome on the chip performance. The junction temperature measurement results obtained with forward voltage change method were validated with thermal imaging method (TIM) and computational models. It was observed that the LED dome may critically affect the thermal, optical, and electrical behaviors of the LED chip. In fact, a 3.7% increase in junction temperature and a 6.1% drop in optical conversion efficiency were found at the normal operation of the red LED after the LED dome was removed. The results were also compared with a blue LED, and lower junction temperatures were measured for the red LED at each driving current. The difference in junction temperature became even more noticeable at higher driving currents. Results have shown a good agreement between three different methods with a maximum variation of 6.9%.
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A novel approach for identification of thermal and optical characteristics for the active layer of high power photonic devices
(2019-12) Muslu, Ahmet Mete; Arık, Mehmet; Arık, Mehmet; Uğurdağ, Hasan Fatih; Budaklı, M.; Department of Mechanical Engineering; Muslu, Ahmet Mete
A vast amount of electricity consumption for general lighting, the implementation of energy efficient technologies is highlighted in many platforms with the use of light emitting diodes (LEDs). Because of its environmentally friendly nature, LEDs offer a promising solution to minimize inefficient use of energy as the demanding operating conditions present new challenges. While they are taking place in a large number of applications, thermal challenges associated with the operation of LEDs have become a significant issue to address since their performance is greatly affected by thermal conditions. Reduction of lumen output, shorter lifetime and degradation of light characteristics with increasing package temperatures are critical issues that need to be addressed with innovative solutions. Especially in high power LEDs exposed to elevated heat fluxes, standard cooling methods fail to remove produced heat effectively. As LED systems have been evolving today in a great number of niche applications including automotive lighting, water purification, and skin imaging etc., extensive studies of scientists and engineers in the field have been constantly looking for ways to reduce generated heat loads and maximize the light output to reach the highest efficiency ratios. While the current systems developed over the last years achieved to reach even a 40% LED light extraction efficiency, a higher portion of the electrical input energy of LEDs is still consumed as heat and it hinders their development potential. In addition, the compact size of the LED systems poses some challenges to the reliable characterization of their performance at low uncertainties. Especially, the performance considerations associated with thermal loads over a limited size of LED chips require the effective characterization of these systems for various operational conditions. One of the techniques used for this purpose is that an LED package is characterized by a decrease in forward voltage with increasing junction temperature. As LEDs are operated at higher junction temperatures, the amount and quality of the light deteriorates significantly, and less efficient use of the LEDs results in additional operating costs and reduced lifetime of LEDs. In fact, accurate identification of thermal behavior of LED packages is one of the essential tasks towards improving the design of LED systems. If thermal characterization of LEDs is accurately done, performance parameters of LED packages are more reliably optimized to yield the highest possible performance ratios. 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. Most reliable method is cited as Forward Voltage Change Method based on the forward voltage drop over the active layer of an LED as a result of the rise in junction temperature. Therefore, FVCM has been regarded as the main method utilized in the study. Since the optical behavior of LEDs is significantly affected by thermal conditions of the high-power lighting systems, a better understanding towards optothermal characterization of LEDs with accurate and reliable junction temperature measurements is needed in order to improve the current LED systems. Current junction temperature measurements applied in industry and literature are based on certain assumptions that are questioned in various studies and user application notes. In addition, some issues need to be addressed in open literature for the development of high-power LEDs designed with enhanced thermal and optical performance. These include insightful analysis of the differences in high power red, green and blue LEDs, the impact of an optical path in an LED package, interaction of optothermal characteristics and the LED performance, reliable thermal testing of multi-LED devices. Thus, this thesis focused on filling these gaps in literature by offering a new measurement approach for junction temperature of multi-LED systems and proposing a highly accurate and rapid junction temperature measurement device. First of all, identifying thermal, optical and electrical characteristics of high-power LEDs was critical to understand the behavior of these LEDs; therefore, an AlGaInP based red LED was chosen to consider the impact of an LED dome on the chip performance. In addition, prediction capability of thermal imaging method in junction temperature measurements was evaluated in comparison with Forward Voltage Change and numerical thermal methods. It was observed that an LED dome may critically affect the thermal, optical and electrical behaviors of an LED chip. In fact, a 3.7% increase in junction temperature and a 6.1% drop in optical conversion efficiency were found at the normal operation of the red LED after the LED dome was removed. The results were also compared with a blue LED and lower junction temperatures were measured for the red LED at each driving current. The difference in junction temperature became even more noticeable at higher driving currents. Results have shown a good agreement between three different methods with a maximum variation of 6.9%. Later, three common types of LEDs (Red, Green, and Blue) were studied and their forward voltage drop (Vf) and optical behaviors with varying junction temperatures were compared. It is found that change in optical power with temperature are much higher in red LEDs compared to blue and green chips. On the other hand, a green LED chip experiences the largest change in forward voltage with respect to a unit change in junction temperature when compared to other type of LED chips. A new approach towards Forward Voltage Change Method was also introduced for the measurement of junction temperatures in multi-LED systems. An Integrating Sphere System was utilized to characterize thermal and optical traits of white multi- LED systems operating at different electrical conditions. Results show that additional electronic components in a multi-LED system can induce considerable thermal issues since it was determined that heat loads may reach up to total radiant power of LEDs and can decrease the conversion efficiency of a lighting unit by 6.1%. It was also shown that junction temperatures of LEDs can be affected by thermal conditions in the circuit and they need to be determined individually. Thermal imaging technique used to identify local hot spots in the study has shown even higher temperatures at the phosphor layer than the junction region so that the significance of thermal control in a phosphor layer has been understood in order to maximize the thermal and optical performance of a white LED package. As a result, with the proposed analysis and measurement technique in the study, a further understanding has been gained for developing better cooling techniques that will lead to more lumen extraction. In order to conduct junction temperature measurements of LEDs, there are several thermal characterization and measurement devices in the market. However, existing temperature measurement devices are quite expensive for most of the LED manufacturers, thermal engineers and designers who need to measure only the junction temperature of LEDs. Existing products use a thermal transient test technique for the junction temperature measurement. This technique involves thermal characterization with high sampling rate and resolution of data collection, such as heat flow path construction, die attach qualification, and material property identification, all of which make the product quite expensive. Moreover, that thermal characterization approach uses a structure function based on the assumption of one-dimensional heat flow path. However, in various types of LEDs, there are thermal masses on top of the LED module such as phosphor and attached lens that change the heat flux symmetry. This issue brings difficulties for the interpretation of the structural function and leads to limitations especially in junction temperature measurements of white LEDs. Consequently, there is a need in the state of the art for affordable, easy to produce and reliable systems which greatly facilitate thermal, optical and electrical design of future LED products with reliable junction temperature measurements. With the experiences gained on junction temperature measurements, a novel junction temperature measurement device with a compact design and robust operation has been developed. In the thesis, the design and manufacturing process of the device have been described along with the used measurement methodology. After the analysis for the heating and cooling system, the required heating and cooling units are determined, and the corresponding design is presented as the final design. Thus, a highly efficient rapid heating and cooling test chamber is offered to users who demand cost effective thermal solutions.
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Rapid heating and cooling chamber for a photonics junction measurement system
(IEEE, 2020) Tarçın, Hüseyin Gökberk; Saygın, Alper; Muslu, Ahmet Mete; Budakli, M.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Tarçın, Hüseyin Gökberk; Saygın, Alper; Muslu, Ahmet Mete
Since many industrial applications require heat treatment processes or validation tests under certain ambient temperatures, thermal design is a key issue to be considered in order to ensure fast heating and cooling capabilities. Although most industrial furnaces provide the required isothermal conditions for various test applications or calibrations, a number of them does not provide rapid heating and cooling inside a closed system and the thermal equilibrium over different regions of the system are not satisfied as intended. This brings a number of challenges for the performance test of most electronics, which are affected by ambient temperatures, such as LEDs, lasers and transistors. Particularly, a test environment that can quickly and accurately adjust a uniformly distributed isothermal domain can be useful for many electronic components in order to test their performance at preselected ambient temperatures. In such systems, the design parameters have to be adjusted depending on the desired conditions. In fact, the design of those systems has to be planned in detail to achieve a system working fast and accurate, which shall contribute to reduction in operating time. Therefore, this study focuses on proposing a new approach to the development of a high-performance and high-resolution heating and cooling chamber used in a junction temperature measurement of light emitting diodes (LEDs). The major objective thereby is to achieve high heating and cooling rates of a controlled chamber that satisfies thermal conditions at a user defined temperature interval between 20°C and 80°C. Therefore, material properties and geometrical dimensions, power requirements and cooling performances of the chamber are analyzed as major design parameters. Numerical models are created for various design options, and simulations are performed for various working conditions under certain design constraints. The relationships between design parameters are determined. A final design is propos...