Browsing by Author "Cengiz, Ceren"

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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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Dynamic opto-electro-thermal characterization of solid state lighting devices: measuring the power conversion efficiency at high current densities
(IOP Publishing, 2022-09-22) Azarifar, Mohammad; Cengiz, Ceren; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Azarifar, Mohammad; Cengiz, Ceren
Solid state lighting devices with high power densities require accurate characterization, as the rise of chip temperature impacts its optical, electrical, and lifetime characteristics. A detailed experimental study is performed in order to analyze opto-electro-thermal behavior as new high-power devices like laser diodes (LDs) are becoming of interest. The outline of this article was inspired by two major issues identified in previous experimental approaches. First, there is some debate in the literature about the linearity of the temperature coefficient of the forward voltage. Second, there is a limited number of experimental reports on the temperature dependence of power conversion efficiency. We have shown that prior variations are the result of a diversity in the selection of electrical parameters during thermal calibrations, as both linear and non-linear relationships for temperature coefficient of voltage can be obtained. On the other hand, it was discovered that report scarcity for temperature-dependent studies can be related to the use of passive approaches for temperature-dependent measurements. In temperature-controlled environments, short pulses with high current densities may not ensure the thermal equilibrium of the device under test due to small thermal capacitance. This issue, as well as the lengthy process of passive measurements can be addressed by implementing a dynamic measurement method presented in this study. Finally, linear power conversion efficiency trends with junction temperature are demonstrated for both blue light emitting diodes and LDs at high current densities.
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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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An experimental and numerical investigation of immersion cooled blue led combined with yag:ce phosphor for thermal, electrical and optical performance
Cengiz, Ceren; Arık, Mehmet; Arık, Mehmet; Uysal, Murat; Başol, Altuğ Melik; Ünal, Ramazan; Nizamoğlu, S.; Department of Mechanical Engineering; Cengiz, Ceren
At the new age of illumination, light emitting diodes (LEDs) have been proven to be the most efficient alternative to conventional light sources. Yet, in comparison to the other lighting systems, LEDs can withstand a lower maximum operating temperature while junction temperature (Tj) is among the main factors dictating their lifespan, reliability, and luminance performance. Thermal concerns in high power LEDs are important not only because of the dramatic failure of the chip, but also lowered performance in terms of light characteristics. Generated heat and increased device temperatures are direct indicators of poor performance. In fact, heat generation in LED packages occurs mainly on the chip and phosphor layers, both of which lead to a decrease in light output due to the reduction in internal and external quantum efficiencies. In addition to the heat losses over an LED die due to the losses in combination of carrier charges, optical losses are also significant. Moreover, with the increase in Tj during the operation of an LED, the transparency of optical components in the package is disrupted and leads to absorption losses. This eventually results in degradation of components in the package and performance losses. However, the highest absorption losses are observed in the LED die due to internal reflections of a portion of emitted light over the chip surface and eventually loss as heat. Although phosphor coating over LED die is a common practice, absorption losses and increased temperature of a phosphor coated layer over the die negatively affect the optical performance. Therefore, removing the phosphor layer from the die and coming up with a novel thermal package design will enhance the optical efficiency of an emission process. In addition to absorptive and reflective losses, certain losses are related to down conversion or Stokes loss of the phosphor layer in the package. During the conversion from blue to white light, a part of light emission is lost as heat, and it affects the amount of light emission and light characteristics. Since the optical performance of LEDs is thermally limited, novel cooling techniques that also favor optical behavior are continuously investigated. Air cooling is very effective in cooling low power LEDs due to availability, low cost, and reliability, although its performance is very limited due to the low thermal capacity of air. On the other hand, the efficiency of indirect liquid cooling is an order of magnitude higher compared to air cooling; however, the cooling performance is negatively affected by additional interfaces between LED to heat sink and heat sink to air. Direct immersion cooling of an LED chip with natural convection is found to be a promising and cost-effective solution for high power LEDs subjected to high heat fluxes. In addition, temperature uniformity of package components is achieved especially at higher driving currents that cause local temperature gradients. Moreover, in recent years, the interaction of unrestricted particles with the dispersed multi-phase flow 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 LEDs (Pc-LEDs), has yet to be thoroughly investigated. Thus, considering the above-mentioned thermal, optical, and electrical concerns with current LED devices, this thesis focused on the accurate thermal characterization of LEDs through experimental and computational measurements and proposed a novel immersion cooling technique combined with YAG:Ce to improve the performance of Pc-LED while light systems. With this objective, first, an extensive literature review was conducted on experimental junction temperature (Tj) measurement of LEDs. For the purpose of attempting to address the governing phenomena, benefits, drawbacks, possibilities, and applications, a wide range of measurement techniques and systems are covered. The literature review section included a large number of published measurement approaches such as Temperature Sensitive Optical Parameters (TSOPs), Optical Temperature Probing, Temperature Sensitive Electrical Parameters (TSEPs), and a few other methods. Some of the corrections noted in non-ideal thermal calibration processes are discussed and presented. In addition, a summary of the experimental parameters employed in the literature is given as a reference. Secondly, based on the outcomes of the literature review, a more accurate and time efficient forward voltage method (FVM) was studied for experimental characterization of transient and opto-thermal behavior of bare and coated LED chips under operation. For pulse duration smaller than 50 ms, pulse currents ranging from 1 to 10 mA were found to be causing an acceptable temperature rise in all short pulse durations. On the contrary, pulse currents as high as 100 mA were suggested to be avoided unless they performed in the 1 ms range. Moreover, according to the bare chip thermal characterizations and reported interrelations between output and input powers, Tj and input currents, efficiency higher than 35% cannot be achieved in useful emission power losses due to the Auger losses that significantly reduce internal quantum efficiency and interfacial thermal resistance in thermal design. Correspondingly, an immersion cooling technique that directly targets heat generation zones was suggested to increase extraction efficiency and further reduce the junction temperature. In the experimental study, a preparation method for an immersion cooled LED package with dispersed phosphor particles was introduced. Details of the package design, step-by-step preparation, and material property specifications were explained in detail. Following that, an experimental setup that enables thermal characterization of immersion cooled LED package simultaneous to the particle velocity measurements was demonstrated. As a result, Tj and fluid temperature near the dome surface were measured to be 100 °C and 40 °C when LED has driven at 1 A current. The effectiveness of the immersion cooling technique was acknowledged by reaching lower junction temperature at higher driving currents. In addition, image processing of particle-based investigation of phosphor has revealed that in the presence of blue light, a 10-fold increase in particle velocity can be observed. This variation was mainly attributed to the photopheresis force acting on the particle when optical effects were incorporated. The origins of this force were discussed, and alterations in particle and fluid motion due to the intense photopheresis effect were reported. The findings presented in that part aim to shed light on future studies for particle trapping and the light attraction of photoluminescence particles. Thirdly, two-phase flow analyzes were conducted computationally by utilizing a discrete phase modeling approach in ANSYS Fluent environment. The impact of different heat generation rates of an LED package was investigated by considering natural convection currents and corresponding phosphor particle trajectories inside a fluid domain. Phosphor location and movement within a buoyancy-driven flow were further analyzed by including the self-heating effect. For that purpose, heat transfer, fluid flow, and energy paths of self-heating phosphor particles were extracted, and the influence of particle sizes was analyzed in detail. The results have shown that with immersion liquid cooling, the highest phosphor particle temperature was recorded to be under 147 °C, while larger size particles introduced higher heat transfer rates to the Pc-LED 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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High brightness illumination based on laser light diffusion with mie scattering
(ASME, 2022) Azarifar, Mohammad; Cengiz, Ceren; Ocaksönmez, Kerem; Onal, A.; Nizamoglu, S.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Azarifar, Mohammad; Cengiz, Ceren; Ocaksönmez, Kerem
Limited luminous flux per wafer area of light emitting diodes (LEDs) for high power solid state illumination causes some packaging real estate issues. This problem can be tackled with laser diodes (LDs). At high current densities, LDs offer higher efficiency, however with very low etendue and divergent angle. This significantly increases the complexity of color conversion for white light generation. Concentrated light can carbonize the color conversion unit and have high speckle contrast. These problems can be addressed by efficient diffusion of the laser beam and this paper is aimed to introduce the first laser diffusion system based on TiO2 Mie particles. Based on a series of ray tracing simulations, an idealized cost-effective system is modeled and results showed an almost lossless diffusion with a guiding system based on reflection resulting in an almost uniform irradiance level with only 17% power loss. Furthermore, offered design can reduce the challenges for the compact packaging of white LDs by eliminating the heat sink for color conversion coating and enabling a safe light intensity for utilizing quantum dots for color engineering.
Open Access
Machine learning to predict junction temperature based on optical characteristics in solid-state lighting devices: A test on WLEDs
(MDPI, 2022-08) Azarifar, Mohammad; Ocaksönmez, Kerem; Cengiz, Ceren; Aydoğan, Reyhan; Arık, Mehmet; Computer Science; Mechanical Engineering; AYDOĞAN, Reyhan; ARIK, Mehmet; Azarifar, Mohammad; Cengiz, Ceren
While junction temperature control is an indispensable part of having reliable solid-state lighting, there is no direct method to measure its quantity. Among various methods, temperature-sensitive optical parameter-based junction temperature measurement techniques have been used in practice. Researchers calibrate different spectral power distribution behaviors to a specific temperature and then use that to predict the junction temperature. White light in white LEDs is composed of blue chip emission and down-converted emission from photoluminescent particles, each with its own behavior at different temperatures. These two emissions can be combined in an unlimited number of ways to produce diverse white colors at different brightness levels. The shape of the spectral power distribution can, in essence, be compressed into a correlated color temperature (CCT). The intensity level of the spectral power distribution can be inferred from the luminous flux as it is the special weighted integration of the spectral power distribution. This paper demonstrates that knowing the color characteristics and power level provide enough information for possible regressor trainings to predict any white LED junction temperature. A database from manufacturer datasheets is utilized to develop four machine learning-based models, viz., k-Nearest Neighbor (KNN), Radius Near Neighbors (RNN), Random Forest (RF), and Extreme Gradient Booster (XGB). The models were used to predict the junction temperatures from a set of dynamic opto-thermal measurements. This study shows that machine learning algorithms can be employed as reliable novel prediction tools for junction temperature estimation, particularly where measuring equipment limitations exist, as in wafer-level probing or phosphor-coated chips.
Open Access
Optical and thermal analysis of secondary optics in light emitting diodes' packaging: Analysis of MR16 lamp
(IOP Publishing, 2021-12-08) Azarifar, Mohammad; Cengiz, Ceren; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Azarifar, Mohammad; Cengiz, Ceren
Optical and thermal control are two main factors in package design process of lighting products, specifically light emitting diodes (LEDs). This research is aimed to study the role of secondary optics in opto-thermal characterization of LED packages. Novel thin total internal reflection (TIR) multifaceted reflector (MR) lens is modelled and optimized in Monte-Carlo ray-tracing simulations for MR16 package, regarded as one of the widely used LED lighting products. With criteria of designing an optical lens with 50% reduced thickness in comparison to commercially available lenses utilized in MR16 packages, nearly same light extraction efficiency and more uniform beam angles are achieved. Optical performance of the new lens is compared with the experimental results of the MR16 lamp with conventional lens. Only 2.3% reduction in maximum light intensity is obtained while lens size reduction was more than 25%. Based on the detailed CAD design, heat transfer simulations are performed comparing the lens thickness effect on heat dissipation of MR16 lamp. It was observed that using thinner lenses can reduce the lens and chip temperature, which can result in improved light quality and lifetime of both lens and light source.
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Particle based investigation of self-heating effect of phosphor particles in phosphor converted light emitting diodes
(Elsevier, 2021-03) Azarifar, Mohammad; Cengiz, Ceren; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Azarifar, Mohammad; Cengiz, Ceren
Since 2003, self-heating effect of phosphor which reduces the efficacy and reliability of phosphor conversion (Pc) light emitting diodes (LEDs), has been a growing research area of opto-thermal Pc-LED modeling. However, few studies have focused on the particles based nature of the phosphor self-heating. Based on a new approach, accurate Monte Carlo ray tracing simulations are performed in regions of interest over discretized control volumes where only a single phosphor particle is exposed to the light radiation. Governed by the Mie scattering effects, heat generation values of phosphor particles showed strong dependency on their optical and geometrical properties. Additionally, based on the emissive behavior of the LED, space above the LED can be divided into two irradiance levels; near and far scale regions. In near scale region, where irradiance levels are above 0.1 W/mm(2), phosphor particles exhibited significant self-heating in milli-Watt scale values. Derived self-heating values are imported to simplified thermal models where phosphor particles showed a temperature rise in excess of 100 degrees C more than LED chip which can lead to considerable conversion efficiency loss and a reduction in lifetime. Higher temperatures are expected in higher irradiance levels where matrix material carbonization can also occur causing fire hazards.
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Thermal and optical characterization of white and blue multi-chip LED light engines
(IEEE, 2021) Cengiz, Ceren; Azarifar, Mohammad; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Cengiz, Ceren; Azarifar, Mohammad
A clear understanding of thermal characteristics for light emitting diodes (LEDs) is at the center of attention for lighting industry. Especially accurate evaluation of the junction temperature of various light engines is the subject matter for last two decades. Nonetheless, viability of the priorly offered solutions is still an argument and only a few of them are practical enough for the extensive practices. Therefore, in this research, utilization of a new multi-LED chip junction temperature measurement technique is studied through thermal characterization of a commercially available MR16 solid-state light lamp. In order to increase the precision of junction temperature measurement for complex LED structures, a newly proposed methodology is applied simultaneously for each LED chip and temperatures of the multi-LED system at each location are determined. Experimental analysis conducted using an advanced junction temperature measurement device that practices the inverse relationship between Forward Voltage (FV) and Junction Temperature (Tj) considering the three-dimensional heat flow from the p-n junction region. Validation of the above-stated measurement device is performed via local temperature maps captured by using an Infrared (IR) thermal camera and detailed CFD numerical models of each LED package is built with commercially available softwares. Optical measurements are then performed, and relevant thermo-optical relationships are established. Findings presented in this paper shown that augmented multi-chip junction temperature measurement method is a promising solution to measure temperature of complex multi-LED devices. In virtue of the established approach, thermal degradation and optical performance assessments of different types of light engines can be accomplished extensively. In future studies, junction temperature data gathered from various multi-LED light engines can be optimized to form a temperature predictive analytical models that can offer a new perspective for the advanced technological applications.
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Thermal and optical performance characterization of bare and phosphor converted LEDs through package level immersion cooling
(Elsevier, 2022-06-15) Azarifar, Mohammad; Cengiz, Ceren; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Azarifar, Mohammad; Cengiz, Ceren
Increased demand for high flux electronics requires creative and affordable thermal management approaches to meet the performance and lifetime expectations. In the present study, optical and thermal improvements of a new package level liquid coolant encapsulation that directly targets heat generation zones are studied for bare and phosphor converted LEDs. As heat both at chip and phosphor composite regions are being generated, a series of detailed experimental and theoretical characterization is performed in order to examine the capability of the new cooling method. The findings can be used to extend the application of precisely controlled liquid packing on the heat source region of optoelectronic components. This method has shown a 15% reduction in thermal resistance and a 7% increase in power conversion efficiency of LED packages. The capacity of this method is mainly limited to the thermal conductivity of the coolant while a significant ability for improvement can be expected by utilizing thermally conductive particles such as hexagonal boron nitride inside the coolant.