Person: TAMDOĞAN, Enes
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Enes
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TAMDOĞAN
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Conference paperPublication Metadata only Thermal and optical performance of eco-friendly silk fibroin proteins as a cavity encapsulation over LED systems(ASME, 2015) Yuruker, Sevket Umut; Arık, Mehmet; Tamdoğan, Enes; Melikov, R.; Nizamoğlu, Sedat; Press, D. A.; Durak, Ilkem; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Yuruker, Sevket Umut; Durak, Ilkem; Tamdoğan, EnesThe demand for high power LEDs for illumination applications is increasing. LED package encapsulation is one of most critical materials that affect the optical path of the generated light by LEDs, and may result in lumen degradation. A typical encapsulation material is a mixture of phosphor and a polymer based binder such as silicone. After LED chips are placed at the base of a cavity, phosphor particles are mixed with silicone and carefully placed into the cavity. One of the important technical challenges is to ensure a better thermal conductivity than 0.2 W/m-K of current materials for most of the traditional polymers in SSL applications. In this study, we investigated an unconventional material of the silk fibroin proteins for LED applications, and showed that this biomaterial provides thermal advantages leading to an order of magnitude higher thermal performance than conventional silicones. Silk fibroin is a natural protein and directly extracted from silk cocoons produced by Bombyx mori silkworm. Therefore, it presents a “green” material for photonic applications with its superior properties of biocompatibility and high optical transparency with a minimal absorption. Combining these properties with high thermal performance makes this biomaterial promising for future LED applications. An experimental and computational study to understand the optical and thermal performance is performed. A computational fluid dynamics study with a commercial CFD software was performed and an experimental set-up was developed to validate the computational findings to determine the thermal conductivity of the proposed material.Conference paperPublication Metadata only Effect of direct liquid cooling on light emitting diode local hot spots: Natural convection immersion cooling(Begell House Inc., 2014) Tamdoğan, Enes; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdoğan, EnesThe increased popularity of solid state systems with the technological developments have led them to be a favorable choice for many lighting applications besides electronics. However, the development of denser high lumen packages has been accompanied by increasing heat fluxes at the LED chip and package levels. Especially, the chips driven at high currents may experience local hot spots, which may cause thermal degradation or even catastrophic failures. As the air cooling has been widely used over the years and significant advances have been made to manage increased heat fluxes. It has been recognized as very difficult to rely solely on it to have an efficient cooling in higher heat fluxes. Moreover, active cooling methods may provide necessary thermal performance but at the expense of high cost and energy consumption. Hence, an efficient cooling capability in high heat fluxes (100 W/cm2) can be accommodated through the use of immersion liquid cooling. Immersion cooling has been studied for electronics circuits since last several decades where the thermal capability of such cooling systems have proved several orders of magnitude higher heat fluxes capability due to phase change heat transfer. Thus, direct liquid cooling with the usage of fluorocarbon liquids, generally considered as the most suitable liquids, has been applied in the current study. The thermal and optical performances of a multi chip LED light engine has been investigated with a series of computational fluid dynamics models and experimental validation studies. Heat transfer mode has been kept at the single phase in dielectric fluids. Effect on the local temperatures, peak and dominant wavelength shifts with respect to temperatures, and impact on total lumen extraction has been presented. Finally, a close form first order correlation has been developed for total lumen extraction depending on driving current and chip temperature.Conference paperPublication Metadata only Direct liquid cooling of high flux LED systems: hot spot abatement(ASME, 2013) Tamdoğan, Enes; Arık, Mehmet; Doğruöz, M. B.; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdoğan, EnesWith the recent advances in wide band gap device technology, solid-state lighting (SSL) has become favorable for many lighting applications due to energy savings, long life, green nature for environment, and exceptional color performance. Light emitting diodes (LED) as SSL devices have recently offered unique advantages for a wide range of commercial and residential applications. However, LED operation is strictly limited by temperature as its preferred chip junction temperature is below 100 °C. This is very similar to advanced electronics components with continuously increasing heat fluxes due to the expanding microprocessor power dissipation coupled with reduction in feature sizes. While in some of the applications standard cooling techniques cannot achieve an effective cooling performance due to physical limitations or poor heat transfer capabilities, development of novel cooling techniques is necessary. The emergence of LED hot spots has also turned attention to the cooling with dielectric liquids intimately in contact with the heat and photon dissipating surfaces, where elevated LED temperatures will adversely affect light extraction and reliability. In the interest of highly effective heat removal from LEDs with direct liquid cooling, the current paper starts with explaining the increasing thermal problems in electronics and also in lighting technologies followed by a brief overview of the state of the art for liquid cooling technologies. Then, attention will be turned into thermal consideration of approximately a 60W replacement LED light engine. A conjugate CFD model is deployed to determine local hot spots and to optimize the thermal resistance by varying multiple design parameters, boundary conditions, and the type of fluid. Detailed system level simulations also point out possible abatement techniques for local hot spots while keeping light extraction at maximum.Conference paperPublication Metadata only Thermal enhancement of an LED light engine for automotive exterior lighting with advanced heat spreader technology(ASME, 2016) Uras, Umut Zeynep; Tamdoğan, Enes; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Uras, Umut ZeynepIn recent years, light emitting diodes (LEDs) have become an attractive technology for general and automotive illumination systems. LEDs have been preferable for automobile lighting due to its numerous advantages such as long life, low power consumption, optical control and light quality as well as robustness for high vibration. Thermal management is one of the main issues due to severe ambient conditions and compact volume. Conventionally, tightly packaged double sided FR4 based printed circuit boards are utilized for both driver electronics components and LEDs. In fact, this approach will be a leading trend for advanced Internet of Things (IOT) applications in near future. A series of numerical models are developed to determine the local temperature distribution on both sides of a light engine. Results showed that FR4 PCB has a temperature gradient of over 63°C while the maximum temperature is 105°C. This causes a significant degradation of lifetime and lumen extraction as many LEDs are recommended to be operated below 100°C. In addition to FR4, Aluminum metal core and vapor chamber based advanced heat spreader substrates are developed to obtain thermal impact on the substrate due to a wide range of thermal conductivity of three boards. To mimic real application, two special flex circuits are developed for LEDs and driver circuit. 10 red and 6 amber LEDs at one flex-PCB, and driver components are populated on the other flex-PCB are mounted. Both flex circuits are attached each side of the substrate. Experimental results showed that the local hotspots occurred at FR4 PCB due to low thermal conductivity. Later, a metal core printed circuit board is investigated to minimalize local hot spots. High conductivity metal core PCB showed a 19.9% improvement over FR4 based board. A further study has been performed with an advanced heat spreader based on vapor chamber technology. Results showed that a thermal enhancement of 7.4% and 25.8% over Al metal core and FR4 based boards with an advanced vapor chamber substrate.Conference paperPublication Metadata only Effect of direct liquid cooling on the light emitting diode local hot spots? A computational and experimental study(Begell House Inc, 2014) Tamdoğan, Enes; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdoğan, EnesThe increased popularity of solid state systems with the technological developments have led them to be a favorable choice for many lighting applications besides electronics. However, the development of denser high lumen packages has been accompanied by increasing heat fluxes at the LED chip and package levels. Especially, the chips driven at high currents may experience local hot spots, which may cause thermal degradation or even catastrophic failures. As the air cooling has been widely used over the years and significant advances have been made to manage increased heat fluxes. It has been recognized as very difficult to rely solely on it to have an efficient cooling in higher heat fluxes. Moreover, active cooling methods may provide necessary thermal performance but at the expense of high cost and energy consumption. Hence, an efficient cooling capability in high heat fluxes (100 W/cm2) can be accommodated through the use of immersion liquid cooling. Immersion cooling has been studied for electronics circuits since last several decades where the thermal capability of such cooling systems have proved several orders of magnitude higher heat fluxes capability due to phase change heat transfer. Thus, direct liquid cooling with the usage of fluorocarbon liquids, generally considered as the most suitable liquids, has been applied in the current study. The thermal and optical performances of a multi chip LED light engine has been investigated with a series of computational fluid dynamics models and experimental validation studies. Heat transfer mode has been kept at the single phase in dielectric fluids. Effect on the local temperatures, peak and dominant wavelength shifts with respect to temperatures, and impact on total lumen extraction has been presented. Finally, a close form first order correlation has been developed for total lumen extraction depending on driving current and chip temperature.ArticlePublication Metadata only Natural convection immersion cooling with enhanced optical performance of light-emitting diode systems(2015-10-15) Tamdoğan, Enes; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdoğan, EnesElectronics driven at high currents may experience local hot spots, which may cause thermal degradation or even catastrophic failures. This common problem occurs at light-emitting diode (LED) chips and it is not easily observed by end-users. Driving over 700 mA over a 1 mm2 chip is expected to generate local temperature gradients. In addition, bonding failures at manufacturing or during operation (cracks, delamination, etc.) may also lead to local hot spots. Therefore, possible hot spots over an LED chip have turned attention to direct cooling with dielectric liquids comprises the current study. Computational and experimental studies have been performed to understand the impact of conduction and alternatively convection with various dielectric fluids to abate local hot spots in a multichip LED light engine. To capture the local temperature distributions over the LED light engine with a dome in the domain especially over the LED chip; first, computational models have been built with a commercial computational fluid dynamics (CFD) software. Later, attention has been turned into experimental validation by using a multichip high brightness LED (HB LED) light engine. An optothermal evaluation has been made at single and multiphase heat transfer modes with dielectric fluids (LS5252, HFE7000, and silicone oil, etc.) to compare with a series of CFD models and experimental studies. While multiphase liquid-cooled LED system has a better cooling performance but lower optical extraction, single-phase liquid-cooled LED system has shown a reasonable thermal performance with a 15% enhancement at light extraction.ArticlePublication Metadata only Thermal performance of a light emitting diode light engine for a multipurpose automotive exterior lighting system with competing board technologies(ASME, 2017-06-12) Uras, Umut Zeynep; Arık, Mehmet; Tamdoğan, Enes; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Uras, Umut ZeynepIn recent years, light emitting diodes (LEDs) have become an attractive technology for general and automotive illumination systems replacing old-fashioned incandescent and halogen systems. LEDs are preferable for automobile lighting applications due to its numerous advantages such as low power consumption and precise optical control. Although these solid state lighting (SSL) products offer unique advantages, thermal management is one of the main issues due to severe ambient conditions and compact volume. Conventionally, tightly packaged double-sided FR4-based printed circuit boards (PCBs) are utilized for both driver electronic components and LEDs. In fact, this approach will be a leading trend for advanced internet of things applications embedded LED systems in the near future. Therefore, automotive lighting systems are already facing with tight-packaging issues. To evaluate thermal issues, a hybrid study of experimental and computational models is developed to determine the local temperature distribution on both sides of a three-purpose automotive light engine for three different PCB approaches having different materials but the same geometry. Both results showed that FR4 PCB has a temperature gradient (TMaxBoard to TAmbient) of over 63 °C. Moreover, a number of local hotspots occurred over FR4 PCB due to low thermal conductivity. Later, a metal core PCB is investigated to abate local hot spots. A further study has been performed with an advanced heat spreader board based on vapor chamber technology. Results showed that a thermal enhancement of 7.4% and 25.8% over Al metal core and FR4-based boards with the advanced vapor chamber substrate is observed. In addition to superior thermal performance, a significant amount of lumen extraction in excess of 15% is measured, and a higher reliability rate is expected.Conference paperPublication Metadata only 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, BurakCommercially 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.ArticlePublication Metadata only A comparative study on the junction temperature measurements of LEDs with raman spectroscopy, microinfrared (IR) imaging, and Forward voltage methods(IEEE, 2018-11) Tamdoğan, Enes; Pavlidis, G.; Graham, S.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdoğan, EnesEnergy efficiency, long life, exceptional color, and performance of solid-state light sources have resulted in a rapidly increasing trend in a number of practical applications especially for general lighting after a long history of incandescent lamps. Besides, light-emitting diodes (LEDs) have thermal limitations that are vital for device quality and lifetime. Specifically, to improve the heat dissipation, one major parameter used to evaluate the LED performance is thermal resistance (R). Reducing the resistance can improve the heat flow from the p-n junction to ambient during operation. To quantify this parameter, the LED junction temperature (TJ) must be determined. In this paper, the junction temperatures are first measured with forward voltage method (FVM), Raman spectroscopy, and infrared (IR) imaging for a 465-nm bare blue LED chip (without any phosphor coating). Then, the same samples have been coated with a phosphor-particles added epoxy mixture (%13, 4300 CCT) to convert blue to white light, and the junction temperatures were measured again experimentally with the previously mentioned three methods and compared to each other. While IR imaging shows better capability on capturing the possible hotspots over the surface, Raman method and FVM were in reasonably good agreement on measuring the junction temperature for 465-nm blue (uncoated) LED chip. However, the measurements performed after coating have shown slightly different results with IR imaging and Raman methods, while FVM has shown consistent results for coated chips.ArticlePublication Metadata only An experimental and computational study on efficiency of white LED packages with a thermocaloric approach(IEEE, 2017) Yuruker, S. U.; Tamdogan, Enes; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdogan, EnesThermal management of light-emitting diode (LED) chips is crucial for light extraction and lifetime. It is well known that the light output of an LED decreases with the elevated temperatures. For higher light extraction, the power input to the chip should be substantially high leading nonuniform current spreading and local joule heating at the chip active layers. However, this leads to a high amount of heat generation at the chip and a considerable amount of increase in the junction temperature. Besides shortening the lifetime of the chip, it also strongly affects the light output of the system. Although nominal driving currents for LEDs are around 350-400 mA, the ideal operating condition for the cost effectiveness at higher driving currents and corresponding efficiency of an LED chip is to be explored. In this paper, LED chips' thermal and optical behaviors were investigated for different driving conditions while the board temperature is controlled using a thermoelectric cooler and the input current to the chip. The system was numerically investigated using a computational fluid dynamics software and validated with experimental studies. Consequently, a correlation for efficiency covering a wide range of operating conditions is presented. The efficiency of the LED that is obtained for 30 °C is 42%, whereas it drops to 30% for 50 °C board temperature. If one assumes a logarithmic relationship between the efficiency and the board temperature, the efficiency is expected to be around 20% for a typical LED operating temperature of between 80 °C and 100 °C.