Işık yayan diotların (LED) optik ışınmalarındaki engellerin azaltılması ve optik verimin artırılması için askılı ve kaplamalı nano-fosfor parçacıklı yeni kaplama ve sıvı soğutma tekniklerinin kıyaslanması

Thesis (M.A.)--Özyeğin University, Graduate School of Sciences and Engineering, Department of Mechanical Engineering, August 2017.Energy efficiency, long life, exceptional color and performance of solid-state light (SSL) 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. However, LEDs, as a solid state lighting technology, have some limitations as typical electronics so thermal management is vital for LEDs. Moreover, predicting the life and the light quality is essential to assess and enhance the performance of LED performance. Specifically, to improve the heat dissipation one major parameter used to evaluate the LED performance is thermal resistance. The major obstacle in estimating the thermal resistance for LEDs is the accuracy of determining the junction temperature especially for high power LEDs. Ideally, the junction temperature is determined reliably by monitoring the device temperature at a position close to the junction. That could be achieved with small temperature sensors that are placed very close to the junction. But there are still physical limitations to this method due to the sensor itself would be larger than the junction, which would result in an additional error to the measurement and will not be very useful in most applications. On the other hand, to convert blue to white light, GaN LEDs are usually encapsulated with a phosphor-epoxy mixture that cures into a soft material at high temperatures. During LED operation, significant self-heating occurs causing the glass-like epoxy to undergo large displacements due to its high thermal expansion coefficient at a critical temperature. This enhanced displacement inside the LED package may fracture the gold wire bonds and ultimately lead to device failure. Thus, the inclusion of phosphor into a high brightness LED package is another complex task. In a typical 450 nm or 470 nm blue LED, YAG phosphor is introduced and combined with the blue emission to create what appears to the eye is white light. While, the common practice for phosphor carrier medium is typically a silicone with high refraction index, the geometry of the phosphor is a primary design variable and can be classified as dispersed (dispersed inside the liquid coolant as particles), remote (remote coated under the dome), and local (coated over chip). In each case the geometry greatly affects the ultimate optical output of the LED color qualities and conversion efficiency. Since there is limited information about individual losses in an LED system and there is no available correlation to predict the total losses, this study is for filling the gap for both of these fundamental problems; estimating the thermal resistance and increasing the ultimate optical output of an LED with different coating technologies. Thus, the junction temperatures are first measured in the current work with Raman Spectroscopy, Infrared (IR) imaging as well as Forward Voltage method (FVM) for a 455 nm bare blue LED chip (without any phosphor coating). Then, the same samples have been coated with a phosphor-epoxy (13%, 4300 CCT) mixture to convert blue light into white light. After that junction temperatures were measured 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 and Forward Voltage methods were in reasonably good agreement on measuring the junction temperature for 455 nm blue (uncoated) LED chip. However, the measurements performed after coating have shown slightly different results with Infrared (IR) imaging and Raman method, while Forward Voltage method has still shown meaningful results for coated chips. To be able to have a reasonable comparison, all cases have been measured with FVM simultaneously with one other mentioned method as in couple. As the second phase of the work, after identifying the junction temperatures accurately, LED effective liquid cooling is examined since it is tackling a major challenge "hot phosphor losses" that provides unique information for both fundamental nano-fluid (phosphor based) heat transfer and improved light conversion efficiency. Thus, topside liquid cooling with optically-transparent liquids is utilized to reduce average chip temperatures and to improve the uniformity of chip and phosphor temperature, leading to higher light extraction efficiencies. Furthermore, computational models and experimental studies of heat transfer and optical behavior to validate modeling results were performed for three proposed coating configurations (coated over chip, dispersed inside the liquid coolant as particles and remote coated under the dome) with di-electric liquid cooling. While, the phosphor is in direct contact with the LED chip in the current coating applications, it has shown higher junction temperatures beside of lower conversion efficiency and possible color shifts. The dispersed phosphor idea inside the liquid coolant as particles has resulted with lower conversion efficiency beside of any important thermal enhancements on the LED junction. However, the phosphor in the remote coating system is not affected by the LED temperature and thus maintained a consistent conversion rate and overall color point. Moreover, the remote phosphor with immersion cooling system has extended the Lumen Extraction Limits of White LEDs in excess of 53%, as long as the remote-phosphor system is well designed.Uzun yıllar akkor lambaların kullanılmasının ardından, yarı-hal ışık kaynaklarının (LED'lerin) sahip oldukları yüksek verim, uzun ömür ve çeşitli renk imkânı sunabilmesi gibi özellikleri sebebiyle özellikle genel aydınlatma olmak üzere birçok uygulamada yer almaktadır. Bununla beraber LED'lerin de tüm elektroniklerde olduğu gibi bazı limitleri mevcuttur. LED'ler de jonksiyon sıcaklığı önemli bir faktör olarak önümüze çıkmaktadır. Ayrıca, LED'lerin ömür ve ışık kalitesinin elde edilmesi ve performansının belirlenebilmesi oldukça önemlidir. Performans ve ışık kalitesinde en önemli etken olan jonksiyon sıcaklığının belirlenmesinde ise kullanılabilecek en önemli parametre ısıl dirençtir dolayısıyla jonksiyon sıcaklığıdır. LED'lerde bu ısıl direncin doğru bir şekilde hesaplanabilmesi ise oldukça zor ve hala tam bir kararlaştırılmış teknik olmamakla beraber, 3 farklı method ile hesaplanabildiği literatürde ki bazı çalışmalarda gösterilmiştir. Normalde, jonksiyon sıcaklığı çok ince sensörlerin jonksiyon bölgesine yerleştirilerek daimi veri alımı ile yapılabilmektedir. Ancak, LED'lerin jonksiyon bölgesi ölçüm sensörlerinden daha ufak olması bu tekniğe imkan sağlamamakla beraber olası bir ölçüm esnadında da ölçülen değerlerdeki hata payı beklenenden fazla olabilecektir. Diğer bir yandan da, GaN yarı iletkenlerinden yapılmış bir LED'den çıkan mavi ışığın beyaza dönüştürülmesi için genelde fosfor malzemesi çip üzerine silikon yardımıyla kaplanır. Bu ışık dönüşümü de kendi içerisinde belli bir verime sahip olduğundan, çalışma esnasında buradaki fosforda oluşan ekstra ısınma bazen buradaki bu silikon yapının erime öncesi yumuşama seviyelerine ulaşabilmektedir. Bu da çip ile elektronik kart arasında elektriksel iletkenliği sağlayan ince tel kabloların kopmasına yol açabilmektedir ve LED çalışmaya devam edememektedir. Bu sebepten jonksiyon sıcaklığının ölçülebilmesinin yanı sıra çip üzerine yapılacak kaplama da kendi içerisinde ayrı bir önem taşımaktadır. Tipik bir 450-470 nm'lik mavi LED çipinden çıkan ışığı beyaza dönüştürmek için genellikle YAG fosforu kullanılmaktadır. Genel kaplama yöntemi ise direk çip üzerine silikon ile sıvı halde döküp orda katılaşmasını sağlamak şeklinde olmaktadır. Bu tekniğe alternatif olarak çip üzerine çipten uzakta bir cam yarım kürenin iç yüzeyine fosfor kaplanması ve elektrik iletkenliği olamayan sıvı dolumu; ve diğer bir teknik olarakta fosfor partiküllerinin bu yarım küre içerisine elektrik iletkenliği olmayan sıvı içerisinde serbest bir biçimde doldurulması düşünülmüştür. Her bir teknik kendi içerisinde ışık çıkışını ve ışık dönüşüm verimini oldukça etkilemektedir. Genel bağlamda, LED'lerde ki kayıpların ilgili yerlerinde ne kadar olduğunun hesaplanması konusunda yeterli çalışma ve korelasyonlar mevcut değildir. Bu çalışma da bu boşluğu doldurabilmek adına sunuyoruz. Bu çalışma da özellikle LED'lerin jonksiyon sıcaklığının ölçülmesi ve sıvı soğutma ile birleştirilmiş farklı fosfor kaplama tekniklerinin kıyaslanması yapılmıştır. Jonksiyon sıcaklığının ölçülmesinde ki en önemli parametre olan ısıl direncin doğru bir biçimde hesaplanabilmesi için literatür de bahsedilen üç farklı method; Raman spektrometresi, Termal kamera ve Voltaj değişim, kullanılmıştır. İlk olarak 465nm'lik bir mavi çipin jonksiyon sıcaklıkları üç method ile çok yakın değerlerle hesaplanabilmiştir. Ardından aynı çip önceden belirlenmiş 4300 CCT değerine göre %13 fosfor ve silikon karışımı ile kaplanarak tekrar ölçülmüştür. Kaplama sonrası yapılan jonksiyon ölçümleri bazı farklılıklar göstermiştir. Termal kamera, silikonun düşük termal iletkenlik katsayısı sebebiyle sebep olduğu merkezde yüksek sıcaklık değerleri okurken; Raman spektrometresi ise kaplama sonrası çipten çıkan yoğun beyaz ışık sebebiyle okunması hedeflenen tepe noktası bu yoğun ışık süzmesi içerisinde kalmakta ve ilgili malzemenin çıkardığı dalga boyundaki tepecik gözlemlenememiştir. Voltaj değişim tekniği ise her iki durum için ise aynı şekilde sıkıntısız ölçüm yapabilmiştir. Jonksiyon sıcaklığının doğru bir biçimde belirlenebilmesi ardından artık sıvı soğutmalı farklı kaplama tekniklerinin kıyaslaması üzerine çalışma devam etmiştir. Daha öncede bahsedildiği üzere fosfordaki ışık dönüşümü esnasında oluşan istenmeyen ısının giderilmesi amacıyla fosfor katmanının çipten uzağa alınması düşünülmüştür. Ardından çip ve fosfor arasına da elektrik iletkenliği olmayan sıvı ile doldurularak optik ve termal etkileri incelenmiştir. Yapılan incelemelerde öncelikli olarak sayısal metotlar kullanılmış ve deneysel ölçümlerle birbirinin kıyaslanması yapılmıştır. Elde edilen sonuçlara göre, direk çip üstü kaplama (sıvılı ve sıvısız) ve cam yarım küre içine sıvı ile serbest bir biçimde bırakılan fosfor partikülleri durumları yüksek jonksiyon sıcaklıklarına beklenen faydayı sağlayamamakla beraber ışık veriminde de kayda değer sonuçlar elde edilememiştir. Cam yarım küre içine yapılan fosfor kaplaması ve elektriksel iletken olmayan sıvı dolumu sonrası yapılan deneylerde ise jonksiyon sıcaklığında kayda değer derecede soğuma elde edilirken, ışık dönüşüm verimimde ilk duruma kıyasla (sıvı soğutmasız, direk çip üstü fosfor kaplamalı durum için) kıyaslandığında %53 oranında ışık çıkışında artış gözlenmiştir

Effect of direct liquid cooling on the light emitting diode local hot spots? A computational and experimental study

Due to copyright restrictions, the access to the full text of this article is only available via subscription.The 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

Thermal enhancement of an LED light engine for automotive exterior lighting with advanced heat spreader technology

Due to copyright restrictions, the access to the full text of this article is only available via subscription.In 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.Istanbul Development Agency ; FARBA Corporation of Burs

Direct liquid cooling of high flux LED systems: hot spot abatement

Due to copyright restrictions, the access to the full text of this article is only available via subscription.With 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

Thermal enhancement of an LED light engine for automotive exterior lighting with advanced heat spreader technology

Due to copyright restrictions, the access to the full text of this article is only available via subscription.In 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.Istanbul Development Agency ; FARBA Corporation of Burs

Thermal performance of a light emitting diode light engine for a multipurpose automotive exterior lighting system with competing board technologies

Due to copyright restrictions, the access to the full text of this article is only available via subscription.In 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.Istanbul Development Agency; Turkish Ministry of Science, Industry and Technology; FARBA Corporation of Burs

A comparative study on the junction temperature measurements of LEDs with raman spectroscopy, microinfrared (IR) imaging, and Forward voltage methods

Due to copyright restrictions, the access to the full text of this article is only available via subscription.Energy 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.EU FP7 Career Integration ; Istanbul Development Agenc

Impact of junction temperature over forward voltage drop for red, blue and green high power light emitting diode chips

Due to copyright restrictions, the access to the full text of this article is only available via subscription.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

Thermal and optical performance of eco-friendly silk fibroin proteins as a cavity encapsulation over LED systems

The 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