Yüksek güçlü fotonik sistemlerin aktif katmanının termal ve optik özelliklerinin karakterizasyonu için yeni bir yaklaşım

Considering 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.