Conclusion

In this project,  the heat dissipation effect of high power LEDs implementing COB architecture  was analyzed using infrared thermography methods and finite element analysis.  Initially an existing LED implementing COB architecture was studied and the  problem statement was identified. The heat sources at the center heat up faster  and achieve a higher steady state temperature due to thermal crossover effect. Further  experiments were performed to explore methods to minimize this thermal  crossover effect. The results of the experiments can be summarized below
Under fixed power  conditions, comparing the temperatures at the LED surface and the point at the  center, it was concluded that a 4mm or a 5mm distance of separation was optimum  for minimum thermal crossover effect while maintaining the compactness of the  module. As power was increased, the temperature increased more rapidly as the  distance between heat sources reduce. For distances under 4mm or 5mm, the  temperature rise is exponential establishing the previous conclusion.  Furthermore, 1W power is optimum for this heat source dimensions to prevent the  module from heating above the critical junction temperature that causes  failure.
 Fixing power at 1W  and varying the thermal resistance of the heat sink and varying the separation  distances between the center of the substrate and the heat source, the  temperature difference at each case indicated that 4mm is the ideal separation  distance. For 4mm separation, the minimum thermal conductivity of the heat sink  to minimize thermal conductivity must be 50W/mK. Below this value, the thermal  crossover effect is significant which was indicated by a greater increase in  the midpoint temperature than the heat source temperature with increase in  thermal resistance of the heat sink. This conclusion was further substantiated  by studying the heat flux distribution.
The major  contribution of the project is to construct a guideline for designing LED  modules that minimizes the thermal crossover effect and the junction  temperature to 85°C. The maximum number of heat sources that can be arranged  in a circular pattern for different values of power and thermal resistances  were determined. Linear arrangement of heat sources is superior to a circular  arrangement as seen in the current LED modules since it gives lower junction  temperatures and thermal crossover effect. The rate of increase in thermal  crossover effect with respect to thermal resistance is higher is in a circular  arrangement pattern. In LED COB arrays, the LEDs are covered with a thin layer  of silicone encapsulate for protection. Low thermal conductivity of silicone  forces heat accumulation around the heat source as demonstrated in the last set  of experiments. Although the increased thermal crossover effect has negligible  effects on the arrangement if heat sources around the center, they cause a rise  in junction temperature and hence their effect cannot be neglected.
In all the  experiments performed, the center of the substrate has no heat sources. This  was done to solve the heat accumulation at the center as stated in the problem  statement drawn by studying the CITIZEN LED COB module. This center space can  be used to clamp the COB module to the PCB substrate. Thus, we can conclude  that heat sink properties and center distances are the 2 most dominant factors  that affect thermal crossover effect in LED COB modules and hence affect the  arrangement of heat sources.
 The research has  focused on studying the thermal dissipation in high power LED packages and ways  to alleviate the thermal crossover effect that affects the junction  temperatures. The discrepancy attained in the heating response comparison  between he infrared thermography experiments and ANSYS thermal simulations show  that practical cases might differ from the simulated and analytical models due  to simplification of modeling, thermal contact resistance, forced convection,  radiation effects etc. This study outlines a method to design an LED COB module  by heat sink selection, heat sources arrangement and density, and power  selection for heat sources. An analytical solution must be developed based on  the experimental solution that reduces the selection to a few set of  mathematical expressions that can be solved instantaneously.