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Thermal Characterization of White Light-Emitting Diodes
Keywords: white light-emitting diodes, phosphor temperature, thermal characterization
Thermal management is of critical importance for high power light-emitting diodes (LEDs) because more than 60% of the electrical power input is converted into heat in LED chips. It is well established that the performance and reliability of monotonic LED emitters can be improved by reducing the junction temperature of the LED chip. In phosphor-converted white LEDs (pcWLEDs), the thermal performance of pcWLEDs is degraded by the generated heat in phosphor materials due to non-unity wavelength conversion efficiency and self-absorption of long-wavelength light by the phosphors. Unlike previous studies, which perform thermal analysis on phosphors without considering the light-heat conversion process, our study includes the optical and thermal interaction in pcWLEDs. In this paper, the thermal characterization of pcWLEDs with different phosphor concentration and arrangement under an input current of 350 mA is investigated by finite element simulation. Heat loads are obtained by 3 dimentional ray-tracing simulation. Key simulation results are verified by experimental data. It is demonstrated that the phosphor temperature is always higher than the junction temperature in all three models with different phosphor arrangement. Therefore, the phosphor temperature is critical in determining the lumen performance and reliability of pcWLEDs. In general, an increase in the phosphor concentration leads to a decrease in phosphor temperature, which is most obvious in the model with phosphor coating. The percolation theory is used to explain a sudden jump in the phosphor temperature as the phosphor concentration in the phosphor-silicone mixture layer is below 60 wt.%. In a word, the phosphor temperature can be effectively reduced by coating the phosphors directly on the chip and maintaining a phosphor concentration above 60 wt.%.
Bohan Yan, Student
University of California, Irvine
Irvine, CA

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