IMPLEMENTATION OF PHOSPHOR SEDIMENTATION TO REDUCE
THERMAL INSTABILITY ISSUE OF WHITE LED
ABSTRACT
In general lighting application, white Light Emitting Diode (LED) usually
exposed to an extreme operating temperature of above 90 °C. It is well known that
spectral characteristics of white LED are dependent on the temperature, causing
thermal effects on the luminous flux and the color-shift of white LED become a critical
application checkpoint to be addressed by white LED manufactures. Thus, this study
aims to minimize the thermal stability issue affecting white LED luminescence during
operation by introducing phosphor sedimentation process. The luminescence
properties of blue LED chip samples, yellow YAG:Ce3+ phosphor samples, red
Sr2Si5N8:Eu2+ phosphor samples and assembled white LED samples were initially
characterized at varying temperatures to understand the seriousness of the
luminescence properties thermal instability issue of both LED materials and assembled
LED. The efficiency of gravitational phosphor sedimentation (GPS) and centrifugal
phosphor sedimentation (CPS) were then compared to determine the best approach to
sediment phosphor particles. Finally, the temperature varied luminescence properties
and the reliability of LED samples after implementing phosphor sedimentation were
investigated to understand the impact of introducing phosphor sedimentation.
Computational fluid dynamics (CFD) thermal simulation was also applied in this study
to understand the temperature distribution of the LED with no phosphor sedimentation
(NPS) and full phosphor sedimentation (FPS) during operation to validate the
hypotheses based on experimental data. The results show that CPS method has greater
efficiency to sediment the phosphor particles compared to GPS method, and this
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method just requires ~60 seconds to fully sediment the phosphor particles with 10 rps
of centrifugation. Moreover, the results show that the color coordination is more stable,
the rate of luminous flux degradation is reduced by around 3.0 %, and the color
rendering index (CRI) shift is reduced by around 0.35 % as temperature increases from
25 °C to 95 °C after implementing phosphor sedimentation process. The results also
show that the degradation of luminous flux and color coordination is not significantly
influenced by phosphor sedimentation in steady state life test (SSLT). In conclusion,
the thermal instability issue of white LED in terms of color-shift, luminous flux
degradation, and CRI shift can be improved by introducing phosphor sedimentation
process without causing any reliability drawbacks. These improvements are mainly
due to the heat generated by phosphor particles during operation can be dissipated
effectively throughout a high thermal conductivity substrate after phosphor
sedimentation. Hence, the phosphor temperature of LED with FPS is lower during
operation as validated with the thermal simulation, which reduces the thermal
quenching and color-shift effects of phosphor particles in LED with FPS at high
temperature. These improvements are practically significant and are valuable to the
LED manufacturing companies that intend to survive against fierce competition and
push for light quality perfection.
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