The interest in light-emitting diodes (LEDs) for illumination applications has been increasing continuously over the last decade due to two key attributes of long lifetime and low energy consumption compared to the conventional incandescent light and compact fluorescent light. Although LEDs are attractive for lighting applications due to the aforementioned advantages, unique technical challenges, such as the extreme sensitivity of luminous output and useful lifetime to LED junction temperature, need to be overcome for their large-scale commercialization.
Among various types of lamps recess downlights are the most common luminaire type in new residential construction. Several LED-based luminaires incorporating more than a single LED chip have been developed to provide the required luminous flux of recess downlight while offering the advantage of higher luminaire efficacy (i.e., higher light output using the same power or lower power consumption for the same light output) over conventional luminaires [1].
Although increasing the number of LEDs in recess downlight results in higher luminous flux (higher total lumens), the overall cost of the final luminaire also increases because of the high cost of LEDs. More importantly the luminaire efficacy remains the same for the same level of drive current. In practice it will be most likely reduced due to the possibly higher junction temperature.
There are two approaches for achieving higher luminous efficacy. An ideal and long-term solution is improvement of the internal and external quantum efficiency of LED chips. An alternative approach relies on lowering the LED junction temperature by utilizing advanced cooling techniques. Indeed, the latter approach has resulted in the development of several cooling solutions for LEDs to enhance the luminaire efficacy of LED-based recess downlights.
Passive cooling solutions have been implemented for several LED-based recess downlights. Due to the limited cooling capacity offered by passive cooling, the maximum total lumen is limited to approximately 600 lumens with the highest luminaire efficacy of about 54 lm/W. In order to be accepted more widely for general illumination, the LED-based luminaires should reach face lumens of 1200-1500 lm with luminaire efficacy higher than 60 lm/W at acceptable cost while maintaining reliability [2].
These requirements necessitate development of active cooling solutions. The basic requirements of active cooling solutions for the recess downlight are cost and reliability. The cooling solutions have to be innovative to satisfy the specific requirements, including (1) low power consumption (the power consumed by cooling solutions reduces the luminaire efficacy); (2) low cost (LED chips are expensive and any substantial extra cost is not desired), (3) compact size (the recess downlight has a limited enclosure) and (4) excellent reliability (reliability of the cooling solution should be at least as good as that of LEDs).
After selecting a suitable active cooling solution, the optimum design of LED-based recess downlights with either passive or active cooling is the use of minimum number of LED chips with an appropriate level of forward current, which meets the requirements of light output, cost and the lifetime (typically time for 70% lumen maintenance). Unlike luminaires with a passive cooling solution, however, the reliability of the luminaire with an active cooling device is dependent not only on the junction temperature but also on the reliability of the cooling method.
This study suggest a novel, hierarchical physics-of-failure (PoF) based reliability model that can be used to assess the reliability of an actively cooled luminaire. The salient considerations for the design of the active cooling solution are offered first, followed by the discussion of the proposed reliability model. The model is implemented to predict the lifetime of a LED-based recess downlight with synthetic jet cooling. The effects of the time-dependent performance degradation mechanisms of the active cooling device on the lifetime of the luminaire are also discussed.
