LED light sources continue to improve in reliability and overall performance, but still require additional research.
The U.S. Department of Energy (DOE) has recently released test results and conclusions from its ongoing reliability testing conducted on solid-state lighting (SSL) drivers, LED packages, and OLEDs.
SSL drivers have long been considered the “weakest link” in a lighting system. In response to this, DOE initiated a long-term LED driver assessment in 2017 that included evaluation of both single- and multi-channel LED drivers.
The overall test strategy involved assessment of two-stage drivers under an accelerated stress condition of a constantly maintained 75°C and 75% humidity environment (referred to as a 7575 environment) with 100% power output and a power cycling protocol chosen to accelerate potential failure mechanisms. LED modules were used as loads but were operated outside the accelerated test chamber at 25°C. Electrical properties of the drivers were monitored (voltage, current, power factor, output waveform, total harmonic distortion) for five dimming levels ranging from 100% (no dimming) to 1% (full dimming). LED flicker was also monitored for the same dimming levels.
This is a schematic of a two-stage, single-channel driver. Source: Accelerated Stress Testing Results on Single-Channel and Multichannel Drivers: Final Report, U.S. Department of Energy
The test results indicated that degradation (vs. complete failure) of the capacitors and inductors in the electromagnetic interference (EMI) filter in stage 1 of the driver circuitry can lead to further failures throughout the driver as a whole. Components in the stage 1 circuitry are exposed to higher voltage levels, making them more susceptible to aging. Further, degradation of stage 2 components appears to lead to observable changes in light flicker.
The report concludes that greater care in design of the incoming power filtering and EMI suppression should provide additional improvement in SSL driver long-term performance.
Luminous flux maintenance and chromaticity shift are the two most important parameters in assessing long-term LED package performance. The standards that govern the methodology for projecting these parameters are published by the Illuminating Engineering Society (IES). IES LM-80-15, IES Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays and Modules, first published in 2008, provides the methodology for setup and data collection of luminous flux and chromaticity data. IES TM-21-19, Technical Memorandum: Projecting Long-Term Lumen, Photon, and Radiant Flux Maintenance of LED Light Sources, first published in 2011, provides a mathematical method for extrapolation of collected data to arrive at a luminous flux maintenance projection. The methodologies specified in these standards have been used in the DOE evaluation program since its inception in 2012.
LED package data collected from 2012 to 2015 indicated that both ceramic and chip-on-board (COB) packages provided the best luminous flux maintenance, and that polymer packages performed worse, on average. Data collected from 2018 onward indicated that ceramic and COB package luminous maintenance continues to improve, but polymer package luminous flux maintenance is gaining on other types, especially at low currents, mainly due to the use of improved polymeric materials. In addition, data on newer chip-scale packages indicate that performance is consistent with other package types.
Luminous flux and chromaticity maintenance data collection that encompasses several generations of each type of product has been ongoing for OLED panels for approximately four years. The test protocol involves operation at slightly elevated temperature (25° C to 45°C) using the methodologies specified in IES LM-84-14, Measuring Luminous Flux and Color Maintenance of LED Lamps, Light Engines, and Luminaires, and in IES TM-28-14, Projecting Long-Term Luminous Flux Maintenance of LED Lamps and Luminaires.
Results thus far have demonstrated a significant improvement in OLED panel luminous flux maintenance. Long-term performance of warm-white OLED panels is typically lower than higher-color-temperature panels and also typically decreases as operating temperature increases. In addition, the chromaticity maintenance of OLED products appears to have improved over the past five years, with warm-white panels exhibiting better chromaticity maintenance than neutral-white.
RTI International, located in Research Triangle Park, NC, has been the organization contracted to conduct the DOE program over its entire course. Dr. Lynn Davis, Fellow of Engineering Research at RTI, summarizes the findings this way: “LED light sources continue to improve in reliability and overall performance, but unlike legacy lighting technologies, we don’t have a 30 to 45-year knowledge base to fall back on. There are still significant gaps in our understanding of the failure mechanisms of LEDs and SSL systems that require additional research and information sharing within the industry.”
For more information, see the full test reports:
—Yoelit Hiebert has worked in the field of LED lighting for over 10 years and has experience in both the manufacturing and end-user sides of the industry.