AN1149-6 Lumileds Lighting, LLC, AN1149-6 Datasheet - Page 9

• Manufacturer Part Number: AN1149-6
• Description: Reliability Considerations for SuperFlux LEDs
• Manufacturer: Lumileds Lighting, LLC

Light output degradation for SuperFlux LEDs is
a function of the current density in the LED
semiconductor chip. The rate of change will be
lower when SuperFlux LEDs are driven at lower
currents. Figure 6.5 shows how the light output
varies over time for LEDs driven at several
currents. Although this particular lot of LEDs
became slightly brighter with time at the lower
forward currents, their light output peaked and
then began the typical logarithmic decline. At
high currents SuperFlux LED light output will
peak within a few hours. At lower currents they
may continue to get brighter for several hundred
hours before beginning the normal gradual
reduction over time.
Ambient temperature has negligible effect on
the light output degradation of SuperFlux LEDs.
During the initial product qualification, HP
monitored the change in light output during the
operating and forward biased weatherability
tests listed in Table 6.1. The average changes
for one of the TS AlInGaP wafers used for these
tests are shown in Tables 6.4 and 6.5. The
different stress temperatures and the presence
of humidity had negligible effect on light output
degradation. In fact, the 100°C test had a lower
light output degradation than the tests run at
cooler temperatures. This illustrates that drive
current has a larger impact than ambient
Table 6.4
Condition A, HTOL, 100°C, 30 mA
Condition E, LTOL, 40°C, 70 mA
Condition B, HTOL, 85°C, 50 mA
Condition C, HTOL, 55°C, 70 mA
Condition D, RTOL, 25°C, 70 mA
Operating Life Test Results for HPWT-MHOO LEDs
Operating Life Tests
9
temperature on the rate of light output
degradation.
Figure 6.6 shows the light output degradation of
some samples mounted in the Arizona Desert.
These samples were directly exposed to the
harsh local weather conditions for over a year.
Some of these samples were mounted on a
vertically oriented, south facing printed circuit
board. Other samples were mounted on a
printed circuit board in a special apparatus that
collects and concentrates sunlight by a factor of
six. A picture of this sun concentrator is shown in
Figure 6.7.
As shown in Figure 6.6, the concentrated
sunlight had only a small effect on device
performance.
The rate of change in light output will vary within
a batch and from one batch of LEDs to another.
Within a lot, the individual standard deviation for
results is approximately 5% after 1000 hours of
operation. Lumileds Quality System is designed
to prevent the use of AlInGaP semiconductor
material that exhibits unacceptable light output
degradation in SuperFlux LEDs. Lumileds only
uses batches of material with an average light
output change of less than 35% after 1000 hours
of operation at 70 mA.
Average Change in Light output
(after 1.000 hours)
10%
21%
24%
24%
16%