AN1149-6 Lumileds Lighting, LLC, AN1149-6 Datasheet - Page 7

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

the LED chip (see Figure 6.2). To verify this root
cause, the junction temperature during a typical
Power & Temperature Cycle should be
measured.
The maximum junction temperature can be
determined by adding the temperature
difference between the LED lead and the LED
junction to the maximum lead temperature. The
temperature difference between the LED lead
and the LED junction is the product of the
forward current flowing through the LED, the
forward voltage of the LED (measured at this
forward current), and the thermal resistance of
the SuperFlux LED (listed in the Technical Data
Sheet). This is expressed as the following
equation:
Where:
T
T
R θ
V
I
If the junction temperature from this evaluation
exceeds the maximum junction temperature
listed in the SuperFlux LED Technical Data
Sheet, then it is unlikely that the assembly will
consistently pass the Power & Temperature
Cycle test.
The LED pin temperature can be measured by
attaching a thermocouple to one of the cathode
leads of the LED on the underside of the PCB
on the solder pad; making sure that it has a
good thermal contact to the metal lead. LEDs
near the center of the assembly and near any
heat sources (such as resistors or other driving
circuitry) should be selected for monitoring.
f
j
p
= Forward current
f
= Junction temperature
= Forward voltage (at I
= Temperature of the LED pin (lead)
jp
= Junction to pin (lead) thermal resistance
T
j
= T
p
+ R θ
jp
V
f
I
f
f
)
(6.1)
7
After all thermocouples have been attached on
the LED assembly, the LED assembly should be
mounted inside the outer housing. Provisions
must be made for the thermocouple leads to exit
the assembly for attachment to a monitoring
device. Place the device in the chamber in the
way it would be placed during the Power &
Temperature Cycle test.
Heat the chamber to the maximum ambient
temperature to be used during the test, and
power the LED assembly (or assemblies).
Monitor and record the pin temperatures of the
SuperFlux LED leads until the temperatures
reach steady state.
The junction temperature can be calculated
using Equation 6.1. An estimate of the maximum
junction temperature can also be obtained by
performing this test on a test bench in normal
ambient temperatures. Note that this test may
not be able to duplicate the thermal conditions
inside the test chamber such as airflow around
the printed circuit assemblies and heating from
adjacent boards. The ambient temperature of the
room during the test should be noted. Using this
approach, the maximum junction temperature is
the sum of the measured pin temperature, the
temperature rise between the pin and the
junction calculated above, and the difference
between the room ambient and the maximum
ambient temperature to be used in the Power &
Temperature Cycle chamber. This is expressed
as the following formula:
Where:
T
T
amax
atest
= Maximum ambient temperature from Power
= Ambient temperature of bench test
& Temperature Cycle test
+
T
j
(
= T
T
amax
p
+
– T
(
R θ
atest
)
jp
V
f
I
f
)
(6.2)