Figure 3.13 Expected Internal Temperature Rise
(TJ -TA) for HPWA-xHOO LED Emitter versus
Forward Current (Linear Scale).
Figure 3.14 Straight-Line Forward Voltage Model
for LED Emitter (Linear Scale).
Besides affecting a number of the electrical and
optical parameters, the junction temperature
and maximum operating current also affect the
reliability of the LED emitter. For AlInGaP
SuperFlux LED emitters, operation of the emitter
near the maximum operating temperature limit
and maximum operating current limit can result
in a small amount of light degradation over time.
In addition, due to the different rates of thermal
expansion of the epoxy material used for the
emitter package and the metal pins and gold
bond-wire within the LED emitter, there are
upper and lower limits to the operating and
storage temperature ranges for each LED
package. Exceeding these limits, especially for
hundred’s of temperature cycles, can result in
premature catastrophic LED emitter failures.
These effects are covered in detail in AB20-6,
titled “Reliability Considerations for SuperFlux
LEDs.”
For these reasons, it is important to understand
the thermal properties of the individual LED
emitter as well as the thermal properties of all the
elements of the LED signal lamp (printed circuit
board, case, etc). AB20-4, titled “Thermal
Management Considerations for SuperFlux
LEDs” discusses the proper thermal design of an
LED signal lamp. In this application note, thermal
modeling will be covered only in sufficient detail
so as to allow proper circuit modeling to maintain
the LED junction temperature within the
recommended operating temperature limits. This
application note will also use thermal modeling to
estimate the change in electrical and optical
parameters of the LED emitters over temperature
and how these effect the operation of the LED
signal light. For additional detail on thermal
modeling please refer to AB20-4.
LED Emitter Modeling
The purpose of modeling is to represent the
electrical, optical, and thermal characteristics of
a component with equations that allow their
interactions with other electronic components to
be expressed mathematically. The process of
modeling also requires that mathematical
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