limiting resistor may be chosen to overly restrict
the forward current through the array. Another
possibility is that if the circuit is designed to
accommodate every LED emitter in a given
array being at their worst case minimum or
maximum limits, then design might eliminate
several potential circuit topologies because of
excessive light output variations. In practice,
the likelihood of these conditions actually
occurring is so small, that the design process
might have eliminated more cost effective circuit
topologies. For this reason, Lumileds Lighting
recommends that worst case design be used
as a development tool in conjunction with
characterization and validation of the signal
lamp assembly.
Following this paper design, a working
prototype LED signal light can be constructed.
This prototype serves several purposes. First,
it allows verification of the electrical design. The
forward current can be measured at different
input voltages and compared with the paper
electrical design. Second, it allows verification
of the optical design. In AB20 1 section,
“Estimating the Number of LED Emitters
Needed,” assumptions were made for the
optical transmission losses. The prototype allows
these assumptions to be measured. Third, the
working prototype allows the thermal properties
of the LED signal light to be evaluated. The
thermal resistance,
Rθ
PIN A
, can be measured by
attaching thermocouples to the cathode pins of
several LED emitters in the array.
Based on the electrical, optical, and thermal
measurements of the working prototype,
additional iterations of the design may be
required. These design iterations will further
refine the estimates for the electrical component
values, the number of LED emitters needed for
the signal lamp, and the thermal resistance of
the signal lamp. Also, it may be necessary to
evaluate improved optical designs and methods
for improving the thermal properties of the LED
signal lamp assembly. For more information on
thermal design, the reader is encouraged to
review AB20 4 “Thermal Management
Considerations for SuperFlux LEDs.” For
more information on optics design, the reader
is encouraged to review AB20 5 “Secondary
Optic Design Considerations for SuperFlux
LEDs.”
7
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
