chosen. Worst-case forward and reverse
transient currents can be estimated using the
linear forward current model for positive
transients and the minimum breakdown voltage
specification, VBR, on the data sheet for negative
transients.
Worst-Case Circuit Analysis and Validation
The next step in the electrical design should be
an analysis of the forward current through the
LED emitters at worst-case input voltage and
operating temperature extremes using worst-
case component tolerances. All of the active
and passive electronic components used in the
circuit design can be modeled with their worst-
case minimum and maximum values. This
analysis serves several purposes. First, it
determines whether the forward current is less
than the maximum dc forward current under all
operating conditions. Secondly, it determines
the change in light output of the signal lamp
under the same conditions. Finally, it can be
used to determine the worst-case matching
within the LED array.
circuit. Thus, the actual occurrence of these
worst-case conditions could be extremely small.
Another approach to worst-case analysis is to
characterize a number of LED emitters and
determine the appropriate forward voltage
model for each one. Then using a Monte-Carlo
simulation, random combinations of these
emitters can be assembled into a “paper”
circuit and the actual forward currents can
be calculated for the circuit based on the
corresponding forward voltage models. Then
the results from multiple simulations can be
tabulated. This approach provides a much better
understanding of the forward current variations
that would occur in actual practice.
This worst-case analysis can be done in several
different ways. One approach is to use worst-
case values for one or more LED emitters in the
array such as to cause worst-case current
matching between LED emitters or to establish
the maximum or minimum forward current
through individual LED emitters. The problem
with this approach is that probability of this
occurrence actually happening can be quite
low. If the probability of getting worst-case LED
emitters is very small, then the probability of
both minimum and maximum worst-case LED
emitters occurring in the same circuit assembly
is even lower. Furthermore, for the worst-case
variations in forward currents to actually occur,
these worst-case LED emitters must both be
randomly assembled into certain parts of the
In general, within arrays of LED emitters, the
maximum forward current occurs at the
maximum input voltage with the minimum value
of the current limiting resistor and minimum
forward voltage model for the LED emitters.
Likewise, the minimum forward current occurs
at the minimum input voltage with the maximum
value of the current limiting resistor and
maximum forward voltage model for the LED
emitters.
The worst-case forward current variations
for different LED emitters within the array is
determined by the circuit topology, the drive
current, and the variation in electrical
characteristics of the individual LED emitters
in the array. When several LED emitters are
5
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
