connected in series, the worst-case minimum
forward current would occur when all LED
emitters in a given series string have the worst-
case maximum forward voltage. Likewise, the
worst-case maximum forward current would
occur when all LED emitters in another series-
string have the worst-case minimum forward
voltage. For these series-string circuits, the
likelihood of all LED emitters being at their
worst-case forward voltage extremes is quite
low. When LED emitters are connected in
parallel, the forward current through each LED
will vary somewhat from the average forward
current so as to generate the same forward
voltage across all LED emitters in the parallel
grouping. The worst-case forward current
variations occur when one LED emitter has
the worst-case minimum forward voltage and
another LED emitter in the same parallel
grouping has the worst-case maximum
forward voltage.
range of 35 to 70 mA (70 to 150 mA for the
SnapLED 150). These matching effects are
covered in more detail in the section “Key
Concepts for Electrical Design of LED Signal
Lamps.”
The reader will need to determine whether the
assumptions used for the worst-case designs are
reasonable. It is possible to design with such
large tolerances, that the worst-case design
results in an over-designed circuit. Over-
designing occurs if significant cost is added to
the assembly in order to protect against the
remote possibility of occurrences that might
never happen in practice. In the case of LED
signal lamps, over-designing might result in many
more LED emitters being added to the array than
needed. The best example might be where the
designer chooses an extremely high worst-case
input voltage and maximum ambient
temperature. Then, the use of the suggested
design process in the “Resistive Current Limiting”
section would result in a fairly small design
current at the design input voltage. This design
current would require a large number of LED
emitters to achieve the desired light output from
the array. Or the assumptions used for the worst-
case input voltage and ambient temperature
might require the use of a more expensive
constant current drive circuit, where with more
reasonable assumptions a resistive circuit could
have been used. These concerns about over-
designing by using excessive tolerances on input
voltage and ambient temperature also can be
applied to LED emitter tolerances. For example,
the probability of all LED emitters in a given array
being at their worst-case minimum or maximum
limits is very small but still is greater than zero. If
every LED emitter is assumed to be at the worst-
case minimum extreme, then the external current
SuperFlux and SnapLED 70 emitters are
categorized for forward voltage at 70 mA.
As might be expected, the smallest forward
current variations within an array of SuperFlux
or SnapLED 70 LED emitters occur at drive
current approaching 70 mA. Similarly, SnapLED
150 emitters are categorized for forward voltage
at 150 mA, so the best matching occurs at 150
mA. At lower forward currents, the variations in
forward current within the LED array become
larger—especially when LED emitters are
connected in parallel. For series-string circuits,
acceptable forward current variations can
usually be achieved over forward currents over
a range of 20 to 70 mA (40 to 150 mA for the
SnapLED 150). However, when LED emitters
are connected in parallel, acceptable forward
current variations can be achieved only over a
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