Figure 3.2 Forward Current Through String of
HPWA-xHOO LED Emitters versus Applied
Voltage.
Figure 3.3 Total Supply Current versus Supply
Voltage for Sixty HPWA-xHOO Emitter LED
Signal Lamp Array Driven in Series Strings
with Two, Three, Four, Five, or Six Emitters
per String.
The number of LED emitters per string also
affects the total supply current. For a fixed
number of LED emitters longer string lengths
result in fewer total strings and thus a lower
total supply current. Figure 3.3 shows the
total supply current for the series-string
configurations in Figure 3.2. Figure 3.3 assumes
a total of 60 LED emitters, thus 2-LED strings
would require 30 strings, 3-LED strings would
require 20 strings, etc. Note that the total
supply current is much higher for series strings
with 2 and 3 emitters.
Variations in the forward voltage characteristics
of the individual LED emitters can lead to forward
current variations within the LED array. These
forward current variations directly affect the
luminous flux output of each LED emitter and can
cause noticeable luminous intensity variations
within the LED array. These random variations in
forward voltage characteristics affect the three
circuits shown in Figure 3.1 differently.
The “series-string” circuit shown in Figure 3.1a
is least affected by random forward voltage
variations between the LED emitters because the
forward voltages of all LED emitters in a given
string are averaged together. In many cases, one
emitter with a high forward voltage can cancel
out another emitter in the same series-string with
a low forward voltage. In addition, the voltage
drop across the current limiting resistor, R, is
much higher than the combined voltage drops
across the series resistors, yRS. Thus, small
variations in RS only cause small variations in the
forward current through the series string.
As shown by Figures 3.2 and 3.3, the choice of
the number of emitters per string is a tradeoff
between the regulation of forward current due
to input voltage variations and the total supply
current for the LED array. Small string lengths
give excellent forward current regulation, but
require a higher supply current. Long string
lengths provide poor forward current regulation
but require less supply current. For these
reasons, most 12 V resistive limited designs
use three or four LED emitters per series
string (y = 3 or 4).
Because SuperFlux and SnapLED 70 emitters
are categorized for forward voltage at 70 mA
(150 mA for the SnapLED 150), the smallest
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