luminous intensity variations even using LED
emitters from the same forward voltage and
luminous flux category. Figure 3.5 shows the
worst-case forward current variations within the
LED array when the array is constructed using
LED emitters from the same forward voltage
category. The worst-case calculations assume
that the LED array consists of 16 HPWT-xH00
emitters constructed using four “minimum” LED
emitters, four “maximum” LED emitters, and
eight “typical” LED emitters. Then each
SuperFlux LED emitters from only one forward
voltage category within the same LED array.
paralleled grouping consists of one “max,” one
“min,” and two “typical” LED emitters. The
typical calculations assume that the LED array
consists of 16 HPWT-xH00 emitters
Figure 3.6 Typical EMC Transient Protection Circuits
for LED Signal Lamps.
constructed using two “minimum” LED emitters,
two “maximum” LED emitters, and twelve
“typical” LED emitters. Then two of the
LED emitters are susceptible to permanent
damage due to high voltage automotive EMC
transients. The addition of a high-voltage
silicon diode in series with the LED array can
effectively protect the array from high-voltage
negative transients. The LED array can be
protected from positive “Load Dump”
paralleled groupings consist of one “max,” one
“min,” and two “typical” LED emitters, and the
other two paralleled groupings consist of four
“typical” LED emitters. Lumileds Lighting
recommends a minimum forward current of
35 mA (70 mA for the SnapLED 150), for the
“paralleled-string” circuit in Figure 3.1b or the
“cross-connected parallel-string” circuit
shown in Figure 3.1c. At drive currents less
than 35 mA (70 mA for SnapLED 150), the
“worst-case” forward current variations
between adjacent LED emitters can exceed
2:1. Because of the averaging effects of several
series-connected LED emitters, the circuit in
Figure 3.1b has somewhat lower typical forward
current variations than the circuit shown in
Figure 3.1c. Note that the forward current
matching can be improved with the addition of
a small resistor (ROPT > RS) in series with each
string for the circuit shown in Figure 3.1b or
“rung” for the circuit shown in Figure 3.1c. For
these circuits, it is important to use
transients with the addition of a transient
suppressor connected in parallel with the LED
array. Figure 3.6 shows the addition of EMC
protection circuitry to the LED array. EMC
transient protection is covered in more detail in
the following section “EMC Transient Protection.”
Some applications require the LED array to
operate at two levels of luminous intensity (i.e.
a rear Stop/ Tail signal). Generally, it is desirable
that the LED emitters should appear matched at
both drive conditions. SuperFlux and SnapLED
70 emitters are categorized for luminous flux at
70 mA (150 mA for the SnapLED 150). As shown
in the section “Electrical, Optical, and Thermal
Characteristics of LED Emitters,” the light output
matching for random combinations of LED
emitters gets progressively worse at lower
12
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