Figure 3.4 Worst-Case and Typical Variations
in Forward Current Between Two Strings of
HPWT-xHOO LED Emitters Driven with
Individual Current-Limiting Resistors per
String (Figure 1a Circuit, with y=4).
Figure 3.5 Worst-Case and Typical Variations
in Forward Current Between 16 HPWT-xHOO
LED Emitters Driven in a Cross-Connected
Paralleled String Configuration (Figure 1c
Circuit with x = y = 4, z = 1).
forward current variations between adjacent
series-strings of LED emitters occur at a drive
current of 70 mA (150 mA for the SnapLED
150). However, the forward current matching
between adjacent strings is quite good even at
forward currents as low as 10 mA (20 mA for
the SnapLED 150). Figure 3.4 shows the worst-
case forward current variation between two LED
strings constructed using four LED emitters per
string. The worst-case calculations assume that
all HPWT-xH00 emitters are from the same
forward voltage category and one string uses
four “minimum” forward voltage emitters and
the other string uses four “maximum” forward
voltage emitters. The typical calculations
SuperFlux and SnapLED 70 emitters and 40
mA for SnapLED 150 emitters (see the section
“Electrical, Optical, and Thermal
Characteristics of LED Emitters).
The “paralleled-string” circuit shown in Figure
3.1b and the “cross-connected series string”
circuit shown in Figure 3.1c do not regulate the
forward current as well as the “series-string”
circuit. When two or more LED emitters are
connected in parallel, the forward current
through each emitter will be somewhat higher or
lower than the average forward current through
them so as to force the forward voltage across
them to be the same. Again, because SuperFlux
LED and SnapLED 70 emitters are categorized
for forward voltage at 70 mA (150 mA for the
SnapLED 150), the smallest forward current
variations between adjacent LED strings (Figure
3.1b circuit) or between LED emitters in the
same “rung” (Figure 3.1c circuit) occur at a drive
current of 70 mA (150 mA for SnapLED 150).
The variations in forward currents become much
worse at lower drive currents. These variations in
forward currents can cause unacceptable
assume that all HPWT-xH00 emitters are from
the same forward voltage category and one
string consists of two “minimum” forward
voltage emitters and two “typical” forward
voltage emitters and the other string consists of
two “maximum “forward voltage emitters and
two “typical” forward voltage emitters. Due to
potential variations in luminous flux output at
low currents, Lumileds Lighting recommends
a minimum forward current of 20 mA for
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