to maintain fixed current through the LED array
if the output voltage of the regulator tracks
the DVF /DT of the LED array (approximately
–2 mV/°C times the number of emitters in each
series string). Finally, if the voltage regulator is
used with parallel-connected LED emitters,
such as shown in Figure 3.1b or 3.1c, there can
still be similar forward current variations within
the LED array as was discussed in the section
“Key Concepts for Electrical Design of LED
Signal Lamps.” 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.
Comparison of Three Constant-Current Circuits
SETUP:
Suppose an LED signal lamp is being designed
using 30 HPWT-DH00 SuperFlux LED emitters
from forward voltage category 3. The circuit will
be designed to operate at 50 mA per emitter at
a design voltage of 12.8 V.
SOLUTION:
For the first two designs, the value of the external
current limiting resistor, R, can be determined
using Equation #3.2. Note, for forward voltage
category 3, the nominal linear forward voltage
model is VO NOM = 1.91 V, and RS NOM = 9.2 ohm.
Thus, for the three-LED string circuit, R = 114
ohm. For the four-LED string circuit, R = 66 ohm.
The detailed designs are shown in Figure 3.22.
Then over an input voltage range of 7 to 18 volts,
the forward current through each LED string
would vary as shown in Figure 3.23.
PROBLEM STATEMENT:
How does the overall power consumption
compare for the following 4 possible circuit
designs over an input voltage range of 9 V
to 18 V?
1. Resistive current limiting (Figure 3.1a circuit)
with ten strings of three emitters per string.
2. Resistive current limiting (Figure 3.1a circuit)
with eight strings of four emitters per string.
3. Series-pass constant-current regulator
driving ten strings of three emitters per string.
4. Switching constant-current regulator driving
ten strings of three emitters per string.
Figure 3.22 Two LED Signal Lamp Designs Using
Resistive Current Limiting.
34
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
