Different mathematical models can be used to
model the forward voltage of the LED emitters
depending on the accuracy and dynamic range
needed.
and LED emitters. With this technique, the
voltages and currents through each component
can be determined based on the expected
minimum and maximum limits of component
values in the circuit. Another technique is to use
a Monte Carlo simulation. With this method the
voltages and currents through each component
are determined for random combinations of
component values in the circuit. Then the results
for a large number of Monte Carlo simulations
are statistically tabulated. The Monte Carlo
simulation method gives a better estimate of the
expected manufacturing variations for the circuit.
Circuit analysis uses the same types of
simultaneous linear equations used in the
electrical design. However, circuit analysis
generally assumes that the external voltage
applied to the circuit, the values of electrical
components, and the ambient temperature can
vary over some predetermined range. Circuit
analysis can be done using “worst-case”
electrical models for the electronic components
Circuit Design Overview
As stated earlier, the first step in the electrical
design is to pick one of the circuit topologies.
Next, the operation of the circuit can be
modeled with a series of simultaneous linear
equations that describe the current through
each electronic component as a function of
component values and applied voltage. For
circuit design, it is usually assumed that all
LED emitters have the same electrical
characteristics, which greatly simplifies the
mathematical modeling.
As will be shown in the section “Key Concepts
for the Electrical Design of LED Signal Lamps”
the different circuit topologies provide different
levels of forward current regulation and overall
electrical power consumption. Circuits with poor
forward current regulation would require the LED
emitters to be driven at a lower forward current
at the nominal input voltage than would circuits
with better forward current regulation (so as not
to exceed the maximum forward current at the
maximum input voltage). Circuits with higher
amounts of power consumption would tend to
have higher internal self-heating (unless the
circuitry is located outside the signal lamp
In order to ensure reliable operation, the
maximum forward current through the LED
emitters should not exceed the maximum value
obtained from Figure 4 in the HPWx-xx00 data
sheet. Note that the maximum forward current
of the LED emitters is based on the maximum
ambient temperature, TA, the maximum input
voltage, and the thermal resistance, R qJA, of the
LED signal light assembly.
housing), which would also tend to reduce the
maximum forward current of the LED emitters. At
this point, it may be desirable to evaluate several
different circuit topologies on paper and see
which one gives the “best” overall results.
If the signal lamp will be exposed to high-voltage
EMC transients, then the appropriate protection
circuitry can be added to the basic circuit
4
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
