This can be done by further subdividing the
thermal modeling equation as shown below:
die will be the first element of the model to heat-
up, followed by the LED emitter package, and
then the rest of the LED signal light.
TJ @ TA + (RqJ LF + RqLF P + RqP A) PD (3.11)
This section discussed the key concepts of
modeling the electrical, optical, and thermal
performance of LED signal lights. Equations #3.3
and #3.7 can be used to model the operation of
an LED emitter at room temperature, ignoring
the effects of self-heating. Equations #3.6, #3.7,
#3.8, and #3.9 can be used together to model
the effects of self-heating of an LED emitter at
room temperature as well as to model the
operation of an LED emitter over temperature.
Equations #3.10 and #3.11 show the various
components of the overall thermal resistance,
RqJA , which can be useful in the thermal
modeling of an LED signal lamp assembly and
the thermal modeling of transient power
conditions.
Where:
Rq J LF = thermal resistance, junction to lead
frame (LED die), °C/W
Rq LF P = thermal resistance, lead frame to pin
(LED package excluding die), °C/W
Please note that each thermal resistance (Rq J LF
Rq LF P, and Rq P A) has a different heating time
constant. The time constant associated with
heating of the LED die is in the order of one
millisecond. The time constant associated with
the heating of the LED emitter package is in the
order of one minute. The time constant
,
associated with the heating of the complete LED
signal lamp is in the order of 10 to 30 minutes.
Thus, for a transient heating condition, the LED
Applications
Resistive Current Limiting
As discussed previously in the section “Key
Concepts for Electrical Design of LED Signal
Lamps,” the choice of the number of LED
emitters per series-string has a large effect on
the forward current regulation and the overall
electrical power consumption of the LED signal
lamp. Most 12V designs commonly use either
three or four emitters per series-string, which is
a good balance of current regulation and
electrical power consumption. Then, the choice
of circuit topology (Figure 3.1 circuits) and the
design current determine the variation in
if the LED signal light will be subjected to
automotive EMC transients.
For a resistive current-limited circuit the electrical
design process consists mainly of picking the
proper value(s) for the current limiting resistor(s).
The key principles of worst-case design are
shown in Figure 3.19. The figure shows the
forward current through one LED string of four
emitters as a function of input voltage. The
equation for this graph (Equation #3.12) is equal
to Equation #3.2 solved for IF:
forward currents for the LED emitters in the
array. Finally, protection circuitry can be added,
Equation #3.2, from “Key Concepts for Electrical
Design of LED Signal Lamps.”
21
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
