Circuit Design
Circuit design can help control the junction
temperature of the LEDs in two important ways:
1) minimize fluctuations in the drive current
(power input), and 2) dissipate a minimum
amount of heat, or dissipate heat in such a
way as to minimize its effect on the LEDs.
Figure 4.7 Schematic of a current control circuit for
LED automotive lamp applications.
Current Control
An ideal drive circuit will provide the same
current to the LEDs even as ambient
temperatures and battery voltages vary.
Inexpensive, simple current control circuits
can be designed to accomplish this task.
A schematic of such a circuit is shown in
Figure 4.7.
Current control circuits are often too expensive
and unnecessary for LED CHMSL applications.
The most common LED CHMSL drive circuit
consists of a current limiting resistor(s) and a
silicon diode for reverse voltage protection in
series with the LEDs. In this circuit design,
the input current into the LEDs varies as the
battery voltage changes. The current control
characteristics of this type of circuit improve
as larger resistor/s are used with fewer LEDs
in series. However, circuits with fewer LEDs in
series will have greater heat generation in the
drive circuit. Figure 4.8 graphs the forward
current provided to the LEDs vs. the input
battery voltage for resistor circuits with three,
four, and five LEDs in series.
Figure 4.8 LED forward current vs. battery voltage
for circuits of two, three, four and five LEDs in
series with a current limiting resistor.
Power Dissipation
If the LED drive circuit is in a remote location
relative to the LEDs (in the wire harness or on a
separate PCB), then the power dissipated by the
drive circuit does not affect the junction
temperature of the LEDs. Drive circuit heating
is a concern when the drive circuit is on the same
PCB as the LEDs. Drive circuit power dissipation,
and thus heat generation is inversely proportional
to the number of LEDs in series. Circuits with
fewer LEDs in series will have greater heat
generation in the drive circuit.
For most automotive applications in which the
battery voltage is approximately 13 V, Lumileds
recommends configuring four LEDs in series. Four
LEDs in series is a good compromise between
forward current control, heat generation, and
minimum turnꢀon voltage for the LED array.
For more information on picking the optimum
design current, and LED drive circuit for your
application, please reference Application Brief
20ꢀ3 Electrical Design Considerations for
SuperFlux LEDs.
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