The resistors should be located in a remote
portion of the PCB (away from the LEDs), on a
separate PCB, or in the wire harness if possible.
If this is not possible, the resistors should be
distributed evenly along the PCB to distribute
the heat generated. In addition, the traces from
resistors to metallized areas surrounding
cathode leads on the LEDs should be
minimized to prevent resistors from heating
adjacent LEDs. This can be accomplished by
thinning down these traces, or by having
metallized areas contacting the LEDs and
resistors only contact the anode leads of the
LED. A portion of an LED CHMSL PCB depicting
the design concepts discussed is shown in
Figure 4.6.
LED Spacing
Most of the electrical power in an LED is
dissipated as heat. Tighter LED spacing
provides a smaller area for heat dissipation,
resulting in higher PCB temperatures and thus
higher junction temperatures. The LEDs should
be spaced as far apart as packaging and
optical constraints will allow. Most CHMSL
applications use only a single row of LEDs at
spacing greater than 15 mm which is ideal, as
opposed to many amber turn signal applications
which use a tightly spaced (less than 10 mm) x y
array of LEDs.
Lamp Housing Design and Mounting of the LED Array
LED lamp housings should be designed to
provide a conductive path from the backside of
the PCB to the lamp housing. This is typically
accomplished by mounting the backside of the
PCB directly to the lamp housing such that they
are contacting one another across the entire
length of the PCB. This mounting scheme can
be improved by applying a thermally conductive
pad between the PCB and the lamp housing.
The thermally conductive pad conforms to the
features on the backside of the PCB and
provides a larger contact area for conduction.
Often the PCB is mounted to the lamp housing
on top of raised bosses. In this case, the area
for conduction into the lamp housing is reduced
to the contact area on the top side of the
bosses, greatly reducing its effectiveness.
Another common configuration mounts the
If the PCB is mounted in such a way that
conduction to the lamp housing is not effective
(trapped air is a very poor conductor of heat),
then allowances for convective cooling should be
made. The most common technique to take
advantage of natural convection is to put holes in
the top and bottom side of the lamp housing to
allow for vertical air flow over the PCB. However,
where the lamp housing must be sealed for
environmental reasons, this type of approach is
impractical.
PCB along its top and bottom edges to slots in
the side of the lamp housing. Again, the area for
conduction into the lamp housing is reduced to
the contact areas of the slots, which reduces the
effectiveness of conduction.
9
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