Thermal Modeling of LED Assemblies
Thermal Resistance of LED Automotive Signal Lamps
Assuming all the electrical power is dissipated in
Thermal resistance is associated with the
the form of heat (approximately 5ꢀtoꢀ10% of the
conduction of heat, just as electrical resistance
is associated with the conduction of electricity.
Defining resistance as the ratio of driving
potential to the corresponding transfer rate,
thermal resistance for conduction can be
power is dissipated optically), the equation for
junctionꢀtoꢀpin thermal resistance (Rθ ) of an LED
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can be written in the form of the equation below:
defined as shown in the equation below:
_
Where:
P = the total electrical power into the LED (If * Vf)
Where:
For LED lamp assemblies, the equation for
Rθ = thermal resistance between two points
∆T = temperature difference between those
two points
junctionꢀtoꢀambient thermal resistance, Rθ ,
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of an individual LED within the assembly can
be written as:
qX = rate of heat transfer between those two
points
The thermal resistance of an LED signal lamp
Where Tj = ∆Tj + Ta .
(junctionꢀtoꢀambient thermal resistance, or Rθ
)
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is made up of two primary components: the
thermal resistance of the LED package
As can be seen from this equation, in order to
determine Rθ of an LED within a lamp assembly,
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(junctionꢀtoꢀpin thermal resistance, or Rθ ) and
the rise in junction temperature, and the electrical
power into the device must be determined. The
electrical power into the LED under test can
easily be determined by multiplying its forward
current and forward voltage. The rise in junction
temperature can be determined by measuring
the change in forward voltage of the LED under
test.
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the thermal resistance of the lamp housing (pinꢀ
toꢀambient thermal resistance, or Rθ ). These
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two components of thermal resistance are in a
series configuration, therefore:
Rθ
+
Rθ
=
Rθ
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(LED emitter)
(lamp housing)
(LED signal lamp)
This is shown graphically in Figure 4.3.
Figure 4.3 Graphic representation of the components
of thermal resistance.
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