AN1149-4 Lumileds Lighting, LLC, AN1149-4 Datasheet - Page 4

• Manufacturer Part Number: AN1149-4
• Description: Thermal Management Considerations for SuperFlux LEDs
• Manufacturer: Lumileds Lighting, LLC

Thermal Modeling of LED Assemblies
Thermal Resistance of LED Automotive Signal Lamps
Thermal resistance is associated with 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
defined as shown in the equation below:
Where:
R
∆
q
The thermal resistance of an LED signal lamp
(junction to ambient thermal resistance, or R
is made up of two primary components: the
thermal resistance of the LED package
(junction to pin thermal resistance, or R
the thermal resistance of the lamp housing (pin
to ambient thermal resistance, or R
two components of thermal resistance are in a
series configuration, therefore:
This is shown graphically in Figure 4.3.
(LED emitter)
R
T = temperature difference between those
X
θ
= rate of heat transfer between those two
θ
= thermal resistance between two points
two points
points
jp
+
(lamp housing)
R
θ
pa
=
(LED signal lamp)
R
θ
ja
θ
pa
). These
θ
jp
_
) and
θ
ja
)
4
Assuming all the electrical power is dissipated in
the form of heat (approximately 5 to 10% of the
power is dissipated optically), the equation for
junction to pin thermal resistance (R
can be written in the form of the equation below:
Where:
P = the total electrical power into the LED (I
For LED lamp assemblies, the equation for
junction to ambient thermal resistance, R
of an individual LED within the assembly can
be written as:
Where T
As can be seen from this equation, in order to
determine R
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.
Figure 4.3 Graphic representation of the components
of thermal resistance.
j
=
∆
θ
T
ja
j
+ T
of an LED within a lamp assembly,
a
.
θ
jp
) of an LED
θ
ja
f
,
* V
f
)