Importance of Thermal Management
for HighꢀPower LED Assemblies
Temperature Induced Effects on LED Light Output
The junction temperature of the LED affects the
device’s luminous flux, the color of the device,
and its forward voltage. Junction temperature
can be affected by the ambient temperature
and by selfꢀheating due to electrical power
dissipation.
Typical temperature coefficients for various highꢀ
brightness LEDs are listed in Table 4.1.
The degradation of flux as a function of
increasing temperature for a typical redꢀorange,
absorbingꢀsubstrate (AS) or transparentꢀ
substrate (TS) AlInGaP LED is shown in Figure
4.1. Note, luminous flux has been normalized at
25°C.
The equation for luminous flux as a function of
temperature (°C) is given below:
Tj
ΦV (T2) = ΦV (T1)e–k
This graph shows the profound affect that
temperatures within the normal operating
guidelines can have on luminous flux. As shown,
an increase in the junction temperature of 75°C
can cause the level of luminous flux to be
reduced to oneꢀhalf of its room temperature
value. From this, it is clear that temperature
effects on luminous flux must be accounted for
in the design of a LED assembly.
∆
Where:
ΦV (T1)= luminous flux at junction temperature T1
ΦV (T2)= luminous flux at junction temperature T2
k = temperature coefficient
∆Tj = change in junction temperature (T2ꢀ T1).
Table 4.1
Temperature Coefficient for High-Brightness LED Materials.
LED Material Type
Temperature Coefficient, k
AS AlInGap, Red-Orange
AS AlInGap, Amber
TS AlInGap, Red-Orange
TS AlInGap, Amber
9.52 x 10-3
1.11 x 10-2
9.52 x 10-3
9.52 x 10-2
Figure 4.1 Luminous flux versus ambient
temperature for a typical red-orange AS/TS AlInGap
LED when operated at a constant current.
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
