Lumileds plots these curves for different values
referred to as the derating curves. The derating
curves for HPWTꢀxx00 devices, are shown in
Figure 4.4. Derating curves for HPWAxx00
devices are provided in the SuperFlux LED
Technical Data Sheet. Refer to sideꢀbar Derating
Example Cases for further explanation.
of Rθ along with their intersection with the
ja
maximum drive current of 70 mA, and their
intersection with the maximum ambient
temperature of 100 °C and includes this graph
in all LED data sheets. This graph is typically
Light Output and Forward Current
The relationship between light output and
decrease as forward current is increased. For
forward current for different thermal resistances
is shown in Figure 4.5. For LED assemblies with
assemblies with high Rθ , a great deal of heating
ja
occurs resulting in high junction temperatures.
In these cases, the effects of increasing junction
temperature can offset the effects of increasing
forward current. Proper thermal management
and drive current selection is critical to
low thermal resistances (Rθ = 200 °C/W), the
ja
relative flux increases almost proportionally to
the forward current. However, for LED
assemblies with high thermal resistances
(Rθ = 600 °C/W), the relative flux can actually
maximizing the performance of LEDs.
ja
Derating Example Cases
Case 1—Class 1 LED CHMSL
From Table 4.2 the thermal resistance can be estimated
as Rθja = 650 °C/W. Using Figure 4.4, the maximum
allowable forward current through each LED is 30 mA at
Ta(max) = 75 °C.
Consider an LED CHMSL application using 12 HPWTꢀ
MH00 LEDs in a row, with a current limiting resistor in the
wire connector. The auto manufacturer has specified a
maximum ambient temperature of 75 °C.
As can be seen from these simplified sample cases, the
Rθja has a major impact on junction temperature, and thus
maximum allowable forward current. The different
applications using the same LED have a difference in
maximum forward current of nearly 2:1.
From Table 4.2 the thermal resistance can be estimated
as Rθja = 325 °C/W. Using Figure 4.4, the maximum
allowable forward current through each LED is 55 mA
at Ta (max) = 75 °C.
Case 2—Class 4 LED Rear Combination Lamp (RCL)
Consider an LED RCL application using 36 HPWTMH00
LEDs in a 6x6 pattern, with the drive circuitry on the same
PCB as the LEDs. The auto manufacturer has specified a
maximum ambient temperature of 75 °C.
A more detailed determination of maximum forward current is
presented in Application Brief 20ꢀ3 Electrical Design
Considerations for SuperFlux LEDs.
7
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