Table 4.2
Typical Rθ Values for the Classes of LED Lamp Assemblies
ja
Typical Rθ (°C/W)
LED Lamp Classification
ja
Class 1
Class 2
Class 3
Class 4
325
400
500
650
Estimating Junction-to-Ambient Thermal Resistance
The procedures described in Junction-to-Ambient Thermal
Resistance Measurement Procedure are accurate
methods for determining the Rθja of an LED within a
plastic lamp assembly. However, in some cases, the time
and/or equipment may not be available to perform such
testing. In these cases, an educated estimate may be the
best method available. Lumileds has developed some
basic classifications of LED lamp assemblies and
corresponding Rθja estimates. Below is an explanation
of the different classes, and the Rθja estimates.
Class 3: Multiple rows, or an x-y arrangement, of LEDs with
the current-limiting resistors/ drive circuitry located
off of the PCB, either in the wire harness assembly
or on a separate PCB.
Class 4: Multiple rows, or an x-y arrangement, of LEDs with
the current-limiting resistors/ drive circuitry located
on the same PCB as the LEDs. This is the most
common situation for LED rear combination lamp
applications.
Table 4.2: lists the typical Rθja values for each class of LED
lamp assembly listed above. These are only
estimates and should not be used for detailed,
worst-case analyses.
Class 1: Single row of LEDs with the current-limiting
resistors/drive circuitry located off of the PCB,
either in the wire harness assembly or on a
separate PCB.
Class 2: Single row of LEDs with the current-limiting
resistors/drive circuitry located on the same PCB
as the LEDs. This is the most common situation
for LED CHMSL assemblies.
Evaluation Junction Temperature and Forward Current
The primary concern when evaluating the
thermal characteristics of an LED assembly
is to ensure that the junction temperature of
the LEDs is kept below the specified maximum
value (125 °C for SuperFlux LEDs). There are
three factors which determine junction
Tj = (Rθ . PLED) + Ta
ja
Tj = (Rθ . If LED . Vf LED) + Ta
ja
Typical values for Ta(max) are shown in Table 4.3.
To determine the worstꢀcase, highest junction
temperature, this equation becomes:
temperature: 1) ambient temperature, 2) Rθ ,
ja
and 3) power into the LED. Below is a sample
junction temperature calculation, which
illustrates how these three factors interact:
Tj max = (Rθ . PLED max ) + Ta max
ja
Tjmax = (Rθ . If max . Vf max ) + Ta max
ja
Tjmax ≤ 125°C
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