AN1149-4 Lumileds Lighting, LLC, AN1149-4 Datasheet - Page 6

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

Table 4.2
Estimating Junction-to-Ambient Thermal Resistance
The procedures described in Junction-to-Ambient Thermal
Resistance Measurement Procedure are accurate
methods for determining the R
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 R
of the different classes, and the R
Class 1: Single row of LEDs with the current-limiting
Class 2: Single row of LEDs with the current-limiting
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
temperature: 1) ambient temperature, 2) R
and 3) power into the LED. Below is a sample
junction temperature calculation, which
illustrates how these three factors interact:
resistors/drive circuitry located off of the PCB,
either in the wire harness assembly or on a
separate PCB.
resistors/drive circuitry located on the same PCB
as the LEDs. This is the most common situation
for LED CHMSL assemblies.
θ
ja estimates. Below is an explanation
Typical R
LED Lamp Classification
θ
ja of an LED within a
θ
θ
ja estimates.
ja
Class 1
Class 2
Class 3
Class 4
Values for the Classes of LED Lamp Assemblies
θ
ja
,
6
Class 3: Multiple rows, or an x-y arrangement, of LEDs with
Class 4: Multiple rows, or an x-y arrangement, of LEDs with
Table 4.2: lists the typical R
T
T
Typical values for T
To determine the worst case, highest junction
temperature, this equation becomes:
T
T
T
j
j
j max
jmax
jmax
= (R
= (R
= (R
Typical R
= (R
≤ 125°C
θ
θ
the current-limiting resistors/ drive circuitry located
off of the PCB, either in the wire harness assembly
or on a separate PCB.
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.
lamp assembly listed above. These are only
estimates and should not be used for detailed,
worst-case analyses.
ja
ja
. P
. I
θ
θ
ja
f LED
. P
ja
LED
. I
325
400
500
650
) + T
. V
f max
LED max
θ
ja
f LED
. V
(°C/W)
a
) + T
) + T
a(max)
f max
) + T
θ
are shown in Table 4.3.
a
a max
ja values for each class of LED
a max