AB05 Datasheet, PDF (4/12 Pages) Lumileds Lighting Company – LUXEON® Power Light Sources

Inputs/Output of the Thermal
Model
You can use a thermal model to predict the junction temper
ature (T_J) for your application. This section discusses setting
a goal for a maximum T_J as well as the variables in the right
hand side of Equation 3 below. You can use variables in the
thermal model as control factors in your application design.
T_Junction = T_Ambient + (P_d)(RΘ_{Junction Ambient})
A. Set Limit for Junction Temperature (T_J)
Good thermal design incorporates TJ limits based on three
factors:
1. Light output with T_J rise
2. Color shift with T_J rise
3. Reliability
Consult LUXEON Custom Design Guide for more detailed
information on light output and color shift with rise in T_J.
1. Light Output with Temperature Rise
LEDs experience a reversible loss of light output as the T_J
increases. The lower the T_J is kept, the better the luminous
efficiency of the product (i.e. the better the light output). Light
output from red, red orange and amber emitters (based on
AlInGaP LED technology) are more sensitive to increases in
junction temperature than other colors.
An example of light output loss associated with temperature
rise occurs with traffic signals. Signals that are simply retro
fitted with LED sources may not account adequately for heat
dissipation. As temperatures rise during the day, the signals
may dim. Redesigning the signal housing to provide airflow
to cool the components alleviates this condition.
The chart on the LUXEON product data sheet will help you
determine a maximum TJ based on the light output require
ments of your application.
2. Color Shift with Temperature Rise
Emitter color can shift slightly to higher wavelengths with T_J
rise. Shift values quantifying this effect are included in the
LUXEON Custom Design Guide. Red, Red Orange and
Amber color emitters are the most sensitive to this effect,
although the human eye is more sensitive to color changes
in the amber region. The importance of this effect depends
on the color range requirements for the application. If the
allowed color range is very small, you will need to account
for color shift when setting your maximum T_J goal.
3. Reliability-Based Temperature Ratings
To ensure the reliable operation of LUXEON Power Light
Sources, observe the absolute maximum thermal ratings for
the LEDs provided in Table 1. The maximum T_J is based on
the allowable thermal stress of the silicone encapsulate that
surrounds die.
Table 1. Maximum Thermal Ratings.
Parameter
LED Junction Temperature
Aluminum Core PCB Temperature
Storage/Operating Temperature:
Maximum
120
105
LUXEON Products without optics
(Star, Star/C)
40 to 105
LUXEON Products with optics
(Star/O, Line, Ring)
40 to 75
B. Assess Ambient Temperature Conditions
The designer must take into account the maximum ambient
temperature (T_A ) the LUXEON Power Light Source will expe
rience over its lifetime. In most cases, you can use product
standards to determine the worst case T_A. Otherwise, use
representative maximum T_A measurements. Please note that
the ambient temperatures should include other sources of
heat such as electronics or heating due to sun exposure.
C. Power Dissipated
The dissipated power (P_d) can be determined as the forward
voltage (V_f) of the emitter times the forward current (I_f). The
portion of power emitted as visible light (about 10%) is negli
gible for thermal design.
D. Add Heat Sink to Model
The RΘ_{B A} component of RΘ_{J A} (see Figure 1A) represents
the heat sink and attachment interface. The responsibility
for the proper selection of the heat sink thermal resistance,
RΘ_{B A}, lies with the engineer using the product. A process
for selecting RΘ_{B A} is explained in the examples that follow.
Many resources exist to assist with this selection. Some are
listed in the resources section at end of this document. The
following section provides additional guidance to help you
determine the most suitable heat sink for your application.
Heat Sink Characterization
A. Explanation of Data Charts
1. Test Set Up
We tested some typical heat sink configurations on LUXEON
Stars and Floods including both finned and flat heat sinks.
We used the following test conditions: free (or natural)
convection environment with no fan (Figures 3A, 3B, 3C and
3D) and forced convection in a small wind tunnel (Figure 3E).
The LUXEON Stars tested did not have optics. The optics do
not affect the RΘ_{J B} of the LUXEON emitter; however,
depending on the orientation, they may affect the convection
flow over the attached heat sink.
Thermal Design Using LUXEON Power Light Sources App Brief AB05 (6/06) 4