AB05 Datasheet, PDF (5/12 Pages) Lumileds Lighting Company – LUXEON® Power Light Sources

Vertical Supports
Fins
vertical
Figure 3.A. Finned Horz.
Insulating foam
Figure 3.B. Finned Vert.
Figure 3.C. Flat Horz.
Wind tunnel
Fan
Figure 3.D. Flat Vert.
HS fins parallel
to forced air flow
Figure 3.E. Finned Horz. in Wind Tunnel
We tested two types of heat sinks: finned heat sinks and flat
plates. All heat sinks were aluminum, which is typically the
best choice because of its excellent thermal conductivity and
ready, low cost availability. We tested several different sizes
of flat heat sinks and two sizes of finned heat sinks.
We tested some samples in free convection oriented both
horizontally and vertically, as illustrated in Figures 3B, 3C
and 3D.
Finned heat sinks were tested in a small wind tunnel
enclosed in a control volume. Figure 3E shows the forced
air set up. We used the same set up to characterize the
finned heat sinks in free convection by turning the fan off
(Figure 3A).
We suspended the finned heat sink so that air could circulate
underneath it.
We used mechanical fasteners to mount the LUXEON Stars.
The mounting surface of the heat sink was smooth and
lightly polished. We did not use thermal grease.
We ran all tests in a closed volume test box to control the
free convection and to improve repeatability. We made all
measurements at steady state conditions. Initial ambient
conditions were nominally 25°C, but the ambient tempera
ture increased as the LEDs reached steady state
temperatures.
2. Heat Sink Characterization Chart Format
The following charts (Figures 4 to 9) are intended to guide
the design engineer in selecting the size and type of heat
sink required for an application. The charts for 25 mm
spaced emitters in Figures 4 to 8 show RΘ_{B A} on the y axis
vs. heat sink area required per emitter on the x axis. The
chart for densely spaced emitters in Figure 9 shows RΘ_{B A}
vs. heat sink area required for the entire array. The heat sink
type and test set up (Figures 3A to 3E) is referenced in the
title and discussion of each chart.
3. Definition of Heat Sink Size
The following charts quantify heat sink size in two ways. The
term "exposed surface area" is the sum total of all surfaces
of the heat sink exposed to convection. The "footprint area"
quantifies the projected area of the heat sink as shown in
following diagram.
A finned heat sink can fit more exposed surface area in a
given foot print than a flat heat sink.
Foot Print Area
Flat Heat Sink
Finned Heat Sink
B. Heat Sink Characterization Charts 25mm
Emitter Spacing
When LUXEON emitters are spaced at least 25 mm apart,
each acts as a discrete heat source. The charts in figures 4
to 8 will help you size heat sinks for the LUXEON Star, Line
and Ring as well as custom boards with individual emitters
spaced 25 mm or further apart. These charts should also be
helpful in characterizing heat sinks for custom boards with
emitter spacing as dense as 20 mm. For boards with more
densely spaced emitters, use the chart in Section C. The
following in Figures 4 to 8 show RΘ_{B A} vs. heat sink area
required per emitter in your application.
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
0
R² = 0.9798
1
2
3
4
5
6
7
8
9 10
FOOT PRINT AREA (=EXPOSED SURFACE AREA) - in²
Flat Heat Sink, 0.09" (2.3 mm) Horizontal on insulating foam
Set up in Figure 3C.
Solid Line: Linear Fit of Data
Figure 4. RΘ_{Board Ambient} per Emitter vs. Foot Print Area
Thermal Design Using LUXEON Power Light Sources App Brief AB05 (6/06) 5