Cree® XLamp® LED
Thermal Management
XLamp LEDs lead the solid-state lighting industry in brightness while providing
a reflow-solderable design that is optimized for ease of use and thermal
management. Lighting applications featuring XLamp LEDs maximize light
output and increase design flexibility, while minimizing environmental impact.
They are used in a broad range of both specialized and general lighting
applications. When designing lighting systems using LEDs, one of the most
critical design parameters should be the system’s ability to draw heat away
from the LED junction. High operating temperatures at the LED junction
adversely affect the performance of LEDs, resulting in decreased output and
lifetime.
This application note serves as a guide to understanding thermal management
of XLamp LEDs and minimizing the effects of elevated junction temperatures.
Introduction
The majority of LED failure mechanisms are temperature-dependent. Elevated junction temperatures cause light
output reduction and accelerated chip degradation. The maximum junction temperature for each product line is
specified in the product data sheet. Junction temperature is primarily affected by three parameters:
•
•
•
Ambient temperature of the LED’s immediate surroundings
Thermal path between the LED junction and ambient conditions
Power dissipated by the LED
5.002
ote: CLD-AP0
Application N
When designing lighting systems using high-power LEDs, the following general guidelines should be followed:
•
The most important consideration for successful thermal design is to minimize the amount of heat that
needs to be removed. It is important to separate the LED drive circuitry from the LED board so that the heat
generated by the driver will not contribute to the LED junction temperature.
•
The next most effective strategy is to minimize the ambient temperature inside the fixture. This goal is
achieved by paying attention to several design parameters such as a conservative packaging design that does
not allow the upper limit on overall system power density to be reached. Maintaining clear and clean airflow
paths for natural convection cooling is vital as well.
•
Enhancing thermal conductivity between the heat sinks and the LED is very preferable for thermal management.
Even though the heat removal from the heat sink is via convection, the path from the LED to the heat sink
depends upon conduction.
•
Finally, the orientation of the LED PCB/heat sink should be considered carefully. It is important to position the
board/heat sink so that the plane is vertical. If the board plane is horizontal, it will block the formation of air
convection currents and substantially reduce the cooling capability of the system.
Subject to change without notice.
www.cree.com/xlamp
1
Thermal Management Technology
Thermal resistance
The thermal resistance between two points is defined as the ratio of the difference in temperature to the power dissipated.
For calculations in this document the units used are °C/W. In the case of LEDs, the resistance of two important thermal
paths affects the junction temperature:
•
•
From the LED junction to the thermal contact at the bottom of the package. This thermal resistance is governed
by the package design. It is referred to as the thermal resistance between junction and solder point (Rth j-sp)
From the thermal contact to ambient conditions. This thermal resistance is defined by the path between the solder
point and ambient. It is referred to as the thermal resistance between solder point and ambient (Rth sp-a)
The overall thermal resistance between the LED junction and ambient (Rth j-a) can be modeled as the sum of the series
resistances Rth j-sp and Rth sp-a.
LED Chip
Solder Point
Rth j-sp
Ambient
Rth sp-a
Tj
Ta
Tsp
Figure 1: Thermal Resistance Model
Power dissipated
The total power dissipated by the LED (Pd) is the product of the forward voltage (Vf) and the forward current (If) of the
LED.
Junction temperature
The temperature of the LED junction (Tj) is the sum of the ambient temperature (Ta) and the product of the thermal
resistance from junction to ambient and the power dissipated.
Tj = Ta + (Rth j-a x Pd)
Calculations
In the most cases, power LEDs will be mounted on metal-core printed circuit boards (MCPCB), which will be attached
to a heat sink. Heat flows from the LED junction through the MCPCB to the heat sink by way of conduction. The heat
sink diffuses heat to the ambient surroundings by convection. In most LED applications, the contact thermal resistance
between LED and MCPCB and/or heat sink is small with respect to the thermal resistance between the junction and
thermal pad and thermal pad to ambient.
LED Chip
Solder Point
Rth j-sp
Tj
Heatsink
Rth sp-h
Tsp
Ambient
Rth h-a
Th
Ta
Figure 2: Thermal Resistance Model Including Heat Sink
When a heat sink is used, the total thermal resistance is the series resistances from the junction to the solder point (Rth
), from the solder point to the heat sink (Rth sp-h) and from the heat sink to ambient (Rth h-a).
j-sp
Rth j-a = Rth j-sp + Rth sp-h + Rth h-a
Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree,
the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property
of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association.
2
CLD-AP05.002
Cree, Inc.
4600 Silicon Drive
Durham, NC 27703
USA Tel: +1.919.313.5300
www.cree.com/xlamp
Note that the direct heat loss from the LED package to ambient is small enough to be neglected for calculations.
The overall design goal in determining the size and nature of the required heat sink is to calculate the maximum heat
sink thermal resistance (Rth h-a) that will maintain the junction temperature below the maximum value at worst-case
operation conditions.
Example 1: Heat Sink Thermal Resistance
In this example, six white 7090 XLamp LEDs are used in an application that sees a maximum ambient temperature
(Ta) of 55°C. Assuming a typical forward voltage (Vf) of 3.25 V at 350 mA and that the power supply is outside of the
fixture, the total power dissipated is:
Ptotal = 6 x 0.350 A x 3.25 V = 6.825 W
LED Chip
Rth j-sp
Tj
LED Chip
Rth j-sp
Tj
LED Chip
Rth j-sp
Heat Sink
Tj
Rth sp-h
Solder Point
LED Chip
Tsp
Ambient
Rth h-a
Th
Ta
Rth j-sp
Tj
LED Chip
Rth j-sp
Tj
LED Chip
Rth j-sp
Tj
Figure 3: Thermal Resistance Model for Six (6) XLamp LEDs
Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree,
the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property
of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association.
3
CLD-AP05.002
Cree, Inc.
4600 Silicon Drive
Durham, NC 27703
USA Tel: +1.919.313.5300
www.cree.com/xlamp
The
)) is
The thermal
thermal resistance
resistance from
from junction
junction to
to solder
solder point
point (R
(Rthth j-sp
is listed
listed in
in the
the data
data sheet
sheet as
as 8°C/W.
8°C/W. The
The maximum
maximum LED
LED
j-sp
junction
(Tj)
provided
the
data
145°C.
junction temperature
temperature
(Tj)2:
provided
in
the calculate
data sheet
sheet is
isthermal
145°C. Therefore:
Therefore:
> Example
Test in
and
resistance
Cree does not recommend operating
XLamp
without
heatsink.
This
example demonstrates the
TTjj =
+ P totalLEDs
(R
/6
+
))
= TT
(Rthth j-sp
/6 +
+R
Ra
+R
Rthth h-a
aa + Ptotal
th
j-sp
th sp-h
sp-h
h-a
procedure for calculating the thermal resistance and maximum operating temperature of one XLamp 7090
,, depends
The
between
The thermal
thermal
between the
the LED
LED solder
solder point
point and
and heat
heat sink,
sink, R
Rthth sp-h
depends on
on the
the surface
surface finish,
finish, flatness,
flatness,
on a resistance
1resistance
inch2 MCPCB.
sp-h
applied
applied mounting
mounting pressure,
pressure, contact
contact area,
area, and
and the
the type
type of
of interface
interface material
material and
and its
its thickness.
thickness. With
With good
good design,
design, it
it can
can
be
be minimized
minimized to
to less
less than
than 1°C/W.
1°C/W.
The
)) can
The maximum
maximum thermal
thermal resistance
resistance from
from the
the heatsink
heatsink to
to ambient
ambient (R
(Rthth h-a
can be
be calculated.
calculated. Using
Using the
the previous
previous equation
equation
h-a
and
:
and solving
solving for
for R
Rthth h-a
:
h-a
R
=
Rthth h-a
= (145°C
(145°C -- 55°C
55°C –8°C/W
–8°C/W xx 6.825
6.825 W/6
W/6 –– 1°C/W
1°C/W xx 6.825
6.825 W)/6.825
W)/6.825 W
W=
= 10.85°C/W
10.85°C/W
h-a
In
In order
order to
to keep
keep the
the junction
junction temperature
temperature below
below 145°C
145°C in
in worst-case
worst-case conditions,
conditions, aa heat
heat sink
sink with
with thermal
thermal resistance
resistance
from
)) less
from heat
heat sink
sink to
to air
air (R
(Rthth h-a
less than
than 10.85°C/W
10.85°C/W must
must be
be chosen.
chosen. A
A heat
heat sink
sink with
with the
the required
required characteristics
characteristics may
may be
be
h-a
selected
selected using
using figures
figures published
published by
by heat
heat sink
sink manufacturers
manufacturers or
or through
through modeling
modeling and
and testing.
testing.
Example
Example 2:
2: Test
Test and
and Calculate
Calculate Thermal
Thermal Resistance
Resistance
Cree
Cree does
does not
not recommend
recommend operating
operating XLamp
XLamp LEDs
LEDs without
without aa heat
heat sink.
sink. This
This example
example
demonstrates
the
thermal
resistance
and
maximum
demonstrates the
the procedure
procedure for
for calculating
calculating
the
thermal
resistance
and
maximum
Figure 4: XLamp
7090 White on MCPCB
operating
operating temperature
temperature of
of one
one XLamp
XLamp 7090
7090 on
on aa 1-inch
1-inch22 MCPCB.
MCPCB.
Since there is no additional heatsink in this example, the MCPCB serves as the heatsink and thermal
Since
Since there
there is
is no
no additional
additional heat
heat sink
sink in
in this
this example,
example, the
the MCPCB
MCPCB serves
serves as
as the
the heat
heat
ambient.to
order to calculate
the
thermal resistance
from
junction to ambient the temperature
sink
thermal
interface
In
the
resistance
sink and
andinterface
thermalto
interface
toInambient.
ambient.
In order
order to
to calculate
calculate
the thermal
thermal
resistance
on theto
back
of thethe
LEDtemperature
must be measured.
In this
case,
the
LEDbe
is measured.
reflow soldered on MCPCB. The
from
ambient,
on
of
LED
must
from junction
junction
to
ambient,
the
temperature
on the
the back
back
of the
the
LED
must
be
measured.
boardthe
temperature
can be
measured
applying
thermocouple
directly
to the
In
LED
soldered
on
MCPCB.
The
can
be
In this
this case,
case,
the
LED is
is reflow
reflow
soldered
on by
MCPCB.
Thea board
board temperature
temperature
can
be back of the MCPCB. In
measured
by
applying
to
back
In
most
most
it is impossibledirectly
to attach
thermocouple
toMCPCB.
the solder
measured
by applications,
applying aa thermocouple
thermocouple
directly
toathe
the
back of
of the
the
MCPCB.
Inpoint
mostof the LED. The test point
applications,
it
to
aa thermocouple
to
the
solder
of
LED.
applications,
it is
is impossible
impossible
to attach
attach
thermocouple
tothe
theback
solder
point
of the
the
LED.
should
be
the point that
is as close
as possible to
of point
the
LED
or the
hottest point on the back of
The
point
should
be
the
point
that
is
as
close
as
possible
to
the
back
of
the
LED
The test
testthe
point
should
be
the
point
that
is
as
close
as
possible
to
the
back
of
the
LED
MCPCB.
or
or the
the hottest
hottest point
point on
on the
the back
back of
of the
the MCPCB.
MCPCB.
Figure 6 shows the MCPCB temperature at different forward currents. The room temperature was con-
Figure
Figure 55 shows
shows the
the MCPCB
MCPCB temperature
temperature at
at different
different forward
forward currents.
currents. The
The room
room Figure
7090
White
Figure 4:
4: XLamp
XLamp
7090
trolled to 23ºC.
Due to the
low thermal
conductance
of the MCPCB to air,
temperatures
quickly
reach
a White
temperature
temperature was
was controlled
controlled to
to 23°C.
23°C. Due
Due to
to the
the low
low thermal
thermal conductance
conductance of
of the
the on
on MCPCB
MCPCB
steady-state.
MCPCB
air,
MCPCB to
to
air, temperatures
temperatures quickly
quickly reach
reach aa steady-state.
steady-state.
Figure
5: MCPCB
temperature
Figure
4:
Temperature
Figure
5: MCPCB
MCPCB
Temperature
Page 7
Subject to change without notice.
www.xlamp.com
L I G H T I N G
Copyright
Copyright ©
© 2004-2006
2004-2006 Cree,
Cree, Inc.
Inc. All
All rights
rights reserved.
reserved. The
The information
information in
in this
this document
document is
is subject
subject to
to change
change without
without notice.
notice. Cree,
Cree,
the
the Cree
Cree logo,
logo, and
and XLamp
XLamp are
are registered
registered trademarks
trademarks of
of Cree,
Cree, Inc.
Inc. Other
Other trademarks,
trademarks, product
product and
and company
company names
names are
are the
the property
property
of
their
respective
owners
and
do
not
imply
specific
product
and/or
vendor
endorsement,
sponsorship
or
association.
of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association.
4
CLD-AP05.002
CLD-AP05.002
Cree,
Cree, Inc.
Inc.
4600
4600 Silicon
Silicon Drive
Drive
Durham,
Durham, NC
NC 27703
27703
USA
USA Tel:
Tel: +1.919.313.5300
+1.919.313.5300
www.cree.com/xlamp
www.cree.com/xlamp
The board temperature (Tb) can be measured at different forward currents. Using a junction-to-solder-pad resistance of
8°C/W and assuming a thermal resistance from solder point to MCPCB of 1°C/W (this will be on the high end based on
the mounting method), the junction temperature at different currents can be calculated by:
Tj = Ta + Ptotal x Rth j-a
Where,
Rth j-a = Rth j-b + Rth b-a
Rth j-b = Rth j-sp + Rth sp-b =8°C/W + 1°C/W = 9°C/W
Rth b-a = (Tb - Ta) / Ptotal
Ptotal = If x Vf
Ta = 23°C
Table 1 shows the test and calculated results.
If (mA)
Vf (V)
Tb (°C)
Rth b-a (°C/W)
Rth j-a (°C/W)
Tj (°C)
200
2.95
51
47
56
56
350
3.11
74
47
56
84
450
3.21
90
46
55
102
Table 1: Junction-Temperature Calculations
From the test results we can conclude that with a 1-inch2 MCPCB as a heat sink, the XLamp 7090 can be safely operated
at room temperature when driven at 350 mA. The maximum allowable ambient temperature is 84°C. However, at roomtemperature conditions, the LED junction will quickly reach a temperature of 84°C. For better results, the designer
should use a secondary heat sink.
Heat Sink Design and Selection
In order to design and select the heat sink, it is useful to know how heat sinks work. Transmission of heat from a heat
source (e.g. the junction of a LED) via the heat sink into the surrounding medium takes place in four successive steps:
1.
2.
3.
4.
Transfer from heat source to the heat sink
Conduction from within the heat sink to its surface
Transfer from surface into the surrounding medium by convection
Radiation depending on the nature of the heat sink’s surface
The efficiency and capability of a heat sink are a function of the heat transfer modes utilized. Heat sinks provide a path
for heat from the LED to flow through conduction. The heat “trapped” in the heatsink must be dissipated in order for
the power from the source to continually flow. If the heat remains trapped in the sink, the temperature will rise and
eventually overheat the source. Heat sinks can dissipate power in three ways: conduction (heat transfer from one solid
to another), convection (heat transfer from a solid to a moving fluid, for most LED applications the fluid will be air), or
radiation (heat transfer from two bodies of different surface temperatures through electromagnetic waves).
There are three common varieties of heat sinks: flat plates, die-cast finned heatsinks, and extruded finned heat sinks.
The material normally used for heat sink construction is aluminum, although copper may be used with an advantage for
flat-sheet heat sinks.
Heat sink thermal radiation is a function of surface finish, especially when the heat sink is at higher temperatures. A
painted surface will have a greater emissivity than a bright, unpainted one. The effect is most remarkable with flat-
Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree,
the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property
of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association.
5
CLD-AP05.002
Cree, Inc.
4600 Silicon Drive
Durham, NC 27703
USA Tel: +1.919.313.5300
www.cree.com/xlamp
plate heat sinks, where about one-third of the heat is dissipated by radiation. The color of the paint used is relatively
unimportant. The thermal resistance of a flat-plate heatsink painted gloss white will be only about 3% higher than that of
the same heat sink painted matte black. With finned heat sinks, painting is less effective since heat radiated from most
fins will fall on adjacent fins, but it is still worthwhile. Both anodizing and etching will decrease the thermal resistance.
A number of important factors need to be considered when selecting a heat sink:
•
Surface area: Thermal transfer takes place at the surface of the heat sink. Therefore, heat sinks should be designed
to have a large surface area. This goal can be reached by using a large number of fine fins or by increasing the
size of the heat sink itself.
•
Aerodynamics: Heat sinks must be designed in a way that air can flow through easily and quickly. Heat sinks with a
large number of fine fins with short distances between the fins may not allow good air flow. A compromise between
high surface area (many fins with small gaps between them) and good aerodynamics must be found.
•
Thermal transfer within the heat sink: Large cooling fins are ineffective if the heat can’t reach them. The heat sink
must be designed to allow adequate thermal transfer from the heat source to the fins. Thicker fins have better
thermal conductivity; so again, a compromise between large surface area (many thin fins) and good thermal
transfer (thicker fins) must be found. The material used has a major influence on thermal transfer within the heat
sink.
•
Flatness of the contact area: The portion of the heat sink that is in contact with the LED or MCPCB must be
perfectly flat. A flat contact area allows the use of a thinner layer of thermal compound, which will reduce the
thermal resistance between the heat sink and LED source.
•
Mounting method: For good thermal transfer, the pressure between the heat sink and the heat source must be
high. Heat sink clips must be designed to provide high pressure, while still being reasonably easy to install. Heatsink mountings with screws or springs are often better than regular clips. Thermoconductive glue or sticky tape
should only be used in situations where mounting with clips or screws is not possible.
For more information regarding to heat sink design and selection, please contact your heat sink manufacturer and
explore these links for further information:
http://www.electronics-cooling.com/
http://www.r-theta.com/
http://www.aavidthermalloy.com
http://www.electronics-cooling.com/html/consultants.html
http://www.coolingzone.com/
The formulas and diagrams given in this application note should be considered as a guide for thermal management of
XLamp LEDs. The thermal resistance of a heat sink depends on numerous parameters that cannot be predetermined.
These parameters include but are not limited to the position of the LED on the heat sink, the extent to which air can flow
unhindered, the screening effect of nearby components, and heating from other components in the fixture. It is always
advisable to check important temperatures in the finished fixture under the worst possible operating conditions and
calculate the LED junction temperature. The probe points should be as close as possible to the back of the LED.
Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree,
the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property
of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association.
6
CLD-AP05.002
Cree, Inc.
4600 Silicon Drive
Durham, NC 27703
USA Tel: +1.919.313.5300
www.cree.com/xlamp