AN 185: Thermal Management Using Heat Sinks

Thermal Management
Using Heat Sinks
March 2002, ver. 2.1
Introduction
Application Note 185
Thermal management is an important design consideration with complex
devices running at high speeds and power levels as these devices can
generate significant heat. Proper thermal management can increase
product performance and life expectancy. The thermal management
requirements for a programmable device depend on its application.
AlteraR packages are designed to minimize thermal resistance
characteristics and maximize heat dissipation. However, in some cases,
complex designs require heat dissipation greater than packages provide.
This application note discusses ways to dissipate heat, how to calculate
the heat dissipation of a device, and how to determine if a device requires
a heat sink in an application.
Heat
Dissipation
Theory
Radiation (device), conduction (heat sink), and convection (fan) are three
ways to dissipate heat from a device. In printed circuit board (PCB)
designs, heat sinks are used to increase the heat dissipation from hot
devices because the heat dissipation between the heat sink and the
surrounding air is more efficient than between the device and the
surrounding air. The thermal energy transfer efficiency of heat sinks is
due to the small thermal resistance between the heat sink and the air.
Thermal resistance is the measure of the heat dissipation capability of a
surface or, in other words, how efficiently heat is transferred across the
boundary between different mediums. A heat sink with a large surface
area and good air circulation gives the best heat dissipation. Limited
vertical space between PCBs in a system constrains airflow and heat sink
dimensions.
A heat sink helps keep a device at a temperature below its specified
maximum operating temperature. To determine whether a device
requires a heat sink for thermal management, calculate its thermal
resistance through the use of thermal circuit models and equations. These
thermal circuit models are similar to resistor circuits, which follow Ohm's
law. With a heat sink, heat from a device flows from the junction to the
case, then from the case to the heat sink, and finally from heat sink to
ambient air.
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A-AN-185-2.1
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AN 185: Thermal Management Using Heat Sinks
Figure 1 shows a thermal circuit for a device with and without a heat
sink. Table 1 defines the thermal circuit parameters.
The thermal resistance of a device depends on the sum of the thermal
resistances from the circuit model. See Table 2 for the thermal resistance
equations for a device with and without a heat sink. Basic thermal
equations are simple. However, the challenge can be finding the required
data to substitute into the equations.
Figure 1. Thermal Circuit
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AN 185: Thermal Management Using Heat Sinks
Table 1. Thermal Circuit Parameters
Parameter
Name
Units
θJA
Junction-to-ambient thermal resistance
°C/W
Specified in the data sheet
Description
θJC
Junction-to-case thermal resistance
°C/W
Specified in the data sheet
θCS
Case-to-heat sink thermal resistance
°C/W
Adhesive compound thermal resistance
specified by manufacturer
θCA
Case-to-ambient thermal resistance
°C/W
Solved for by equation
θSA
Heat sink-to-ambient thermal
resistance
°C/W
Solved for by equation and specified by the
heat sink manufacturer
TJ
Junction temperature
°C
The maximum junction temperature as
specified for the device
TA
Ambient temperature
°C
Usually the maximum ambient air
temperature specified for the device on the
device’s data sheet
TS
Heat sink temperature
°C
The heat sink’s maximum measured
temperature near the device
Tc
Device case temperature
°C
The device package’s maximum measured
temperature
P
Power
W
Total rate of heat dissipation from the device
while operating. Use the maximum value for
selecting a heat sink
Table 2. Device Thermal Resistance Equations
Device
Equation
Without a heat sink
TJ – TA
θ JATotal = θ JC + θ CA = ----------------P
With a heat sink
TJ – TA
θ JATotal = θ JC + θ CS + θ SA = ----------------P
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AN 185: Thermal Management Using Heat Sinks
Thermal
Equation
Parameters
Many parameters contribute to a design's thermal circuit, including the
device's maximum power consumption for the design, the maximum
environment temperature, package characteristics, and airflow at the
device.
Maximum Power Consumption (P)
Use the power calculator values from design simulations in the Altera
Quartus® II software (or the device's power calculator at
http://www.altera.com) to estimate the maximum power consumption
of the device. Once a prototype design is available, measure the actual
power consumption and use this value for thermal calculations.
Maximum Temperature (TJ & TA)
The maximum ambient and junction temperatures are found in the data
sheet for the device under Device Absolute Maximum Rating and the
operating junction temperature is found under Device Recommended
Operating Conditions. The temperature must be kept within the
maximum conditions or damage could occur. The junction temperature
should be kept within the recommended operating conditions to ensure
the device achieves the performance reported by the Quartus II software.
Package Characteristics (θJC & θJA)
Table 3 is an example of package thermal characteristics as found on the
Altera device data sheets.
Table 3. Example Package Characteristics
θJC (°C/W) θJA (°C/W) θJA (°C/W) θJA (°C/W) θJA (°C/W)
Still Air 100 ft./min. 200 ft./min. 400 ft./min.
Device
Pin
Count
Package
EP20K400E
652
BGA
0.5
9.0
8.0
7.0
6.0
672
FineLine BGA
0.2
11.7
9.7
8.0
6.7
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AN 185: Thermal Management Using Heat Sinks
Example
Applications
An Example Application Not Requiring a Heat Sink
First, use the operational conditions shown in Table 4 in the device's
power calculator on the web page to get the maximum power
consumption. If the design is complete and simulation vectors are
available, you can also use the Quartus II software’s Power Gauge feature.
Table 4. Operational Conditions for Example Application
Parameter
Value
Device
EP20K400EBC652
VCCINT
1.8 V
VCCIO
3.3 V
fmax
50 MHz
Flip flops
8000
Carry chain logic elements (LE)
4000
Non-carry chain LE
3500
Number of I/O pins
300
Toggle rate
20%
Air flow rate
100 ft./min.
Maximum TJ
85° C
Maximum ambient temperature (TA)
50° C
From the power calculator, these values give a maximum power
consumption of P= 1.32 W. Then substitute the parameter values in
Table 3 and Table 4 into the equation in Table 2 to determine the P
allowable without a heat sink.
θ JA
Max Commercial
TJ – T
= -----------------AP
TJ – TA
P ≤ ------------------------------------------------θ JA Max Commercial
85°C – 50°C
P ≤ -------------------------------8°C ⁄ W
P ≤ 4.375 W
The 1.32 W consumed is less than the 4.375 W this device can
accommodate. Therefore, this design does not require a heat sink.
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AN 185: Thermal Management Using Heat Sinks
Example Application Requiring a Heat Sink
First, use the operational conditions shown in Table 5 in the device's
power calculator on the web page to get the maximum power
consumption. If the design is complete and simulation vectors are
available, you can also use the Quartus II software’s Power Gauge feature.
Table 5. Operational Conditions for Example Application
Parameter
Value
Device
EP20K400EBC672
VCCINT
1.8 V
VCCIO
3.3 V
fmax
70 MHz
Flip flops
6500
Carry chain logic elements (LE)
3000
Non-carry chain LE
3000
Number of I/O pins
300
Toggle rate
25%
Air flow rate
200 ft./min.
Maximum TJ
85° C
Maximum ambient temperature (TA)
70° C
Therefore, P=2.00 W.
Next, use the equation for thermal resistance without a heat sink:
TJ – TA
P ≤ ------------------------------------------------θ JA Max Commercial
85°C – 70°C
P ≤ -------------------------------8°C ⁄ W
P ≤ 1.85 W
The 2.0 W consumed is greater than the 1.85 W this device can
accommodate. Therefore, this design requires a heat sink. An alternative
to a heat sink would be to increase the air flow across the device.
Finally, for this example, calculate the appropriate thermal resistance for
a heat sink. θ CS is typically extremely small. As such, the number to use
is the adhesive compound thermal resistance that runs between 0.15 and
0.8 °C/W. For this example, use θCS= 0.5 °C/W.
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AN 185: Thermal Management Using Heat Sinks
TJ – TA
θ JA = ----------------P
85°C – 70°Cθ JA = ------------------------------2W
θ JA = 7.5°C ⁄ W
Use the θ CS just calculated to solve for the required heat sink.
θ JATotal = θ JC + θ SA + θ CS
7.5°C ⁄ W > 0.2°C ⁄ W + θ SA + θ CS
θ SA < 7.3°C ⁄ W – 0.5°C ⁄ W
θ SA < 6.8°C ⁄ W
Selecting a
Heat Sink
Selection of a heat sink manufacturer is typically straightforward. Once
the maximum thermal resistance has been calculated, locate available heat
sinks with that characteristic for the size of package. A few iterations may
be required to get the final design. Altera also recommends a guard band
of 10 to 15% less thermal resistance than calculated rather than only
matching the maximum thermal resistance value.
The heat sink to choose is one that meets a design's thermal, packaging,
power, and cost requirements. A few heat sinks types include stampings,
extrusions, folded fin, or active heat sinks with fans. Figure 2 apply to the
application in the “Example Application Requiring a Heat Sink” section.
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Guard band was added for the 6.8 °C/W heat sink required at
the 200 ft./min. in the example application.
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AN 185: Thermal Management Using Heat Sinks
Figure 2. Heat Sink & Thermal Resistance Curve for Example Application
Requiring a Heat Sink
Conclusion
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The equations in this application note help determine the heat dissipation
requirements for a design. The easiest ways to improve a system's thermal
characteristics are to increase the airflow, lower the power consumed, or
reduce the maximum ambient temperature. When the environmental
conditions cannot be modified enough to remove the need for a heat sink,
then these equations can be used to find the heat sink required.
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AN 185: Thermal Management Using Heat Sinks
References
For a power consumption calculator worksheet and more information
about thermal management, refer to AN 74: Evaluating Power for Altera
Devices.
Please evaluate any vendor’s products for compatibility with Altera
devices. The vendors listed at the time of the writing of this paper had heat
sink characteristics in the range of the application in the “Example
Application Requiring a Heat Sink” section. For more information on heat
sinks visit the following heat sink vendor web sites:
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Revision
History
The information contained in AN 185: Thermal Management Using Heat
Sinks version 2.1 supersedes information published in previous versions.
AN 185: Thermal Management Using Heat Sinks version 2.1 contains the
following change:
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101 Innovation Drive
San Jose, CA 95134
(408) 544-7000
http://www.altera.com
Applications Hotline:
(800) 800-EPLD
Literature Services:
lit_req@altera.com
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Malico Inc. (http://www.malico.com.tw)
Cool Innovations (http://www.coolinnovations.com)
Aavid Thermalloy (http://www.aavidthermalloy.com)
Dynatron Corp (http://www.dynatron-corp.com)
Alpha (http://www.micforg.co.jp)
Heat Technology (http://www.heattechnology.com)
Updated text in the paragraph below Table 4 from “typical power” to
“maximum power.”
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