Power Dissipation Limits for
High-Performance Tablet Cooling
Guy R. Wagner
Electronic Cooling Solutions, Inc.
2344B Walsh Ave, Bldg F
Santa Clara, California, 95051
USA
(408) 738-8331
[email protected]
www.ecooling.com
Advancements in Thermal Management 2016
August 3-4, 2016
Denver, Colorado
Outline of Presentation
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Abstract
Handheld Device Background
Power Dissipation and Surface Temperature
Use of Air Movers to Increase Power Dissipation
Experimental Results Compared to CFD Modeling
Use of Heat Spreaders to Increase Power Dissipation
Summary
References
2
Abstract
As the performance of tablets continues to increase, some of the most recent
introductions contain Intel I7 processors and have performance levels rivaling
laptop and desktop computers. Because of touch temperature limitations, these
tablets can no longer depend upon natural convection for their cooling.
This presentation will show the power dissipation limits imposed upon these
high performance tablets and will demonstrate how the addition of a small
internal air mover can greatly increase power dissipation while still keeping the
external touch temperature within acceptable limits.
The presentation includes thermal modeling using Computational Fluid
Dynamics as well as test results and IR images.
3
The Future of Computing
Computing is moving from the desktop to handheld devices and cloud
computing using large datacenters filled with servers. With the popularity of
smart phones and tablets, a large percentage of the world’s computing power is
represented by the millions of handheld devices. This represents a major shift in
data generation and the location where the data is processed.
4
Handheld Device Background
Handheld devices are increasingly capable of running applications that used to
require laptop and desktop computers. The requirement that these devices
provide similar performance with a smaller form factor presents significant
challenges, especially when one considers that passive cooling or noiseless
active cooling is a requirement.
Thermal design of next generation handheld tablet devices will need to address
both a comfortable surface touch temperature and maximum temperature
limitations of internal critical components while also meeting aggressive
industrial design requirements.
5
Power Dissipation and
Surface Temperature
6
Handheld Device Limiting Factor
In most cases, the limiting factor in the thermal design of handheld devices is
not the temperatures of the internal components but the temperature of the
external surfaces since these surfaces are in direct contact with the skin of the
user. There have been studies that address the maximum allowable comfortable
touch temperature of a handheld device.
Thermal design of next generation handheld tablet devices needs to address
both a comfortable surface touch temperature and maximum temperature
limitations of internal critical components while also meeting aggressive
industrial design requirements.
7
Constraints – Touch Temperatures
Surface Touch Temperatures
Energy Transfer
• Temperature
• Time in contact
• Rate of transfer – Material Conductivity
Increasing Conductivity
IEC 60950 - Touch Temperature Limits ('C)
Increasing Time
Metal
Glass, porcelain & vitreous
materials
Plastic & rubber
Handles, knobs, grips, etc.
continuously held in normal use
55
65
75
Handles, knobs, grips, etc. held or
touched for short periods only
60
70
85
External surfaces that may be
touched
70
80
95
Internal parts that may be
touched
70
80
95
8
Constraints – Touch Temperatures
Limits for Consumer Comfort
“Ergonomic Temperature Limits for
Handheld Electronic Devices” - Berhe
• 41C for Metal Cases
• 45C for Plastic Cases
[1] Berhe, M.K., Ergonomic Temperature Limits for Handheld Electronic
Devices, Proceedings of ASME InterPACK’07
9
Surface Temperature of a 10 Inch
Vertical Isothermal Tablet
Surface Temperature Rise
the
of an Isothermal Tablet
In a 25°C ambient condition,
maximum total power dissipation is
calculated with a requirement that the
surface temperature does not exceed
a touch temperature of 41°C (16°C
Temperature Rise).
This is the
maximum
aluminum
enclosure
comfort
touch
temperature as
presented by Berhe (2007). Use of
low conductivity case materials has
the effect of increasing the maximum
comfortable touch temperature by
about 5C.
The chart on the right shows the
surface temperature rise of an
isothermal tablet vs power dissipation
when the device is suspended
vertically in midair with heat transfer
occurring from all surfaces.
16°C Temperature Rise
These calculations assume perfect
heat spreading and a surface
emissivity of 1.0. In actual practice,
there will be hot spots on the device
which have the effect of lowering the
maximum allowable power.
Over half of the power is dissipated
by radiation
11
Power Dissipation vs Surface Area
If the power dissipation at 41C
maximum touch temperature is
calculated using CFD for three
devices ranging from smart phone
size through mini tablet size and a
full-size
tablet,
the
power
dissipation versus surface area is
shown for the device in both the
vertical position and the horizontal
position with an adiabatic lower
surface.
12
Altitude Effect on the Surface
Temperature Rise of an Isothermal Tablet
This chart shows the
effect of altitude on the
surface temperature of an
ideal isothermal tablet
suspended in the vertical
position. As the density of
the air decreases with
increasing altitude, less
heat is transferred from
the
surface
due
to
convection while the heat
transferred by radiation
remains constant.
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Heat Transfer due to Radiation for an
Isothermal Tablet
Note that as the air density
decreases with altitude,
the percentage of heat
dissipated by radiation
increases
as
the
percentage
of
heat
dissipated by convection
decreases.
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The Problem
•As consumers move from their desktop and laptop computers, they are not willing to
give up performance (processing power).
•A 10 inch tablet is limited to around 17 W of power dissipation under natural convection
conditions for a 16C external temperature rise or about 22W if a 20C temperature rise
is acceptable.
•Assuming that the emissivity is already close to 1.0 for maximum radiation heat
transfer, the only way to dissipate more power is to increase the surface area which
means an increase in the physical size of the tablet.
•If the tablet size is to remain the same size, then the only solution is to resort to forced
convection using a small internal blower.
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Forced Convection Tablet
• Forced Convection Tablet
• Blower / Heatpipe System
16
Surface Temperature Rise
of an Isothermal Tablet with Airflow
These calculations assume perfect
heat spreading and a surface
emissivity of 1.0. In actual practice,
there will be hot spots on the device
which have the effect of lowering the
maximum allowable power.
The percentage of natural convection
power
dissipation
and
power
dissipated by radiation decreases as
the airflow increases.
Tablet Temperature Rise vs. Power and Airflow
10 inch tablet, vertical position
30.0
Temperature Rise above Ambient (C)
The chart on the right shows the
surface temperature rise of an
isothermal tablet vs power dissipation
when the device is suspended
vertically in midair with heat transfer
occurring from all surfaces and with an
internal air-mover blowing air through
the tablet. The air is being sucked into
vents at the bottom and blown out of
vents at the top.
0.0 CFM
0.2 CFM
0.4 CFM
0.6 CFM
0.8 CFM
1.0 CFM
1.2 CFM
25.0
20.0
16°C Temperature Rise
15.0
10.0
5.0
0.0
0
5
10
15
20
Tablet Power (W)
25
30
35
17
Surface Temperature Rise
of an Isothermal Tablet with Airflow
The internal blower increases airflow
by increasing the rotational speed of
the impeller.
For high-performance tablets, many
manufacturers are allowing the touch
temperature to go to 45C with a 25C
ambient. A 20C temperature rise is
allowable since the tablet will be
running at much lower power levels
and consequently with a much lower
touch temperature most of the time.
Even when running at full power, it
takes 20 to 30minutes for the tablet to
reach a steady state temperature.
Since the fan speed is temperature
controlled in these tablets, it will be
inaudible most of the time.
10 inch tablet, vertical position
30.0
Temperature Rise above Ambient (C)
As the speed increases, there is an
increase in the acoustic noise
generated by the impeller. At 0.6 cfm,
the acoustic noise in barely audible in
a quiet environment. However, the
noise level becomes objectionably
loud as the blower approaches 1.0
CFM.
Tablet Temperature Rise vs. Power and Airflow
0.0 CFM
0.2 CFM
0.4 CFM
0.6 CFM
0.8 CFM
1.0 CFM
1.2 CFM
25.0
20°C Temperature Rise
20.0
16°C Temperature Rise
15.0
10.0
Acoustic noise
becomes objectionable
5.0
0.0
0
5
10
15
20
Tablet Power (W)
25
30
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Allowing the temperature rise of the system to go from 16C
to 20C also allows the power dissipation to increase from
22W to 28W while keeping the airflow at 0.6 CFM to keep
the acoustic noise low.
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Power Dissipation as a function of
Airflow
The chart on the right shows the
power dissipation by category for a
tablet dissipating 25W total power.
Note that there is still more power
dissipated by the combination of
natural convection and radiation
than is removed by the air mover
due to the acoustic noise limit.
10 inch tablet, vertical position
16.00
14.00
Radiation
Heat removed by blower
12.00
Natural Convection
10.00
Power (W)
Notice that as the internal airflow
increases, the power removed by
the airflow through the tablet
increases and the power dissipated
by radiation and natural convection
from
the
external
surfaces
decreases. This is assuming an
isothermal tablet with an emissivity
of 1.0
Power vs. Airflow at 25W Total Power
8.00
6.00
Increasing acoustic noise
4.00
2.00
0.00
0.0
0.2
0.4
0.6
0.8
Tablet Airflow (CFM)
1.0
1.2
1.4
19
Experimental Results Compared to
CFD Modeling
20
iPAD Internal Components
Speaker
Batteries
Camera
Cabling
The touch screen display has been removed
Main PCB with EMI Shields over ICs
21
Tablet Modeling using CFD
22
Natural Convection Tablet
IR Imaging vs FloTHERM Simulation
IR Camera Image, Emissivity = 0.90
Numerical Simulation
The agreement between IR temperature measurements and simulation is very good
23
Use of Heat Spreaders to
Increase Power Dissipation
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Use of Heat Spreaders
Heat spreaders may be either internal or part of the case structure.
Through the use of high-conductivity heat spreaders, the maximum hot
spot temperature is reduced. Due to reducing hot spot temperature,
the average case temperature may be raised allowing increased power
dissipation while not exceeding the maximum surface temperature
requirement.
The following screen captures show the effect of increasing the thermal
conductivity of a 0.8 mm thick case from 0.2 W/mK (plastic) to 200
W/mK (aluminum).
This study assumes an internal power dissipation of 8.9 watts and an
ambient temperature of 24C. The tablet is in the vertical orientation.
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Temperatures of Back Surface of Case
K = 0.2 W/mK
K = 2.0 W/mK
Temperatures of Back Surface of Case
K = 20 W/mK
K = 200 W/mK
Effect of Case Thermal Conductivity
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Effect of Case Thermal Conductivity
Effect of Conductivity on Tablet Power Dissipation
41C Hot Spot Temperature, 24C Ambient, Vertical Orientation
12.0
Tablet Power Dissipation (W)
11.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
0.1
1
10
100
Heat Spreader Thermal Conductivity (W/mK)
1000
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Summary
The maximum power dissipation of the internal components is not only governed by the size of
the tablet but is a strong function of how well that heat is spread internally to reduce hot-spot
temperatures. Few engineers realize the importance played by radiation in dissipating the heat
from the exposed surfaces of a tablet. It is not until precise calculations are made that the
importance of radiation is realized in the thermal design of the tablet. If the emissivities of the
various surfaces are high, over half of the heat transfer to the surroundings is due to radiation.
Overall heat transfer is maximized by reducing hot spot temperatures and spreading the heat
so that all surfaces are effectively providing maximum heat transfer through convection and
radiation.
Once the touch temperature limit has been reached due to the internal power dissipation of the
tablet, the only way to increase the performance and the power dissipation and without
increasing the size of the tablet is to add an internal air mover such as a miniature blower.
While the air mover does not consume a great deal of power, the addition of the air mover
comes at the cost of acoustic noise. Even with the addition of an air mover, there is still more
power dissipated by radiation and natural convection off the exterior surface than the power
removed due to forced convection.
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References
[1]
Berhe, M.K., Ergonomic Temperature Limits for Handheld Electronic Devices, Proceedings
of ASME InterPACK’07, Paper No. IPACK2007-33873
[2]
Brown, L., Seshadri, H., Cool Hand Linux® - Handheld Thermal Extensions, Proceedings of
the Linux Symposium, Vol. 1, pp 75 – 80, 2007
[3]
Gurrum, S.P., Edwards, D.R., Marchand-Golder, T., Akiyama, J., Yokoya, S., Drouard, J.F.,
Dahan, F., Generic Thermal Analysis for Phone and Tablet Systems, Proceedings of IEEE Electronic
Components and Technology Conference, 2012
[4]
Huh, Y., Future Direction of Power Management in Mobile Devices, IEEE Asian Solid-State
Circuits Conference, 2011.
[5]
Lee, J., Gerlach, D.W., Joshi, Y.K., Parametric Thermal Modeling of Heat Transfer in
Handheld Electronic Devices, Proceedings of the 11th IEEE Intersociety Conference on Thermal and
Thermomechanical Phenomena in Electronic Systems, I-THERM, pp 604-609, 2008
[6]
Mongia, R., Bhattacharya, A., Pokharna, H., Skin Cooling and Other Challenges in Future
Mobile Form Factor Computing Devices, Microelectronics Journal, Vol. 39, pp 992 – 1000, 2008
[7]
Wagner, G.R., Maltz, W., Thermal Management Challenges in the Passive Cooling of
Handheld Devices, Proceedings of Therminic 2013, pp 135-140.
[8]
Wagner, G.R., Maltz, W., On the Thermal Management Challenges in Next Generation
Handheld Devices, Proceedings of the ASME InterPACK 2013, Paper No. InterPACK2013-73237
[9]
Wagner, G.R., Cooling Challenges in Handheld Electronic Devices, Presentation at the
AIAA Propulsion and Energy Forum 2015, July 27-29, 2015, Orlando Florida
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