Download Thermal Design for NOTEBOOK PC by using thermal analysis

Thermal Design for NOTEBOOK PC by using thermal analysis
Man-In Baek*, Jung-Mi Lee
LG Electronics Inc., LG Production Research center
[email protected]
Key word : Thermal design, RHE(Remote Heat Exchanger), Thermal analysis, PCB(Printed circuit
board),
Abstract
The portable personal computer is more popular than ever. On the other hand, the power dissipation of
notebook PC is higher. So many portable personal computer makers have a problem in thermal
management. In this paper, we will review the various thermal enhancements from the component, board,
and system level for the use of the Pentium processor in the ball grid array(BGA) in notebook PC. CFD
and heat transfer simulations are applied to the layout design of the notebook PC. The CPU location is
decided by the result of optimization of the fan and inlet air ventilation. To escape hot spots in the outcase
we suggested Magnesium. And to maintain the device temperature and the surface temperature of the
enclosure within limits, the thermal models have been developed for compact simulation and equivalent
accuracy with experiments.
1. Introduction
Electronic portable devices, especially portable
computers, have become pervasive and popular
nowadays. As the technology progresses and the
market demands, more functions have become
integrated and packed into these devices in
conjunction
with
higher
performance
semiconductor chips, while the size of the devices
is decreasing. The CPU in a personal computer
has the largest power consumption of all devices
and its consuming trend towards increasing. In
addition, Chipset, audio, graphic processor,
memory, and etc. are also beginning to generate
more heat due to higher performance priority. On
the other hand, there is a trend toward portable
computer becoming thinner and lighter.
They offer users the advantage of full personal
computer capabilities in a portable system. And
the disparity between notebook and desktop
performance continues to lessen. The consumer’s
need to get hold of portable property continues to
escalate. Sales of portable units will grow twice as
fast as those of desktops and account for a third of
PC market in past ten years. Needless to say, this
trend has not been lost on the industry’s major
players.
High performance processor and new features in
notebooks, such as L2 cache, more DRAM, larger
hard disk drivers, PCMCIA, etc., lead to higher
power dissipation inside the notebook. This
creates a greater need for high performance
thermal solutions without compromising the
notebook’s size and weight. While advancing
processor technology is reducing the power
consumption of components, notebook computer
manufacturers are pushing the performance
envelope to produce even higher performance
notebook PC’s. The system designer is caught in
the dilemma of having less time to design thermal
management solutions for more complex, more
compact, and higher powered notebook systems.
The peripheral products for notebook computers
are progressing very fast, such as larger TFT
LCD, larger capability hard disk and added many
additional devices. In this way the competition
between vendors are so intense that make the
price of notebook computers motivates the market
to expand very fast. There are some problems
when notebook computer design. Space
allocation, definition and area of printed circuit
boards, heat handling, noise interruption are the
topics which determine whether notebook
computers are successful or not. Among them,
the heat problem plays a more important role than
before. In the past, there was no overheat
consideration, for the performance of CPU is not
as high as it is now. However, the average power
consumed by CPU reaches 14-16watts. It is an
inevitable trend that CPU must include more and
more power inside for stronger ability and more
function. More power input means more heat
generated. The reliability, functionality and even
consumer satisfaction are concerned with thermal
management. Thus, many kinds of heat control
technology, theory and device are developed.
In order to accomplish the principle of “TIME TO
PROFIT”, designers take the double pressure of
low cost and high performance on their shoulders.
Usually, the failure of final layout in notebook
computers is due to bad thermal considerations.
This is because thermal aspect is always put in the
last part of design.
2. Model Concept
Heat Dissipation of CPU
Figure 1 shows the power dissipation of Intel
microprocessors as a function of time. The scatter
10
in data clearly suggests technology advancements
where (T1-T2)and h are temperature difference
and heat transfer coefficient between the two
walls, g,ß,v and Pr are gravitational acceleration,
volumetric thermal expansion coefficient,
kinematic viscosity, and Prandtl number,
respectively.
to reduce power. But the overall trend is the
exponential increase in power dissipation with Conductio
time. This trend is not particular to one
company’s technology.
Nu
Convectio
Rac
Nu =1
Fig.4 Heat transfer through horizontal fluid layer
1
Fig. 1 power dissipation of microprocessors
Model of heat transfer
Figure 3 shows the property of heat transfer
between two horizontal surfaces which confront
each other as illustrated in Figure 2 X-axis
represents the Rayleigh number Ra and Y-axis
represents the Nusselt number Nu. The
characteristic length for these numbers is the gap
of the two surfaces L.
6
2
3 where Rayleigh
4
5
number
10
10 In the region
10
10
10 is smaller
than its critical value Ra
Racr, the Nusselt number is
1.0. This indicates that for small Ra. Natural
convection does not occur. Fluid between two
walls does not move and heat transfer takes place
only by conduction and radiation.
Notebook PCs are designed so as to minimize
dead space. Hence, in many cases, the Rayleigh
number inside the PC is lower than its critical
value and the thermal analysis for this type of PCs
can be performed on the basis of heat conduction
analysis. Heat conduction analysis does not need
much computational power compared with that
for fluid analysis.
Even for the case that the Rayleigh number
exceeds its critical value, if the corresponding part
is independent or the domain of natural
convection is small, it is not difficult to simulate
the heat transfer by stuffing material, of which
thermal conductivity k is represented by
equation(3), instead of air in the corresponding
portion of the numerical model.
k = k air Nu
Ai
L
Q
Fig.2 Confronting surfaces
Ra = Gr Pr
Gr = gβ (T1 − T2 ) L3 / v 2
T1
Assuming that the view factor is one or gaps
between PCBs or between PCB and other parts
are small enough. Heat transfer Q due to thermal
conduction and radiation between parallel
surfaces is
k
Q = A( (T1 − T2 ) + σf 12 (T14 − T24 ))
L
1
f 12 =
1/ ε 1 + 1 / ε 2 − 1
where
ε i is emissivity of each surface and s is
Stefan-Boltsmann constant.
To simplify the calculation, heat transfer is
converted to a form of thermal conductions as
follows.
RHE thermal solutions
The mechanisms and functions of heat pipes are
described in various publications. A brief review
is introduced below on the basic structures and
key properties of heat pipe.
Heat pipes are sealed and evacuated vacuum tight
vessels which are partially backfilled with a fluid.
The fluid vessel are lined with aporous media(the
wick) which acts as a passive pump, via capillary
action, to circulate the condensate within the heat
pipe.
Earlier heat pipe thermal solutions have tried to
avoid the use of miniature fans, due to the cost,
reliability, power consumption of the fans. The
past few years have seen a lot of improvements in
miniature fans. Cost has been continually
decreasing, making it quite affordable in the
notebook PCs environment. Although miniature
fan’s reliability is still not as good as the bigger
fans used in desktop PCs, the use of ball bearing
and seal of the bearing have improved miniature
fans’s reliability of operation. Power consumption
is also dropping, with some fans consuming only
0.2watts of power for typical operation.
The RHE thermal solution was then developed to
take advantage of the emergence of miniature fans
in notebook PCs cooling. Figure 4 shows a setup
of an RHE thermal solution of mechanical design
that has been adopt in our new notebook PC.
finned heat sink and a miniature fan to provide air
flow for the heat sink. A special vent is created
by the bottom of the notebook to provide inlet air
flow to the heat sink. Inlet air flows through the
heat sink, carries away the heat from the heat sink
fins and finally exits through an exhaust fan
dissipating heat to the ambient.
The material of the case.
As the technology progresses and the market
demands, more functions have become integrated
and packed into these devices in conjunction with
higher performance semiconductor chips, while
the size of the devices is decreasing. For instance,
notebook PCs are getting faster and thinner.
However most customers demand more
reliability.
In this notebook PC, we adopt a Mg case that is
more than 100 times more conductive than a
plastic case.
3. Numerical analysis
Simplified Keyboard
The keyboard has a plate which consists of a
PCB and a aluminum plate. On the base plate,
there are many switches with key-caps.
Components beneath the keyboard heat up the
base plate. From the base plate and key-caps, heat
dissipates under natural convection. The
phenomenon involved in the effect of key rows on
the heat transfer is complicated.
Fig.5 is the real mechanical design and the
simulation model of keyboard. Since the
contribution of key array to horizontal thermal
conduction is small, the keyboard is simplified to
a base plate and modified heat transfer coefficient
which includes the influence of the key array is
applied on the base plate.
The keyboard is assumed to be a plain plate on
which equivalent heat transfer coefficient hk is
calculated as follows.
hk =
Figure 4: RHE thermal solution of mechanical
design
As shown in the figure, the heat generated from
the CPU is conducted to a metal block directly in
contact with the CPU through thermal interface
material. The heat is then transferred to two paths
for heat dissipation. One is a passive solution
represented by the heat pipe to keyboard thermal
solution on the top side of the block.
The other is an active solution represented by the
heat exchanger/fan assembly which consists of a
Q
where
LW (Tk − Ta )
- Q,L,W are heat dissipation from key side of the
keyboard, keyboard length and width
respectively.
- Tk and Ta are temperature of base plate and
ambient temperature.
Since the contribution of the keys to horizontal
thermal conduction is small, it is appropriate to
use the base plate as the simplified keyboard.
Battery
Card Bus
PCI
440Zx Grph.
Pix
HDD
Equivalent
heat
Figure
Plain
5 : Keyboard
plate design
Equivalent PCB Model
Printed Circuit boards(PCBs) consist of glassepoxy layers and copper layers. In the present
analysis, the PCBs are simplified as simple plates
with anisotropic thermal conductivity for the
whole domain analysis model.
The equivalent horizontal thermal conductivity
and normal thermal conductivity are calculate by
use of the Flotherm web site.
Numerical and Experimental result
Fig.6 illustrates the numerical model for the
whole domain of the notebook PC. This
numerical model was created using of Flotherm.
The boundary conditions are turbulent flow and
radiation effects attached to each component.
Table 1 shows heat dissipation of each
component and was obtained from Intel
corporation and other component manufactures.
Total heat dissipation from the notebook PC
model was 19.35watts. CFD-base thermal
simulation were carried out using the software
Flotherm.
Components
Heat dissipation(watts)
CPU
11.2
Graphic chip
2.01
Audio chip
0.38
Video chip
2.23
Card bus
0.35
Dimm socket
0.33
Memory chips
0.2
Modem card
1.2
PC card
1.35
HDD
0.5
Table 1 : Heat dissipation of each components
CPU
Fan
Figure 6 : Simulation model of notebook PC
Two simulation cases were run for each of the
without docking station and with docking station,
slice
Figure7 and Figure 8 are simulation result of each
case. In each components temperatures were
recorded for the two cases in table 2.
Components
Sim. Result
Exp. Result
CPU
97.5 C
95.0 C
Graphic chip
83.0 C
80.2 C
Audio chip
82.5 C
81.0 C
Bottom case1
45.3 C
42.9 C
Bottom case2
46.8 C
43.2 C
HDD
46.4 C
44.6 C
Keycap
38.1 C
34.0 C
Palm rest
35.8 C
31.5 C
Ambient
25.0 C
25.3 C
Table2: Comparison of experimental and
simulation results
Conclusion
With the continuing increase of CPU and other
system power dissipation in notebook PC. Metal
outcase and RHE (remote heat exchanger) use in
notebook PC has been quite widely within the
introduction of the high power microprocessor.
Thermal analysis and experimental for the
notebook PC were carried out. The main purpose
is to decrease the CPU temperature and case
temperature.
When we used metal case in notebook PC, the
skin temperature and main heat dissipate
components temperatures are decreased. The
design of RHE (remote heat exchanger) is carried
by result of simulation and experimental result.
As a result of applying this method, the CPU with
fan and other components were located on a
unique position.
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