Enhance MOSFET Cooling
with Thermal Vias
Bottom-side cooling enables MOSFET heat
transfer through the pc board to a heatsink. This
decreases MOSFET junction temperatures while
reducing the required area and thermal stress
on the board.
By Christopher Hill, Senior Applications Engineer, PowerMOS
Product Line, and Norman Stapelberg, International
Product Marketing Manager, PowerMOS Product Line, Philips
Semiconductors, Hazel Grove, United Kingdom
I
n most modern power semiconductor applications
there is a need to carefully manage heat. This is true
in fields as diverse as mobile communications, PC
motherboards, telecom power converters and industrial motor drives. Typically, the MOSFETs used as
power switches in such applications are a significant source
of heat, and the heat energy dissipated by these components
must be carefully controlled if safe operating temperatures
are to be maintained.
To remove the heat from a power MOSFET, a thermally
conductive pathway from the MOSFET to ambient is needed,
which usually includes some form of heatsink. There are
two conventional approaches to achieving this goal. The
first approach is to use a MOSFET in a leaded package such
as a TO-220. The package may be mounted on a heatsink
or chassis, which allows for efficient removal of heat energy
from the device. While this is an effective approach, there
are some disadvantages. One disadvantage is that the assembly of the device and the associated insulating material or
heatsink compound may be labor intensive. Another is that
the increased lead and track lengths may create difficulties
where a low-inductance design is required.
The second approach is to use a
MOSFET in a surface-mount package,
Top copper
such as D2PAK. A surface-mount package is soldered directly to the pc board,
using it as a heatsink. This approach
FR4
overcomes the two main disadvantages
Bottom copper
of the leaded-package approach: assemPower MOSFET in
Electrical insulator
bly is straightforward and devices may
2
D PAK package
Aluminum plate
be mounted close together to minimize
stray inductances. Unfortunately, this
approach also has disadvantages. One
major problem is that a pc board will
never be as effective at heat removal as
a chassis or an aluminum heatsink. A
closely related issue is that a temperature
rise in one region of the pc board may
Thermal vias region
Printed circuit board
have an adverse effect on other components mounted nearby. Furthermore, the
pc board itself may suffer delamination
Aluminum plate
and other adverse effects if continuously
operated at elevated temperatures.
Topside cooling, an alternative to bottom-side cooling, involves mounting a
Fig. 1. A D2PAK MOSFET mounted on a pc board employing bottom-side cooling transfers heat heatsink on the top of a device in order to
to the aluminum plate heatsink through the thermal vias region.
provide an improved thermal pathway to
Power Electronics Technology February 2006
28
www.powerelectronics.com
BOTTOM-SIDE COOLING
are taken from Flomerics’ Flotherm
thermal simulation software. The software uses computational fluid dynamics
(CFD) techniques to analyze complex
thermal scenarios involving coupled
heat transfer by conduction, convection
and radiation. Although the principles
of CFD are generally applicable to many
fields of engineering, Flomerics’ Flotherm package is specifically targeted at
users in the field of electronic and electrical engineering. The following thermal
analyses were carried out with Flotherm
software. All the device and pc board
models used in the simulations have been
validated against real empirical data.
Device silicon
(heat source)
Printed circuit board
Aluminum plate
Ambient
Thermal Simulation
For the purposes of this demonstration, we will consider the power stage
of an H-bridge motor control circuit
comprised of eight D2PAK MOSFETs
(two parallel MOSFETs in each arm of the bridge). The
circuit is constructed on a double-sided FR4 pc board with
2-oz copper on both layers. The topside of the circuit board
is shown in Fig. 3.
The pc board shown in Fig. 3 comprises the motor control power stage only. The power stage represents the area of
greatest power dissipation and, therefore, is the focus of the
thermal analysis. The upper copper layer (in green) is laid out
in a way that represents the actual electrical connections that
would be made in the real circuit. This is important because
the upper copper layer plays a role not only in the electrical
configuration of the circuit but also in its thermal behavior.
Also note that areas of thermal vias have been incorporated
beneath the tabs of each of the D2PAK packages, as represented in Fig. 2.
Two simulations were initially carried out. The first used
the pc board in Fig. 3 with the power dissipation per MOSFET
set to a constant 1 W. Thermal vias were incorporated into
the pc board, as stated previously. However, the aluminum
plate was omitted so that bottom-side cooling did not apply
Fig. 2. The primary heat path from device silicon to ambient for a D PAK MOSFET in a bottomside cooling scheme diverts the majority of heat directly to ambient via the pc board and
heatsink.
2
ambient. Depending on the nature of the application, topside
cooling can be highly effective. In certain other cases, it may
make little difference or even make the thermal situation
marginally worse.
The bottom-side approach to cooling power MOSFETs
retains the benefits of surface-mounted components while
avoiding their main disadvantages. Typically, bottom-side
cooling involves mounting one or more power MOSFETs on
a pc board in the usual manner. A pattern of thermal vias is
incorporated in the pc board under each power MOSFET, and
the opposite side of the pc board is mounted on a chassis or
heatsink. In this way, the pc board acts as a pathway for the
heat energy to reach the main heatsink, rather than serving as
the heatsink itself. This arrangement is illustrated in Fig. 1.
The primary heat path in the bottom-side cooling scheme
is shown in Fig. 2.
Note that the images shown in Figs. 3 and 4, as well as Fig. 6,
Fig. 4. Simulated temperature profiles for the H-bridge motor control
power stage show how bottom-side cooling (omitted at left; enabled
at right) can reduce device temperatures.
Fig. 3. In this implementation of an H-bridge motor control power stage,
the tab of each MOSFET is mounted over a group of thermal vias.
Power Electronics Technology February 2006
30
www.powerelectronics.com
BOTTOM-SIDE COOLING
The bottom-side approach to cooling power MOSFETs
retains the benefits of surface-mounted components
while avoiding their main disadvantages.
to this case. In the second simulation,
all parameters were the same, but an
aluminum plate was attached to the
bottom of the pc board enabling bottom-side cooling.
The device junction temperatures
were recorded for these two simulation
cases. For the case with no bottom-side
cooling, an average device junction temperature of 105.6°C
was recorded. For the second case, in which bottom-side
cooling was enabled, the average device junction temperature
was 68.8°C, a significant decrease. The
temperature profiles for the two cases are
shown in Fig. 4.
In Fig. 4 it is clear the application of
bottom-side cooling results not only in a
large reduction in MOSFET temperatures
but also in a significant reduction in pc
board temperatures. This occurs mainly
because the pc board is no longer being
used as a heatsink, and hence, problems
with pc board delamination and the
heating of adjacent components are also
greatly reduced.
This fact also opens the possibility
of further reducing the pc board space
occupied by the power MOSFETs by
using physically smaller devices and
placing those devices closer together.
To investigate this possibility, another
simulation was carried out where each
pair of D2PAK MOSFETs in the H-bridge
circuit was replaced with three LFPAK
devices.
The LFPAK package is much smaller
than the D2PAK, occupying the same
pc board footprint as the familiar SO-8
package. However, unlike the SO-8, the
LFPAK is a true power package that incorporates a bottom metal contact, which
provides an effective heat path out of the
device. There is an additional thermal
pathway between the top of the device
silicon and ambient through the top part
of the encapsulation (Fig. 5).
Although the LFPAK solution increases the total number of power packages used, the total board area occupied
by this solution is significantly less than
for the D2PAK case because the LFPAK
package is much smaller than the D2PAK
(Fig. 6). Note that the overall pc board
size in Fig. 6 is the same as in Figs. 3 and
4, though the copper coverage in Fig. 6
is much less. The bare FR4 pc board surrounding the power devices in Fig. 6 does
www.powerelectronics.com
not play a significant role in cooling the power MOSFETs
and therefore is available for mounting other components
such as driver ICs.
31
Power Electronics Technology February 2006
BOTTOM-SIDE COOLING
An Emerging Solution
Bottom-side cooling is not a totally
new concept. For several years it has
developed as a suitable solution in
dc-dc power supplies for telecom applications. However, the concept has
not yet fully matured in the telecom
industry and is still in its infancy in
the industrial market segment.
Industrial electronics forms a large
part of the semiconductor industry,
and dc motor control in turn forms
a main portion of the industrial electronics market. These motor-control
applications vary from stepper motor
controllers the size of a matchbox
(four to eight MOSFETs) to forklift
motor controllers having between 80
and 300 MOSFETs per controller. According to a report by EA technology
(Cheshire, United Kingdom), “Fuel
Cells Niche Markets Applications
Solution
Average Junction
Temperature (°C)
8x D2PAK
68.8
12x LFPAK
8x LFPAK
70.4
74.1
Table. Average device junction temperatures
for various MOSFET thermal solutions. Note
that the 8x LFPAK solution may be used in
place of the 12x LFPAK if a slightly higher
average junction temperature is acceptable.
and Design Study,” the estimated
2005 demand for heavy industrial
battery-powered vehicles is 260,000
and for light industrial/commercial
vehicles is 250,000. At an average of
110 MOSFETs per vehicle, the total
estimated market for these MOSFETs
is 56 million pieces.
In the case of heavy industrial
vehicles, physical space is not always
a constraint and TO-220 devices can
thus be used for these applications. In
light industrial and commercial applications, however, there is a push for
better efficiency and higher power density. Furthermore, in heavy and light
industrial applications, there is also
a move toward lower-cost solutions,
and this is where the mounting cost of
TO-220 packages and their associated
heatsinks become a burden.
There also are several practical
problems associated with using these
packages in industrial motor controllers. The challenge of mounting 20 to
40 MOSFETs in parallel while ensuring that they are all aligned with the
heatsink and the mounting holes in
the heatsink is one example. Another
example is the electrical challenge of
driving several MOSFETs simultaneously when using packages with high
inductance and resistance.
The aim of using surface-mount
packages is to reduce the pc board
size, if not, more importantly, to
simplify the electrical behavior of
these MOSFETs in parallel. Mounting
D2PAK devices on a pc board holds
the promise of reduced pc board
space, shorter track lengths and easier
driving of the MOSFETs. In practice,
however, this is not so easily achieved.
In choosing D2PAK devices, sufficient
Power Electronics Technology February 2006
32
www.powerelectronics.com
BOTTOM-SIDE COOLING
Encapsulation
Upper die attach
Top metal contact
Silicon (die)
Pins 1 – 4
���������
��
���
������
Lower die attach
(Printed circuit board)
Solder
Printed circuit board copper
Drain tab
Solder
Fig. 5. The cross-section view of the LFPAK package shows the two thermal paths from the
top and bottom of the device silicon.
throug h the pc
board, even when
smaller power
packages like the
LFPAK are used in
place of the D2PAK.
The package onresistance and inductances for these
smaller package
Fig. 6. The overlaid temperature profile (right) of the LFPAK devices
types are also sig(left) shows that the pc board area around the devices remains
nificantly lower.
relatively cool.
The total losses in
a system caused by these sources are
copper space is required around each
therefore reduced significantly, even
MOSFET to act as a heatsink. Thus,
with the additional devices needed
additional components cannot be
when using the smaller package types
placed in the copper area and the pc
in place of larger MOSFETS.
board space is ultimately not decreased
The greatest advantage when
as much as desired.
switching from D2PAK to LFPAK is
The concept of bottom-side coolthe resulting reduction of board space
ing may hold some of the answers to
occupied by the MOSFETs, since pc
these problems. Bottom-side cooling
board top copper is not needed to radiallows the heat from MOSFETs to
ate heat. The smaller MOSFETs can be
flow more effectively into a heatsink
placed closer together, and the previmounted on the opposite side of the
ously occupied board space is made
host pc board. The heatsink can quite
available for other components.
often be the chassis of the vehicle beIt is ideal for any module manuing driven, such as the cast-iron body
facturer to do more with the same pc
of a forklift.
board space while reducing thermal
In the last few years, there have
and electrical losses, and system and
been significant advances in the
manufacturing costs. With bottompackaging of MOSFETs, including the
side cooling and design optimization
introduction of the power SO-8 packthrough thermal simulation, this goal
age. Bottom-side cooling can now
is one step closer.
PETech
be used successfully to transfer heat
���
�������������
������������������������������
����������������������������
���������������
� ���������������������
����������������
��
��
����������������������������������
�������������������������������������
���������������������������
��
��������� �����������������������������
�����������������������������������
��
���������������������������������
����������������������������������
����������
��
���������������������������������
���������������������������������
��
������������������������������
��������������
��
����������������������������
��
���������������������������������
���������������������
������������
������������������������
����������������������
www.powerelectronics.com
33
Power Electronics Technology February 2006