PowerPAK
®
SO-8 Mounting and Thermal Considerations
APPLICATION NOTE
Application Note AN821
www.vishay.com
Vishay Siliconix
Revision: 16-Mai-13
3
Document Number: 71622
For technical questions, contact: powermosfettechsupport@vishay.com
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
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THERMAL PERFORMANCE
Introduction
A basic measure of a device’s thermal performance
is the junction-to-case thermal resistance, R
thJC
, or the
junction-to-foot thermal resistance, R
thJF
This parameter is
measured for the device mounted to an infinite heat sink and
is therefore a characterization of the device only, in other
words, independent of the properties of the object to which
the device is mounted. Table 1 shows a comparison of
the DPAK, PowerPAK SO-8, and standard SO-8. The
PowerPAK has thermal performance equivalent to the
DPAK, while having an order of magnitude better thermal
performance over the SO-8.
Thermal Performance on Standard SO-8 Pad Pattern
Because of the common footprint, a PowerPAK SO-8
can be mounted on an existing standard SO-8 pad pattern.
The question then arises as to the thermal performance
of the PowerPAK device under these conditions. A
characterization was made comparing a standard SO-8 and
a PowerPAK device on a board with a trough cut out
underneath the PowerPAK drain pad. This configuration
restricted the heat flow to the SO-8 land pads. The results
are shown in figure 5.
Fig. 5 PowerPAK SO-8 and Standard SO-0 Land Pad Thermal
Path
Because of the presence of the trough, this result suggests
a minimum performance improvement of 10 °C/W by using
a PowerPAK SO-8 in a standard SO-8 PC board mount.
The only concern when mounting a PowerPAK on a
standard SO-8 pad pattern is that there should be no traces
running between the body of the MOSFET. Where the
standard SO-8 body is spaced away from the pc board,
allowing traces to run underneath, the PowerPAK sits
directly on the pc board.
Thermal Performance - Spreading Copper
Designers may add additional copper, spreading copper, to
the drain pad to aid in conducting heat from a device. It is
helpful to have some information about the thermal
performance for a given area of spreading copper.
Figure 6 shows the thermal resistance of a PowerPAK SO-8
device mounted on a 2-in. 2-in., four-layer FR-4 PC board.
The two internal layers and the backside layer are solid
copper. The internal layers were chosen as solid copper to
model the large power and ground planes common in many
applications. The top layer was cut back to a smaller area
and at each step junction-to-ambient thermal resistance
measurements were taken. The results indicate that an area
above 0.3 to 0.4 square inches of spreading copper gives no
additional thermal performance improvement. A
subsequent experiment was run where the copper on the
back-side was reduced, first to 50 % in stripes to mimic
circuit traces, and then totally removed. No significant effect
was observed.
Fig. 6 Spreading Copper Junction-to-Ambient Performance
TABLE 1 - DPAK AND POWERPAK SO-8
EQUIVALENT STEADY STATE
PERFORMANCE
DPAK
PowerPAK
SO-8
Standard
SO-8
Thermal
Resistance R
thJC
1.2 °C/W 1 °C/W 16 °C/W
Si4874DY vs. Si7446DP PPAK on a 4-Layer Board
SO-8 Pattern, Trough Under Drain
Pulse Duration (sec)
)
s
ttaw/
C
( e
cn
adep
m
I
0.0001
0
1
50
60
10
100000.01
40
20
Si4874DY
Si7446DP
100
30
R
th
vs. Spreading Copper
(0 %, 50 %, 100 % Back Copper)
Spreading Copper (sq in)
)sttaw/C(
ecn
adep
m
I
0.00
56
51
46
41
36
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
0 %
50 %
100 %
