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Document Number 71622
28-Feb-06
Vishay Siliconix
AN821
SYSTEM AND ELECTRICAL IMPACT OF
PowerPAK SO-8
In any design, one must take into account the change in
MOSFET r
DS(on)
with temperature (Figure 7).
A MOSFET generates internal heat due to the current
passing through the channel. This self-heating raises
the junction temperature of the device above that of the
PC board to which it is mounted, causing increased
power dissipation in the device. A major source of this
problem lies in the large values of the junction-to-foot
thermal resistance of the SO-8 package.
PowerPAK SO-8 minimizes the junction-to-board ther-
mal resistance to where the MOSFET die temperature is
very close to the temperature of the PC board. Consider
two devices mounted on a PC board heated to 105 °C
by other components on the board (Figure 8).
Suppose each device is dissipating 2.7 W. Using the
junction-to-foot thermal resistance characteristics of the
PowerPAK SO-8 and the standard SO-8, the die tem-
perature is determined to be 107 °C for the PowerPAK
(and for DPAK) and 148 °C for the standard SO-8. This
is a 2 °C rise above the board temperature for the Pow-
erPAK and a 43 °C rise for the standard SO-8. Referring
to Figure 7, a 2 °C difference has minimal effect on
r
DS(on)
whereas a 43C difference has a significant effect
on r
DS(on)
.
Minimizing the thermal rise above the board tempera-
ture by using PowerPAK has not only eased the thermal
design but it has allowed the device to run cooler, keep
r
DS(on)
low, and permits the device to handle more cur-
rent than the same MOSFET die in the standard SO-8
package.
CONCLUSIONS
PowerPAK SO-8 has been shown to have the same
thermal performance as the DPAK package while hav-
ing the same footprint as the standard SO-8 package.
The PowerPAK SO-8 can hold larger die approximately
equal in size to the maximum that the DPAK can accom-
modate implying no sacrifice in performance because of
package limitations.
Recommended PowerPAK SO-8 land patterns are pro-
vided to aid in PC board layout for designs using this
new package.
Thermal considerations have indicated that significant
advantages can be gained by using PowerPAK SO-8
devices in designs where the PC board was laid out for
the standard SO-8. Applications experimental data gave
thermal performance data showing minimum and typical
thermal performance in a SO-8 environment, plus infor-
mation on the optimum thermal performance obtainable
including spreading copper. This further emphasized the
DPAK equivalency.
PowerPAK SO-8 therefore has the desired small size
characteristics of the SO-8 combined with the attractive
thermal characteristics of the DPAK package.
Figure 7.
MOSFET
r
DS(on)
vs. Temperature
Figure 8.
Temperature of Devices on a PC Board
0.6
0.8
1.0
1.2
1.4
1.6
1.8
-50 -25 0 25 50 75 100 125 150
V
GS
= 10 V
I
D
= 23 A
On-Resistance vs. Junction Temperature
T
J
- Junction Temperature (°C)
)dezilamroN(
( ecnatsiseR-nO -r
)no(SD
)
0.8 °C/W
107 °C
PowerPAK SO-8
16 C/W
148 °C
Standard SO-8
PC Board at 105 °C
