SI7634BD Datasheet, PDF (11/13 Pages) Vishay Telefunken – N-Channel 30-V (D-S) MOSFET

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Application Note AN821
Vishay Siliconix
PowerPAK^® SO-8 Mounting and Thermal Considerations
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).
On-Resistance vs. Junction Temperature
1.8
1.6
V_GS = 10 V
I_D = 23 A
1.4
1.2
1.0
0.8
0.6
- 50 - 25 0
25 50 75 100 125 150
T_J - Junction Temperature (°C)
Fig. 7 MOSFET R_DS(on) vs. Temperature
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 thermal
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
temperature 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 PowerPAK
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
43 °C difference has a significant effect on R_DS(on).
Minimizing the thermal rise above the board temperature 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 current 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 having 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 accommodate implying no
sacrifice in performance because of package limitations.
Recommended PowerPAK SO-8 land patterns are provided
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
information 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.
PowerPAK SO-8
107 °C
Standard SO-8
148 °C
0.8 °C/W
PC Board at 105 °C
16 C/W
Fig. 8 Temperature of Devices on a PC Board
Revision: 16-Mai-13
4
Document Number: 71622
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