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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

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

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

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 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

On-Resistance vs. Junction Temperature

0.8 °C/W

107 °C

PowerPAK SO-8

16 C/W

148 °C

Standard SO-8

PC Board at 105 °C