Application Note AN821

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Vishay Siliconix

Revision: 16-Mai-13

4

Document Number: 71622

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

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 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).

Fig. 8 Temperature of Devices on a PC Board

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,

Minimizing the thermal rise above the board temperature by

using PowerPAK has not only eased the thermal design but

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.

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