Vishay Siliconix

AN821

Document Number 71622

28-Feb-06

www.vishay.com

3

THERMAL PERFORMANCE

Introduction

A basic measure of a device’s thermal performance is

the junction-to-case thermal resistance, R

θ

junction-to-foot thermal resistance, R

θ

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 stan-

dard SO-8. The PowerPAK has thermal performance

equivalent to the DPAK, while having an order of magni-

tude 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 pat-

tern. The question then arises as to the thermal perfor-

mance 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.

Because of the presence of the trough, this result sug-

gests 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 Power-

PAK sits directly on the pc board.

Thermal Performance - Spreading Copper

Designers may add additional copper, spreading cop-

per, 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 cop-

per.

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.

TABLE 1.

DPAK and PowerPAK SO-8

Equivalent Steady State Performance

DPAK

PowerPAK

SO-8

Standard

SO-8

Thermal

Resistance R

θ

jc

1.2 °C/W

1.0 °C/W

16 °C/W

Figure 5. PowerPAK SO-8 and Standard SO-0 Land Pad Thermal Path

Si4874DY vs. Si7446DP PPAK on a 4-Layer Board

SO-8 Pattern, Trough Under Drain

Pulse Duration (sec)

0.0001

0

1

50

60

10

10000

0.01

40

20

Si4874DY

Si7446DP

100

30

Figure 6. Spreading Copper Junction-to-Ambient Performance

(0 %, 50 %, 100 % Back Copper)

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 %