2

where P

shown in the data sheet, a transient temperature change can be calculated. The transient thermal

behavior is a complicated subject because R

tions of the power pulse. A more thorough treatment of transient power analysis is beyond the

scope of this document and the reader can refer to [13] for details. Nevertheless, Fairchild provides

a Discrete SPICE Thermal Model (LIT#570240-002) for general thermal evaluation. The user may

find these models helpful in determining the dynamic power and temperature limits in the applica-

tion.

R

R

tact between the drain lead of the package and the mounting surface.

R

method. Due to the lack of an industry standard, the value of R

affect R

facturers. Under such conditions, it becomes difficult to define R

turer standpoint. On the other hand, R

rately measured by the component manufacturer.

Therefore, in this paper an effort has been made to define a procedure which can be used to

quantify the junction-to-ambient thermal resistance R

designer.

3. Result

The scope of the investigation has been limited to the size of copper mounting pad and its relative

surface placement on the board. In still air with no heatsink, the application of these heat dissipa-

tion methods is the most cost effective thermal solution. A total of sixteen different combinations

of 2 Oz copper pad sizes and their placement were designed to study their influence on R

θJA

thermal resistance. The configurations of the board layout are shown in figure 2 and table 1.

Layouts 1 to 6 have the copper pad sizes from 0.001 to 0.4 square inches on the top side of the

board (top side is defined as the component side of the board). Layouts 7 to 11 have copper pad

sizes from 0.02 to 0.4 square inches on the bottom side of the board. Layouts 12 to 16 have

copper pad sizes from 0.02 to 0.4 square inches divided equally on both sides of the board.