©2004 Fairchild Semiconductor Corporation

FDD8880 Rev. B1

Thermal Resistance vs. Mounting Pad Area

thermal resistance of the heat dissipating path determines

application.

Therefore

the

application’s

ambient

Equation 1 mathematically represents the relationship and

serves as the basis for establishing the rating of the part.

In using surface mount devices such as the TO-252

package, the environment in which it is applied will have a

significant influence on the part’s current and maximum

complex and influenced by many factors:

1. Mounting pad area onto which the device is attached and

whether there is copper on one side or both sides of the

board.

2. The number of copper layers and the thickness of the

board.

3. The use of external heat sinks.

4. The use of thermal vias.

5. Air flow and board orientation.

6. For non steady state applications, the pulse width, the

duty cycle and the transient thermal response of the part,

the board and the environment they are in.

Fairchild

provides

thermal

information

to

assist

the

designer’s preliminary application evaluation. Figure 21

copper (component side) area. This is for a horizontally

positioned FR-4 board with 1oz copper after 1000 seconds

of steady state power with no air flow. This graph provides

the necessary information for calculation of the steady state

junction

temperature

or

power

dissipation.

Pulse

applications can be evaluated using the Fairchild device

Spice thermal model or manually utilizing the normalized

maximum transient thermal impedance curve.

Thermal resistances corresponding to other copper areas

can be obtained from Figure 21 or by calculation using

Equation 2 or 3. Equation 2 is used for copper area defined

in inches square and equation 3 is for area in centimeters

square. The area, in square inches or square centimeters is

the top copper area including the gate and source pads.

(EQ. 1)

PDM

T

JM

T

A

–

()

-----------------------------

=

Area in Inches Squared

(EQ. 2)

RθJA

33.32

23.84

0.268

Area

+

()

-------------------------------------

+

=

(EQ. 3)

RθJA

33.32

154

1.73

Area

+

()

----------------------------------

+

=

Area in Centimeters Squared

25

50

75

100

125

0.01

0.1

1

10

Figure 21. Thermal Resistance vs Mounting

Pad Area

(0.645)

(6.45)

(64.5)

(0.0645)