FDS8978 Rev. A

www.fairchildsemi.com

7

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.

(EQ. 1)

PDM

TJM TA

–

()

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

=

In using surface mount devices such as the SO8 package,

the environment in which it is applied will have a significant

influence on the part’s current and maximum power

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. The area, in square inches is the top copper

area including the gate and source pads.

(EQ. 2)

26

0.23 Area

+

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

+

=

varied top copper board area. Figure 22 shows the effect of

copper pad area on single pulse transient thermal

impedance. Each trace represents a copper pad area in

square inches corresponding to the descending list in the

graph. Spice and SABER thermal models are provided for

each of the listed pad areas.

Copper pad area has no perceivable effect on transient

thermal impedance for pulse widths less than 100ms. For

pulse widths less than 100ms the transient thermal

impedance is determined by the die and package.

Therefore, CTHERM1 through CTHERM5 and RTHERM1

through RTHERM5 remain constant for each of the thermal

models. A listing of the model component values is available

in Table 1.

100

150

200

0.001

0.01

0.1

1

10

50

Figure 21. Thermal Resistance vs Mounting

Pad Area

0

30

60

90

120

150

Figure 22. Thermal Impedance vs Mounting Pad Area

t, RECTANGULAR PULSE DURATION (s)

COPPER BOARD AREA - DESCENDING ORDER