©2008 Fairchild Semiconductor Corporation
F
DD8878 / FDU8878 Rev. A
4
www.
fairchildsemi.com
FDD8878 / FDU8878 N-Channel PowerTrench
®
MOSFET
8
Thermal Resistance vs. Mounting Pad Area
The maximum rated junction temperature, T
JM
, and the
thermal resistance of the heat dissipating path determines
the maximum allowable device power dissipation, P
DM
, in an
application. Therefore the application’s ambient
temperature, T
A
(
o
C), and thermal resistance R
θJA
(
o
C/W)
must be reviewed to ensure that T
JM
is never exceeded.
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
power dissipation ratings. Precise determination of P
DM
is
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
defines the R
θJA
for the device as a function of the top
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)
P
DM
T
JM
T
A
–()
R
θJA
-----------------------------=
Area in Inches Squared
(EQ. 2)
R
θJA
33.32
23.84
0.268 Area+()
--------- ----------------- -----------+
=
(EQ. 3)
R
θJA
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
R
θJA
= 33.32+ 23.84/(0.268+Area) EQ.2
R
θJA
(
o
C/W)
AREA, TOP COPPER AREA in
2
(cm
2
)
R
θJA
= 33.32+ 154/(1.73+Area) EQ.3
(0.645) (6.45) (64.5)(0.0645)
