FDS8880 N-Channel PowerTrench
®
MOSFET
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.
P
DM
(
T JM
–
T A
)
= ------------------------------
-
R
θJA
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.
R
θJA
=
64
+
-------------------------------
26
0.23 +
Area
(EQ. 2)
(EQ. 1)
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
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 design-
er’s preliminary application evaluation. Figure 21 defines the
R
θJA
for the device as a function of the top copper (compo-
nent 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 in-
formation for calculation of the steady state junction temper-
ature or power dissipation. Pulse applications can be
evaluated using the Fairchild device Spice thermal model or
manually utilizing the normalized maximum transient
150
120
90
60
30
0
10
-1
10
0
COPPER BOARD AREA - DESCENDING ORDER
0.04 in
2
0.28 in
2
0.52 in
2
0.76 in
2
1.00 in
2
The transient thermal impedance (Z
θJA
) is also effected by
varied top copper board area. Figure 22 shows the effect of
copper pad area on single pulse transient thermal imped-
ance. 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.
200
R
θJA
= 64 + 26/(0.23+Area)
R
θJA
(
o
C/W)
150
100
50
0.001
0.01
0.1
1
AREA, TOP COPPER AREA (in
2
)
10
Figure 21. Thermal Resistance vs Mounting
Pad Area
Z
θJA
, THERMAL
IMPEDANCE (
o
C/W)
10
1
t, RECTANGULAR PULSE DURATION (s)
10
2
10
3
Figure 22. Thermal Impedance vs Mounting Pad Area
©2007 Fairchild Semiconductor Corporation
FDS8880 Rev. B
8
www.fairchildsemi.com
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
