HUF75531SK8
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.
Displayed on the curve are R
θJA
values listed in the Electrical
Specifications table. The points were chosen to depict the
compromise between the copper board area, the thermal
resistance and ultimately the power dissipation, P
DM
.
Thermal resistances corresponding to other copper areas can
be obtained from Figure 20 or by calculation using Equation 2.
R
θJA
is defined as the natural log of the area times a coefficient
added to a constant. The area, in square inches is the top
copper area including the gate and source pads.
R
θJA
=
83.2
–
23.6
×
R
θJA
(
o
C/W)
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.
240
R
θJA
= 83.2 - 23.6*
ln
(AREA)
200
189
o
C/W - 0.0115in
2
160
152
o
C/W - 0.054in
2
120
ln
(
Area
)
(EQ. 2)
80
0.01
0.1
AREA, TOP COPPER AREA (in
2
)
1.0
The transient thermal impedance (Z
θJA
) is also effected by
varied top copper board area. Figure 21 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
150
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
FIGURE 20. THERMAL RESISTANCE vs MOUNTING PAD AREA
120
Z
θJA
, THERMAL
IMPEDANCE (
o
C/W)
90
60
30
0
10
-1
10
0
10
1
t, RECTANGULAR PULSE DURATION (s)
10
2
10
3
FIGURE 21. THERMAL IMPEDANCE vs MOUNTING PAD AREA
©2001 Fairchild Semiconductor Corporation
HUF75531SK8 Rev. B
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
