Micrel, Inc.
MIC2593
September 2008
23
M9999-092208
50ms, I
LOAD(CONT, MAX)
is 5.0A, the slow-trip threshold is
50Mv nominal, and the fast-trip threshold is 100mV. If the
output is connected to a 0.60& load, the output current
from the MOSFET for the slot in question will be regulated
to 5.0A for 50ms before the MIC2593 circuit breaker trips.
During that time, the dissipation in the MOSFET is given
by:
[
]
2V
5A(0.6&A
5V
E
I
E
P
MOSFET
=
=
?/DIV>
=
(
)
50ms
for
10W
5A
2V
P
MOSFET
=
?/DIV>
=
At first glance, it would appear that a really hefty MOSFET
is required to withstand this sort of fault condition. This is
where the transient thermal impedance curves become
very useful. Figure 13 shows the curve for the Vishay
(Siliconix) Si4430DY, a commonly used SO-8 power
MOSFET.
Taking the simplest case first, well assume that once a
fault event such as the one in question occurs, it will be a
long time, several seconds, before the fault is isolated and
the channel is reset. In such a case, we can approximate
this as a single pulse event, that is to say, theres no
significant duty cycle. Then, reading up from the X-axis at
the point where Square Wave Pulse Duration is equal to
0.1sec (=100msec), we see that the Z
?JA)
of this MOSFET
to a highly infrequent event of this duration is only 7% of
its continuous R
?JA)
.
This particular part is specified as having an R
?JA)
of
35癈/W for intervals of 10 seconds or less. Thus:
Assume T
A
= 55癈 maximum, 1 square inch of copper at
the drain leads, no airflow.
Recalling from our previous approximation hint, the part
has an R
ON
of (0.014/2) = 7m& at 25癈.
Assume it has been carrying just about 5A for some time.
When performing this calculation, be sure to use the
highest anticipated ambient temperature (T
A(MAX)
) in which
the MOSFET will be operating as the starting
temperature, and find the operating junction temperature
increase (T
J
) from that point. Then, as shown next, the
final junction temperature is found by adding T
A(MAX)
and
T
J
. Since this is not a closed-form equation, getting a
close approximation may take one or two iterations, but
its not a hard calculation to perform and tends to
converge quickly.
Then the starting (steady-state) T
J
is:
J
A(MAX)
J
擳
T
T
)
C)(R
)(0.005
T
(T
R
T
ON
A
A(MAX)
ON
A(MAX)
?/DIV>
?JA)
2
R
I ?/DIV>
?/DIV>
]
7m&m
C)(0.005)(
25
C
(55
7m&
C
55
T
J
?/DIV>
?/DIV>
?/DIV>
C/W)
(35
(5A)
2
?/DIV>
?/DIV>
?/DIV>
C)
(35
(0.20125W)
C
55
T
J
?/DIV>
?/DIV>
C
62.0?nbsp
Iterate the calculation once to see if this value is within a
few percent of the expected final value. For this iteration
we will start with T
J
equal to the already calculated value
of 62.0癈:
]
7m&m
C)(0.005)(
25
C
(62.0
7m&
T
T
A
J
?/DIV>
?/DIV>
C/W)
(35
(5A)
2
?/DIV>
?/DIV>
?/DIV>
C
62.35
C)
(35
(0.20125W)
C
55
T
J
?/DIV>
E
?/DIV>
?/DIV>
So our original approximation of 62.0癈 was very close to
the correct value. We will use T
J
= 62癈.
Finally, add (10W)(35癈/W)(0.07) = 24.5癈 to the steady-
state T
J
to get T
J(TRANSIENT MAX.)
= 86.5癈. This is an
acceptable maximum junction temperature for this part.
10
-4
10
-3
10
-2
10
-1
1
10
100
600
2
1
0.1
0.01
0.2
0.1
0.05
0.02
Single Pulse
Duty Cycle = 0.5
Normalized Thermal Transient Imperance, Juction-to-Ambient
1. Duty Cycle, D =
2. Per Unit Base = R
qJA
= 67癈/W
3. T
JM
T
A
= P
DM
Z
qJA
(t)
4. Surface Mounted
t
1
t
2
t
1
t
2
Notes:
P
DM
Square Wave Pulse Duration (sec)
Figure 13. Si4430DY MOSFET Transient Thermal Impedance Curve
发布紧急采购，3分钟左右您将得到回复。
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