March 2005 25 MIC2584/2585
MIC2584/2585 Micrel
higher pulsed power without damage than its continuous
dissipation ratings would imply. The reason for this is that, like
everything else, thermal devices (silicon die, lead frames,
etc.) have thermal inertia.
In terms related directly to the specification and use of power
MOSFETs, this is known as “transient thermal impedance,”
or Z
θ(J-A)
. Almost all power MOSFET data sheets give a
Transient Thermal Impedance Curve. For example, take the
following case: V
IN
= 12V, t
OCSLOW
has been set to 100msec,
I
LOAD(CONT. MAX)
is 1.2A, the slow-trip threshold is 50mV
nominal, and the fast-trip threshold is 100mV. If the output is
accidentally connected to a 6Ω load, the output current from
the MOSFET will be regulated to 1.2A for 100ms (t
OCSLOW
)
before the part trips. During that time, the dissipation in the
MOSFET is given by:
P = E x I E
MOSFET
= [12V-(1.2A)(6Ω)] = 4.8V
P
MOSFET
= (4.8V x 1.2A) = 5.76W for 100msec.
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) Si4410DY,
a commonly used SO-8 power MOSFET.
Taking the simplest case first, we’ll assume that once a fault
event such as the one in question occurs, it will be a long time,
10 minutes or more, 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, there’s 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
θ(J-A)
of this MOSFET to a highly infrequent
event of this duration is only 8% of its continuous R
θ(J-A)
.
This particular part is specified as having an R
θ(J-A)
of
50°C/W for intervals of 10 seconds or less. Thus:
Assume T
A
= 55°C 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 (0.0335/2) = 17mΩ at 25°C.
Assume it has been carrying just about 1.2A 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 it’s not a hard calculation to perform, and
tends to converge quickly.
Then the starting (steady-state)T
J
is:
T
J
≅ T
A(MAX)
+ ∆T
J
≅ T
A(MAX)
+ [R
ON
+ (T
A(MAX)
– T
A
)(0.005/°C)(R
ON
)]
x I
2
x R
θ(J-A)
T
J
≅ 55°C + [17mΩ + (55°C-25°C)(0.005)(17mΩ)]
x (1.2A)
2
x (50°C/W)
T
J
≅ (55°C + (0.02815W)(50°C/W)
≅ 54.6°C
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 54.6°C:
T
J
≅ T
A
+ [17mΩ + (54.6°C-25°C)(0.005)(17mΩ)]
x (1.2A)
2
x (50°C/W)
T
J
≅ ( 55°C + (0.02832W)(50°C/W) ≅ 56.42°C
So our original approximation of 56.4°C was very close to the
correct value. We will use T
J
= 56°C.
Finally, add (5.76W)(50°C/W)(0.08) = 23°C to the steady-state
T
J
to get T
J(TRANSIENT MAX.)
=79°C. This is an acceptable
maximum junction temperature for this part.
2
1
0.1
0.01
10
—4
10
—3
10
—2
10
—1
11030
0.2
0.1
0.05
0.02
Single Pulse
Duty Cycle = 0.5
1. Duty Cycle, D =
2. Per Unit Base = R
thJA
= 50
¡
C/W
3. T
JM
— T
A
= P
DM
Z
thJA
(t)
t
1
t
2
t
1
t
2
Notes:
4. Surface Mounted
P
DM
Normalized Thermal Transient Impedance, Junction-to-Ambient
Normalized Effective Transient
Thermal Impedance
Square Wave Pulse Duration (sec)
Figure 13. Transient Thermal Impedance
