MIC2583 Micrel Semiconductor, MIC2583 Datasheet - Page 19

• Manufacturer Part Number: MIC2583
• Description: Single Channel Hot Swap Controllers
• Manufacturer: Micrel Semiconductor

MOSFET Steady-State Thermal Issues
The selection of a MOSFET to meet the maximum continuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
The data sheet will almost always give a value of on resis-
tance given for the MOSFET at a gate-source voltage of 4.5V,
and another value at a gate-source voltage of 10V. As a first
approximation, add the two values together and divide by two
to get the on-resistance of the part with 8V of enhancement.
Call this value R
as an ohmic (resistive) device, almost all that’s required to
determine steady-state power dissipation is to calculate I
The one addendum to this is that MOSFETs have a slight
increase in R
approximation for this value is 0.5% increase in R
rise in junction temperature above the point at which R
initially specified by the manufacturer. For instance, if the
selected MOSFET has a calculated R
T
at 110 C, a good first cut at the operating value for R
would be:
The final step is to make sure that the heat sinking available
to the MOSFET is capable of dissipating at least as much
power (rated in C/W) as that with which the MOSFET’s
performance was specified by the manufacturer. Here are a
few practical tips:
April 2003
MIC2582/MIC2583
J
= 25 C, and the actual junction temperature ends up
• The value of I
• The manufacturer’s data sheet for the candidate
• The maximum ambient temperature in which the
• Any knowledge you can get about the heat
1. The heat from a surface-mount device such as
2. Airflow works. Even a few LFM (linear feet per
3. The best test of a surface-mount MOSFET for
R
question (see
MOSFET.
device will be required to operate.
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane,
if using a surface-mount part? Is any airflow
available?).
an SO-8 MOSFET flows almost entirely out of
the drain leads. If the drain leads can be sol-
dered down to one square inch or more, the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board
as the MOSFET drain.
minute) of air will cool a MOSFET down sub-
stantially. If you can, position the MOSFET(s)
near the inlet of a power supply’s fan, or the
outlet of a processor’s cooling fan.
an application (assuming the above tips show it
to be a likely fit) is an empirical one. Check the
MOSFET's temperature in the actual layout of
the expected final circuit, at full operating
ON
10m [1 + (110 - 25)(0.005)]
ON
ON
with increasing die temperature. A good
. Since a heavily enhanced MOSFET acts
LOAD(CONT, MAX.)
Sense Resistor Selection
for the output in
ON
14.3m
of 10m
).
ON
ON
per C
at a
was
2
ON
R.
19
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the imposed
voltage stresses, and the worse case continuous I
dissipation which it will see, it remains only to verify the
MOSFET’s ability to handle short-term overload power dissi-
pation without overheating. A MOSFET can handle a much
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
Transient Thermal Impedance Curve. For example, take the
following case: V
I
nominal, and the fast-trip threshold is 100mV. If the output is
accidentally connected to a 3 load, the output current from
the MOSFET will be regulated to 2.5A for 100ms (t
before the part trips. During that time, the dissipation in the
MOSFET is given by:
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 10 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
event of this duration is only 8% of its continuous R
This particular part is specified as having an R
50 C/W for intervals of 10 seconds or less. Thus:
Assume T
drain leads, no airflow.
Recalling from our previous approximation hint, the part has
an R
Assume it has been carrying just about 2.5A for some time.
When performing this calculation, be sure to use the highest
anticipated ambient temperature (T
MOSFET will be operating as the starting temperature, and
find the operating junction temperature increase ( T
that point. Then, as shown next, the final junction temperature
is found by adding T
form equation, getting a close approximation may take one or
LOAD(CONT. MAX)
ON
(J-A)
P = E x I E
P
current. The use of a thermocouple on the drain
leads, or infrared pyrometer on the package, will
then give a reasonable idea of the device’s
junction temperature.
MOSFET
of (0.0335/2) = 17m at 25 C.
. Almost all power MOSFET data sheets give a
A
= 55 C maximum, 1 square inch of copper at the
= (4.5V x 2.5A) = 11.25W for 100msec.
IN
MOSFET
(J-A)
is 2.5A, the slow-trip threshold is 50mV
= 12V, t
A(MAX)
of this MOSFET to a highly infrequent
= [12V-(2.5A)(3 )] = 4.5V
OCSLOW
and T
J
. Since this is not a closed-
has been set to 100msec,
A(MAX)
MIC2582/MIC2583
) in which the
2
OCSLOW
R power
(J-A)
(J-A)
J
) from
Micrel
.
of
)