Micrel, Inc.
MIC2085/2086
May 2006
24
M9999-050406
(408) 955-1690
Figure 11. Zener Clamped MOSFET GATE
2. Airflow works. Even a few LFM (linear feet per
minute) of air will cool a MOSFET down
substantially. If you can, position the
MOSFET(s) near the inlet of a power supplys
fan, or the outlet of a processors cooling fan.
3. The best test of a surface-mount MOSFET for
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
current. The use of a thermocouple on the drain
leads, or infrared pyrometer on the package, will
then give a reasonable idea of the devices
junction temperature.
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the
imposed voltage stresses, and the worse case
continuous I2R power dissipation which it will see, it
remains only to verify the MOSFETs ability to handle
short-term overload power dissipation 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
?J-A)
. Almost all power MOSFET data
sheets give a Transient Thermal Impedance Curve. For
example, take the following case: VIN = 12V, t
OCSLOW
has been set to 100msec, I
LOAD(CONT. MAX)
is 2.5A, the
slow-trip threshold is 48mVnominal, and the fast-trip
threshold is 95mV. 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
OCSLOW
)
before the part trips. During that time, the dissipation in
the MOSFET is given by:
P = E x I E
MOSFET
= [12V-(2.5A)(3&)]=4.5V
PMOSFET = (4.5V x 2.5A) = 11.25W 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 12 shows the curve
for the Vishay (Siliconix) Si4410DY, 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 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,
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
?J-A)
of this MOSFET to a highly infrequent event of this
duration is only 8% of its continuous R
?J-A)
.
This particular part is specified as having an R
?J-A)
of
50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
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