MIC2590B_08 Datasheet, PDF (20/23 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

Micrel, Inc.
the MOSFET drain.
2. Since the rating for the part is given as “for
10seconds,” derate the maximum junction
temperature by 35°C. This is the standard good
practice derating of 25°C, plus another 10°C to
allow for the time element of the specification.
3. Airflow, if available, works wonders. This is not
the place for a dissertation on how to perform
airflow calculations, but even a few LFM (linear
feet per minute) of air will cool a MOSFET down
dramatically. If you can position the MOSFET(s)
in question near the inlet of a power supply’s fan,
or the outlet of a processor’s cooling fan, that’s
always a good free ride.
4. Although it seems a rather unsatisfactory
statement, 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.
The ideal evaluation is in the actual layout of the
expected final circuit, at full operating current. The
use of a thermocouple on the drain leads, or in
infrared pyrometer on the package, will then give
a reasonable idea of the device’s junction
temperature.
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the imposed
voltage stresses, and be able to handle the worst-case
continuous I²R power dissipation which it will see, it
remains only to verify the MOSFET’s ability to handle
short-term overload power dissipation without
overheating. Here, nature and physics work in our favor: a
MIC2590B
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,
semiconductor devices (silicon die, lead frames, etc.)
have thermal inertia. This is easily understood by all of us
who have stood waiting for a pot of water to boil.
In terms related directly to the specification and use of
power MOSFETs, this is known as “transient thermal
impedance.” Almost all power MOSFET data sheets give
a Transient Thermal Impedance Curve, which is a handy
tool for making sure that you can safely get by with a less
expensive MOSFET than you thought you might need.
For example, take the case where t_FLT for the 5V supply
has been set to 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 0.60Ω load, the
output current from the MOSFET for the slot in question
will be regulated to 5.0A for 50ms before the part’s circuit
breaker trips. During that time, the dissipation in the
MOSFET is given by:
P = E × I E_MOSFET = [5V − 5A(0.6ΩA] = 2V
P_MOSFET = (2V × 5A) = 10W for 50ms
Wow! Looks like we need a really hefty MOSFET to
withstand just this unlikely—but plausible enough to
protect against—fault condition. Or do we? This is where
the transient thermal impedance curves become very
useful. Figure 10 shows those curves for the Vishay
(Siliconix) Si4430DY, a commonly used SO-8 power
MOSFET.
Normalized Thermal Transient Imperance, Juction-to-Ambient
2
1
Duty Cycle = 0.5
0.2
Notes:
0.1
0.1
0.05
0.02
Single Pulse
0.01
10^-4
10^-3
10^-2
10^-1
1
P_DM
t₁
t₂
1.
2.
Duty Cycle, D
Per Unit Base
=
=
t₁
R^t2qJA
=
67°C/W
3. T_JM – T_A = P_DMZ_qJA(t)
4. Surface Mounted
10
100
600
Square Wave Pulse Duration (sec)
Figure 10. Si4430DY MOSFET Transient Thermal Impedance Curve
September 2008
20
M9999-091808