MIC2590B Datasheet, PDF (20/24 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

MIC2590B
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 empiri-
cal one. The ideal evaluation is in the actual
layout of the expected final circuit, at full operat-
ing 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 continu-
ous 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 phys-
ics work in our favor: 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, 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.
Micrel
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(0.6Ω)] = 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 com-
monly used SO-8 power MOSFET.
2
1
Duty Cycle = 0.5
Normalized Thermal Transient Impedance, Junction-to-Ambient
0.2
0.1
0.1
0.05
0.02
0.01
10^–4
Single Pulse
10^–3
10^–2
10^–1
1
Square Wave Pulse Duration (sec)
Notes:
P_DM
t₁
t₂
1. Duty Cycle, D =
t₁
t₂
2. Per Unit Base = R_thJA = 67°C/W
3. T_JM – T_A = P_DMZ_thJA(t)
4. Surface Mounted
10
100
600
Figure 10. Si4430DY MOSFET Transient Thermal Impedance Curve
Using this graph is not nearly as daunting as it may at first
appear. 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 slot 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 (=100ms),
we see that the effective thermal impedance of this MOSFET
to a single pulse event of this duration is only 6% of its
continuous R_θ(JA).
This particular part is specified as having an R_θ(JA) of
50°C/W for intervals of 10 seconds or less. So, some further
math, just to get things ready for the finale:
Assume T_A = 55°C maximum, 1 square inch of copper at the
drain leads, no airflow.
MIC2590B
20
August 2002