MIC2590B-5BTQ TR【PDF 20/23 页】IC PCI HOT PLUG CTLR DUAL 48TQFP

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Sheet目录45

16页 17页 18页 19页 [20页]21页 22页 23页

Micrel, Inc.

MIC2590B

September 2008

M9999-091808

the MOSFET drain.

2. Since the rating for the part is given as for

10seconds, derate the maximum junction

temperature by 35癈. This is the standard good

practice derating of 25癈, plus another 10癈 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 supplys fan,

or the outlet of a processors cooling fan, thats

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 devices 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 I

R power dissipation which it will see, it

remains only to verify the MOSFETs ability to handle

short-term overload power dissipation without

overheating. Here, nature and physics 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.

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 0.60& load, the

output current from the MOSFET for the slot in question

will be regulated to 5.0A for 50ms before the parts circuit

breaker trips. During that time, the dissipation in the

MOSFET is given by:

[

] 2V

5A(0.6&A

MOSFET

)

50ms

for

10W

MOSFET

Wow! Looks like we need a really hefty MOSFET to

withstand just this unlikelybut plausible enough to

protect againstfault 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.

-4

-3

-2

-1

100

600

0.1

0.01

0.2

0.1

0.05

0.02

Single Pulse

Duty Cycle = 0.5

Normalized Thermal Transient Imperance, Juction-to-Ambient

1. Duty Cycle, D =

2. Per Unit Base = R

qJA

= 67癈/W

3. T

= P

qJA

(t)

4. Surface Mounted

Notes:

Square Wave Pulse Duration (sec)

Figure 10. Si4430DY MOSFET Transient Thermal Impedance Curve

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