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| 型號： | MIC2583R-MBQS |
| 廠商： | MICREL INC |
| 元件分類： | 電源管理 |
| 英文描述： | Single Channel Hot Swap Controllers |
| 中文描述： | 1-CHANNEL POWER SUPPLY MANAGEMENT CKT, PDSO16 |
| 封裝： | QSOP-16 |
| 文件頁數(shù)： | 19/22頁 |
| 文件大小： | 190K |
| 代理商： | MIC2583R-MBQS |
April 2003
19
MIC2582/MIC2583
MIC2582/MIC2583
Micrel
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 value of I
LOAD(CONT, MAX.)
for the output in
question (see Sense Resistor Selection.
The manufacturer
’
s data sheet for the candidate
MOSFET.
The maximum ambient temperature in which the
device will be required to operate.
Any knowledge you can get about the heat
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).
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
ON
. Since a heavily enhanced MOSFET acts
as an ohmic (resistive) device, almost all that
’
s required to
determine steady-state power dissipation is to calculate I
2
R.
The one addendum to this is that MOSFETs have a slight
increase in R
ON
with increasing die temperature. A good
approximation for this value is 0.5% increase in R
ON
per
°
C
rise in junction temperature above the point at which R
ON
was
initially specified by the manufacturer. For instance, if the
selected MOSFET has a calculated R
ON
of 10m
at a
T
J
= 25
°
C, and the actual junction temperature ends up
at 110
°
C, a good first cut at the operating value for R
ON
would be:
R
ON
10m
[1 + (110 - 25)(0.005)]
14.3m
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:
1. The heat from a surface-mount device such as
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.
2. Airflow works. Even a few LFM (linear feet per
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.
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 device
’
s
junction temperature.
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the imposed
voltage stresses, and the worse case continuous I
2
R power
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
θ
(J-A)
. Almost all power MOSFET data sheets give a
Transient Thermal Impedance Curve. For example, take the
following case: V
IN
= 12V, t
OCSLOW
has been set to 100msec,
I
LOAD(CONT. MAX)
is 2.5A, the slow-trip threshold is 50mV
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
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
P
MOSFET
= (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 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
θ
(J-A)
of this MOSFET to a highly infrequent
event of this duration is only 8% of its continuous R
θ
(J-A)
.
This particular part is specified as having an R
θ
(J-A)
of
50
°
C/W for intervals of 10 seconds or less. Thus:
Assume T
A
= 55
°
C maximum, 1 square inch of copper at the
drain leads, no airflow.
Recalling from our previous approximation hint, the part has
an R
ON
of (0.0335/2) = 17m
at 25
°
C.
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
A(MAX)
) in which the
MOSFET will be operating as the starting temperature, and
find the operating junction temperature increase (
T
J
) from
that point. Then, as shown next, the final junction temperature
is found by adding T
A(MAX)
and
T
J
. Since this is not a closed-
form equation, getting a close approximation may take one or
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