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參數資料
| 型號： | MIC2583R-MBQS TR |
| 廠商： | Micrel Inc |
| 文件頁數： | 20/25頁 |
| 文件大小： | 826K |
| 描述： | IC CTRLR HOT SWAP 200MV 16-QSOP |
| 標準包裝： | 2,500 |
| 類型： | 熱交換控制器 |
| 應用： | 通用 |
| 內部開關： | 無 |
| 電源電壓： | 2.3 V ~ 13.2 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 16-SSOP（0.154"，3.90mm 寬） |
| 供應商設備封裝： | 16-QSOP |
| 包裝： | 帶卷 (TR) |
| 其它名稱： | MIC2583R-MBQSTR MIC2583R-MBQSTR-ND |
Micrel, Inc.
MIC2582/MIC2583
April 2009
20
M9999-043009-C
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 manufacturers 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
resistance 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 thats
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 篊 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篊, and the actual junction
temperature ends up at 110篊, a good first cut at the
operating value for R
ON
would be:
(
)(
)
[
]
?/DIV>
E
+
?/DIV>
E
m
m
R
ON
3
.
14
005
.
0
25
110
1
10
(13)
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 篊/W) as that with which
the MOSFETs performance was specified by the
manufacturer. Here are a few practical tips:
1. The heat from a surface-mount device such as
an SOIC-8 MOSFET flows almost entirely out
of the drain leads. If the drain leads can be
soldered 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
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 MOSFETs 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 I
2
R 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
?JA)
. 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
SOIC-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
?JA)
of this MOSFET to a highly infrequent event of this
duration is only 8% of its continuous R
?JA)
.
This particular part is specified as having an R
?JA)
of
50癈/W for intervals of 10 seconds or less.
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