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參數(shù)資料
| 型號： | ADDC02828SA |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 28 V/100 W DC/DC Converter with Integral EMI Filter |
| 中文描述： | 中西區(qū)區(qū)議會28 V/100 /直流轉(zhuǎn)換器積分EMI濾波器 |
| 文件頁數(shù)： | 14/16頁 |
| 文件大?。?/td> | 204K |
| 代理商： | ADDC02828SA |
ADDC02828SA
REV. 0
–14–
power supply’s internal EMI filter. T he values were chosen to
achieve the results shown in Figures 12 and 14. T o increase the
margin between the specification limits and the measured emis-
sions, larger external component values could be used.
T o do this it is useful to know that most of the emissions below
10 MHz, whether conducted or radiated, are due to differential-
mode currents flowing in the input power leads. T o make the
emissions in this frequency range smaller, the differential
capacitor value should be increased above 2
μ
F. Conversely,
most of the emissions above 10 MHz are due to common-mode
currents, and to make them smaller the common-mode capaci-
tors should be increased above the 82 nF value. In both cases it
is important to minimize the parasitic inductance of the capaci-
tors; the use of several smaller capacitors connected in parallel is
one way to achieve this.
Using larger valued capacitors than those shown in Figure 15 is
a good solution if an additional 6 dB–10 dB of margin is de-
sired. However, if in an extremely sensitive application it is
desired to increase the margin by 20 dB or more, it may be
better to add both differential- and common-mode inductors to
the external components to make a higher order filter.
RE LIABILIT Y CONSIDE RAT IONS
MT BF (Mean T ime Between Failure) is a commonly used
reliability concept that applies to repairable items in which
failed elements are replaced upon failure. T he expression for
MT BF is
MTBF = T/ r
where
T
= total operating time
r
= number of failures
In lieu of actual field data, MT BF can be predicted per
MIL -HDBK -217.
MT BF, Failure Rate, and Probability of Failure:
A proper
understanding of MT BF begins with its relationship to lambda
(
λ
), which is the failure rate. If a constant failure rate is assumed,
then MT BF = 1/
λ
, or
λ
= 1/MT BF. If a power supply has an
MT BF of 1,000,000 hours, this does not mean it will last
1,000,000 hours before it fails. Instead, the MT BF describes
the failure rate. For 1,000,000 hours MT BF, the failure rate
during any hour is 1/1,000,000, or 0.0001%. T hus, a power
supply with an MT BF of 500,000 hours would have twice the
failure rate (0.0002%) of one with 1,000,000 hours.
What users should be interested in is the probability of a power
supply not failing prior to some time t. Given the assumption of
a constant failure rate, this probability is defined as
R
(
t
)
=
e
±
λ
t
where
R(t)
is the probability of a device not failing prior to some
time t.
If we substitute
λ
= 1/MT BF in the above formula, then the
expression becomes
R
(
t
)
=
e
±
t
MTBF
T his formula is the correct way to interpret the meaning of
MT BF.
If we assume t = MT BF = 1,000,000 hours, then the probability
that a power supply will not fail prior to 1,000,000 hours of use
is e
–1
, or 36.8%. T his is quite different from saying the power
supply will last 1,000,000 hours before it fails. T he probability
that the power supply will not fail prior to 50,000 hours of use is
e
–.05
or 95%. For t = 10,000 hours the probability of no failure
is e
–.01
or 99%.
T emperature and E nvironmental Factors:
Although the
calculation of MT BF per MIL-HDBK -217 is a detailed process,
there are two key variables that give the manufacturer signi-
ficant leeway in predicting an MT BF rating. T hese two vari-
ables are temperature and environmental factor. T herefore, for
users to properly compare MT BF numbers from two different
manufacturers, the environmental factor and the temperature
must be identical. Contact the factory for MT BF calculations
for specific environmental factors and temperatures.
ME CHANICAL CONSIDE RAT IONS
When mounting the converter into the next higher level assem-
bly, it is important to ensure good thermal contact is made
between the converter and the external heat sink. Poor thermal
connection can result in the converter shutting off, due to the
temperature shutdown feature (Pin 9), or reduced reliability for
the converter due to higher than anticipated junction and case
temperatures. For these reasons the mounting tab locations
were selected to ensure good thermal contact is made near the
hot spots of the converter, which are shown in the shaded areas
of Figure 24.
Figure 24. Hot Spots (Shaded Areas) of DC/DC Converter
T he pins of the converter are typically connected to the next
higher level assembly by bending them at right angles, either
down or up, and cutting them shorter for insertion in printed
circuit board through holes. In order to maintain the hermetic
integrity of the seals around the pins, a fixture should be used
for bending the pins without stressing the pin-to-sidewall seals.
It is recommended that the minimum distance between the
package edge and the inside of the pin be 100 mils (2.54 mm)
for the 40 mil (1.02 mm) diameter pins; 120 mils (3.05 mm)
from the package edge to the center of the pin as shown in
Figure 25.
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