ADDC02812DA 데이터 시트보기 (PDF) - Analog Devices

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ADDC02812DA

28 V/100 W DC/DC Converters with Integral EMI Filter

Analog Devices 

ADDC02812DA/ADDC02815DA

RELIABILITY CONSIDERATIONS

MTBF (Mean Time Between Failure) is a commonly used reli-

ability concept that applies to repairable items in which failed

elements are replaced upon failure. The expression for MTBF is

MTBF = T/r

where

T = total operating time

r = number of failures

In lieu of actual field data, MTBF can be predicted per

MIL-HDBK-217.

MECHANICAL CONSIDERATIONS

When mounting the converter into the next higher level assem-

bly, it is important to insure 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 insure good thermal contact is made near the

hot spots of the converter which are shown in the shaded areas

of Figure 26.

MTBF, Failure Rate, and Probability of Failure: A proper

OBSOLETE understanding of MTBF begins with its relationship to lambda

(␭), which is the failure rate. If a constant failure rate is assumed,

then MTBF = 1/␭, or ␭ = 1/MTBF. If a power supply has an

MTBF of 1,000,000 hours, this does not mean it will last

1,000,000 hours before it fails. Instead, the MTBF describes the

failure rate. For 1,000,000 hours MTBF, the failure rate during

any hour is 1/1,000,000, or 0.0001%. Thus, a power supply

with an MTBF 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–λt

where R(t) is the probability of a device not failing prior to some

time, t.

If we substitute ␭ = 1/MTBF in the above formula, then the

Figure 26. Hot Spots (Shaded Areas) of DC/DC Converter

The 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

expression becomes

circuit board through holes. In order to maintain the hermetic

integrity of the seals around the pins, a fixture should be used

–t

for bending the pins without stressing the pin-to-sidewall seals.

R(t ) = e MTBF

It is recommended that the minimum distance between the

This formula is the correct way to interpret the meaning of

package edge and the inside of the pin be 100 mils (2.54 mm)

MTBF.

for the 40 mil (1.02 mm) diameter pins; 120 mils (3.05 mm)

If we assume t = MTBF = 1,000,000 hours, then the probability from the package edge to the center of the pin as shown in

that a power supply will not fail prior to 1,000,000 hours of use Figure 27.

is e–1, or 36.8%. This is quite different from saying the power

supply will last 1,000,000 hours before it fails. The 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%.

Temperature and Environmental Factors: Although the

calculation of MTBF per MIL-HDBK-217 is a detailed process,

there are two key variables that give the manufacturer significant

leeway in predicting an MTBF rating. These two variables are

temperature and environmental factor. Therefore, for users to

properly compare MTBF numbers from two different manufac-

turers, the environmental factor and the temperature must be

identical. Contact the factory for MTBF calculations for specific

environmental factors and temperatures.

0.100"

(2.54mm)

0.120"

(3.05mm)

Figure 27. Minimum Bend Radius of 40 Mil (1.02 mm) Pins

REV. 0

–15–

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