DSP 104-plus, Справочное руководство

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164004.C03 H3
areas subject to minor shock, vibration and temperature or atmospheric variations. In
marine applications, this is equivalent to a fixed installation inside the superstructure of
the vessel. It is recommended that, unless very rigorous thermal management is a
factor of the TP400 installation, that this is the most benign environment that should be
used for reliability estimation.
Mobile (vehicular mounted or portable and uncontrolled): Conditions more severe
than Fixed, mostly for shock and vibration. More maintenance limited and susceptible
to operator abuse. Typical applications are the mobile IT, portable operating
equipment and portable test equipment. In marine applications, this is equivalent to a
fixed installation inside the engine room of the vessel.
Note that Table H1 and Figures H1 and H2 provide separate data for each of these
usage environments.
H.2.4 Sensitivity Analysis
The highest failure rate item in the TP400 at 25ºC is the Geode GXm processor,
accounting for 3.7% of the overall failure rate. The next highest, the 32M byte SODIMM
SDRAM, accounts for 3.2% of the failure rate.
The highest failure rate item in the TP400 at 75ºC is the switch mode PSU chip
(LT1506CR-3.3SYNC), accounting for 5.5% of the overall failure rate. The next
highest, the Geode GX1 processor, accounts for 5.1% of the failure rate.
These figures are not at all excessive, given the range of complexity of the
components used.
H.3 RELIABILITY DATA
The reliability calculations are presented in Table H1 and Figures H1 and H2. It should
be noted that Mean Time to Failure and Unit Failure Rate per Annum have a fixed
relationship: (Unit Failure Rate per Annum = Reciprocal [Mean Time to Failure in
hours/(24 x 365)]).
The data represent the reliability performance under the assumption that usage is
continuous. If the usage is not continuous then the environment “Benign” is not
applicable; the Fixed environment is the most benign that can be achieved. Calculation
of failure rates should then be conducted by estimating the calendar reliability on the
basis of the elapsed calendar time taken to reach the applicable MTTF.
For example: a TP400 is in use within a control room environment. It runs for 6 hours
per day, 7 days per week. The operating temperature of the TP400 is found to be
70ºC:
MTTF (at 70ºC in Fixed environment): 31220 hours.
Mean calendar time to failure = 36767/6 = 6128 days = 16.79 years