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Do You Need Worst Case Circuit Analysis?
Have a question? As WCCA experts, we are here to help.
There is no charge for initial consultations.
We are happy to discuss
radiation, part tolerance, mission life, space,
specification, reliability, failure, and SDRL
related questions along with any other project requirements.
If you have any questions about WCCA or need to find out more about our Worst Case Analysis services please
contact AEi Systems via email, info@aeng.com, or by phone at (310) 216-1144.
How Worst Case Circuit Analysis Saves $$$
Worst Case Circuit Analysis (WCCA or WCA) is a cost effective means of screening a design to
ensure with a high degree of confidence that potential defects and deficiencies
are identified and eliminated PRIOR TO and DURING test, production, and delivery.
WCCA is not an after-the-fact exercise but a cost-effective integral part
of the design process.
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WCCA is not a candidate for elimination when there are cost overruns.
When a WCCA is performed properly, the results often save companies millions of
dollars in lost revenue, dramatically lessen the possibility of human disaster,
and avert potential disasters both monetary and political.
Conducting a WCCA during the design phase should be a top consideration.
The computer models should be constructed in parallel with the breadboards and/or EM’s.
The models should be reviewed and updated for EVERY circuit or requirement change.
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Should you do a WCCA? Here are some questions to ask yourself.
How much risk can the program accept?
What is the cost of failure?
What is the heritage of the circuitry?
What is the cost of doing or not doing a worst case analysis
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The typical cost of performing a rigorous WCCA is
generally less than 1% of the program cost. The cost of not doing the WCCA can
cost 100% of the program cost. We have seen many program failures, which required
at the least, major redesigns AFTER completion of qualification testing.
This costs enormous amounts of time and money.
Can't electrical testing be used as a less expensive an alternative?
The answer is generally "No".
Testing normally only determines Beginning of Life (BOL) performance. In many cases
extended testing needs to be performed with extreme operating conditions such as
temperature, voltage, power, etc. in order to determine End-Of-Life (EOL) margins. This can
overstress the hardware. Testing is only valid for the measured lot and may vary lot to
lot and manufacturer to manufacturer. It requires the parts to be procured PRIOR to
completion of the WCCA, which can be Very RISKY!! And it can be very costly if many
measurements are required.
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Benefits From WCCA
Assure acceptable operation throughout the entire product life cycle under the
most unfavorable combination of anticipated conditions [Worst Case
Extreme Value Analysis (EVA)]
Define Critical Components and Spec. Control Drawing (SCD) Limits
Provide Acceptance Test Procedure (ATP) Limits
Define Need for and Range of Select-At-Test (SAT) components
Improve Reliability through Parts Stress and Derating analysis
Identify design concerns which during test, alignment, and use could result
in circuit damage or premature degradation.
WCCA helps ensure increased product reliability. We accomplish this through
rigorous mathematical and simulation-based models along with hardware correlation.
Correlated models are then used to determine part stress margins, and EOL/BOL product
operating specifications. A single over-stressed component can cost your company millions
of dollars. A thorough Worst Case Analysis can eliminate this from happening.
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Background & Definition
A Worst Case Circuit Analysis is a quantitative assessment of the equipment
performance, accounting for manufacturing, environmental and aging effects.
In addition to a circuit analysis, a WCCA often includes stress and derating
analysis, Failure Modes and Effects
Criticality (FMECA) and Reliability
Prediction (MTBF).
The specific objective is to verify
that the design is robust enough to provide operation which meets the
system performance specification over design life under worst case conditions
and tolerances (initial, aging, radiation, temperature, etc.).
The Stress and Derating Analysis is intended to increase reliability by providing
sufficient margin compared to the allowable stress limits. This reduces overstress
conditions that may induce failure, and reduces the rate of stress-induced parameter
change over life. It determines the maximum applied stress to each component in the
system.
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Methodology
A WCCA follows this general form:
Generate/Obtain circuit model
Obtain Correlation to validate model
Determine sensitivity to each component parameter
Determine component tolerances
Calculate the variance of each component parameter as sensitivity times
absolute tolerance
Use at least two methods of analysis (eg. hand analysis and SPICE or Saber, SPICE and
measured data) to assure the result
Generate a formal report to convey the information produced
The design is broken down into the appropriate functional sections.
A mathematical model of the circuit is developed and the effects of various
part/system tolerances (see below) are applied. The circuit's EVA and RSS
results are determined for Beginning-of-Life and End-of-Life states.
Two methods of analysis and/or hardware correlation are always used to
confirm results.
These results are used to calculate part stresses and are applied to other
analyses. In order for the WCCA to be useful throughout the product’s life cycle,
it is extremely important that the analysis be documented in a clear and concise
format. This will allow for future updates and review by other than
the original designer. A compliance matrix is generated that clearly
identifies the results and all issues.
Factors Addressed During WCCA
Analog Circuit Analysis
Some of the factors to be considered during analysis include:
Maximum line voltage variations and line transients
Maximum input and output variation
Maximum part parameter variation
Maximum performance demands and variations
Maximum and minimum environmental conditions
Fail-safe provisions
Redundancy provisions
Radiation effects, as applicable
Parameter drift due to aging
Transients due to turn-on, turn-off, and state changes
Fatigue due to cyclical loading and temperature cycling
Interface conditions between modules and modules to test equipment
Digital Circuit Analysis
Digital circuit worst case analyses involves one or more of the following:
Timing Margin Analysis
Transmission Line Effects
Noise Due to Crosstalk and Grounding
Meta-Stability Analysis
Decoupling Analysis
Fanout Analysis
Logic Compatibility/Interface Analysis
Supply Power Application and Sequencing Analysis.
State Machine Analysis
Unused/Tri-stated Input Analysis
No Connection Analysis
Test Point Current Limiter Analysis
Physical Layout Analysis
One-Shot Margin Analysis
Component Tolerances
Initial
Temperature and Environmental Factors
Radiation
Aging or End-of-Life Factors
A Partial List of AEi Systems Clients
Aeroflex
Allied Signal
Boeing Space
Brookhaven National Labs
Hamilton Standard
Hughes Space and Communications
International Rectifier
Interpoint
ITT Industries
Lockheed Martin Astronautics
Lockheed Sanders
Lucix
Omnirel
Raytheon
Sierra Microwave
Signal Technology
Smith-Nephew
Space Systems Loral
Starsys/SpaceDev
TRW Space and Technology
Watkins Johnson
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