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DIABLO MOUNTAIN RESEARCH, LLC
Walnut Creek, CA 94597
925.788.4807
Worst Case Circuit Analysis (WCCA) White Paper
Worst case circuit analysis (WCCA) is a technique which, by accounting for component
variability, determines circuit performance under a worst case scenario, i.e., under extreme
environmental or operating conditions. Environmental conditions are defined as external
stresses applied to each circuit component, and can include temperature, humidity or
radiation. Operating conditions include external electrical inputs, but must also consider
factors such as component quality level, interaction between parts, and drift due to
component aging. The output of a WCCA allows an assessment of actual applied part
stresses against rated part parameters. This can help ensure sufficient part stress derating
to meet design requirements. WCCA should be considered for all circuitry that is safety
and/or financially critical.
Performance of a WCCA, and implementation of its results, can help identify design
problems and alternatives that can reduce financial, legal and safety risks to the
manufacturer, and help ensure satisfactory performance for the customer under virtually all
operating conditions. The advantages and disadvantages of the three major WCCA methods
are presented in Table 1. A capacitor example is shown in Table 2. The WCCA process is
outlined in Table 3.
One of the most critical steps involved in completing a meaningful WCCA is the development
of a part characteristic database. This database contains a composite of information
necessary for quantifying sources of component parameter variation. Once these sources
have been identified, the database can be used to calculate worst case component drift for
critical parameters. Quantifying the contribution of environmental effects on component
variability (as will be illustrated in an example) is also a critical step in the development of a
WCCA. A number of starting places can be used to establish random and biased
contributions to variability. They maybe summarized as:
• Company data (historical test data from other products, or special test programs)
• Vendor data (documentation of test conditions, sample size, number of lots, etc.,
is needed)
• Military specifications (tend to be very conservative)
• Outside sources (e.g., Jet Propulsion Lab for radiation data)
Actual field performance indicates that components tend to drift beyond initial tolerance
levels. The magnitude of component tolerance variation is dependent on a variety of
sources, as illustrated in Table 4. When conducting a worst case circuit performance
analysis, the key elements to be examined within the system are dependent upon the
intended function of the circuit. Critical timing of digital circuits, transfer functions of
filtering networks, and characteristics of amplifiers are examples of circuit performance
elements. Table 5 describes those parameters which should be analyzed in a worst case
performance analysis for digital and analog circuits.