Worst Case Analysis (WCA) is performed on a newly developed circuit to attempt to reveal  circuit weaknesses that could contribute to failure. Failure modes are determined, and then  each part parameter is varied in a max. or min. direction that would contribute the most to  circuit failure or out-of-spec condition. Some parts of the circuit might contribute little,  but usually most do, in some way. WCA is a statistical survey of circuit operation. Analysis  software is available, but other than massive number crunching, it can be done manually.

 How done?

 First, circuit failure or out-of-spec conditions are determined, using the given design spec  and/or system spec.

 Next, circuits in the direct path to the result are outlined in red on the schematic.

 Then each part type is given a list of parameters important to operation of the circuit. A  data sheet is originated made up of the part parameters. Using the given temperature range of  the product, any external stresses (i.e. radiation effects) and the manufacturer's data sheet,  the worst case values for each parameter are enumerated. This can be an extensive process:  included are manufacturing tolerance; end-of-life tolerance (if desired); thermal variations;  max./min. loading; and max./min. input voltages and currents.

 Each part of the circuit is mathematically described, with resistance values, transistor hfe,  etc., plugged into the equations. The equations can get long and complex, and are best placed  in software such as Mathcad, where results are instant. Usually a "nominal" case is written to  get a feel for accuracy and proper equations. Then max./min. values are inserted into the  equations to produce the desired results.

 When complete, the analyst observes whether the circuit produces failure or out-of-spec  conditions. It may not, which is fine. If it does, then changes are in order. The analyst can  suggest the changes to the circuit designer.

 Value of all this effort

 It is far better to know in-house that there could be problems with a product rather than  waiting for your customers to find out for you. In space applications more so, because repairs  are not possible in a satellite, for example. It is true that WCA could be too critical of a  circuit, and maybe a better way would be to use a Monte Carlo analysis of variations. But in  most companies, time restricts most effort and the simplest approach is called for. If a  circuit passes the WCA, it should be successful on the market. Also, circuit weaknesses can be  designed out at the beginning by a thoughtful designer, so when a WCA is performed, changes  would be minimal to non-existent. Wouldn't it be nice!