Analytically Unobservable Failure Events

Based on computational approximations, product designers routinely simplify algebraic expressions of complex transfer functions, which, in turn, simplify post-design analysis. This practice is justified because both the simplified and actual transfer functions yield approximately equal average performance values. However, utilizing the design of a switching regulator circuit, it is shown that such simplified expressions underestimate output variation. The discrepancy between the actual and underestimated output variation spans an analytically unobservable failure event. Since the observable portion constitutes a partial characterization of product unreliability and quality problems, only a partial solution is possible at the design stage. This severely limits the effectiveness of concurrent product design, reliability, and quality assurance methodologies that are based on variation modeling and analysis.