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Reliability Software, Safety and Quality Solutions / Sneak Circuit Analysis / Methodology and Tool

Methodology and Tool

Conventional SCA Techniques

The original SCA techniques depended on recognition of circuit patterns or "clues" for the detection of potential sneak circuits.  The most common of these circuit patterns are the H-Pattern, Y-Pattern and Inverted-Y:
 

 


The box symbols represent arbitrary circuit elements; in many cases the individual legs of the patterns include switches. (The CARGO OPEN switch is the middle horizontal leg of an H-pattern).  The inverted Y is also called a ground dome; note that the two bottom legs terminate in different ground levels, such as chassis ground and signal ground.  The Y-pattern is also called a power dome. The two upper legs terminate at different power sources, such as V1 and V2.

To facilitate the recognition of these patterns or clues, the schematic diagrams were redrawn as "network trees", with power sources at the top and grounds at the bottom. In sneak circuit analysis both positive and negative sources will be shown at the top of the figure.  Despite the aid of computers, SCA remained a very expensive and lengthy activity, and it is usually conducted only after the circuit design was frozen to avoid having to repeat it after changes.  However, at this point usually the circuit board or cabling are already in production and it becomes very expensive to fix.  In order to reduce the effort involved in SCA and thus enable its use earlier in the design SoHaR developed (within a USAF Research Laboratory contract) as technique that would permit SCA to be conducted as part of the design activity. The technique is based on a "bi-path" methodology which focuses on bi-directional paths rather than particular topologies. The technique reduces the effort by an order of magnitude and has allowed for the development of our automated tool, SCAT.

 

The SCAT Approach to Sneak Circuit Analysis

SCAT is an automated Sneak Circuit Analysis Tool developed by SoHaR for the Air Force Research Laboratory and currently marketed to the entire industry as a SCA tool. SCAT differs from conventional SCA (Sneak Circuit Analysis) techniques in that the latter are based upon the generation (usually automated) and analysis (mostly manual) of network trees to identify sneak paths. In contrast, SCAT does not require or even make use of the traditional network trees.

The automated procedure provides the design engineer or reliability analyst with a simple yet powerful tool for rapidly identifying and correcting sneak paths. The automated procedure is based on the fact that sneak paths involve circuit components which can conduct current in either direction depending upon the switching state of the circuit. SCAT exhaustively searches for these bidirectional branches, which is more readily automated than searching for specific topological circuit patterns, as done by conventional SCA techniques. Furthermore, the analyst's task is reduced to evaluating the significance of specific potential sneak paths rather than applying "clue lists" to circuit patterns for identifying the sneak paths.

A significant issue that arises is the assurance that sneak paths associated with interconnecting assemblies or subsystem interfaces are not overlooked. This issue is addressed in two ways. First, the system compels the user to identify each interface port of a switching circuit in terms of it being a power input. Interfaces to power and ground are labeled as such regardless of whether they respectively go to power and ground directly or through switched or un-switched loads, and they are included within the sneak path search. SCAT was developed to enable design engineers to perform SCA early in the project life so that problems can be corrected early and at minimal cost.

 
 
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