1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2007-10-30a71 Center Point of Contact: JPLa71 Submitted by: David OberhettingerSubject: Design and Analysis of Electronic Circuits for Worst Case Environments and Part Variations Practice: Design all circuits to perform within def
2、ined tolerance limits over a given mission lifetime while experiencing the worst possible variations of electronic piece parts and environments.Abstract: Preferred Practice for Design & Test. Worst Case Analysis (WCA) is a uniform, disciplined, systematic approach to circuit analysis that determines
3、 whether each circuit and each assembly meets its specified electrical performance attributes over the most extreme but realizable combinations of part variation sources. The lack of a structured WCA activity jeopardizes the integrity of the initial circuit design over the project life cycle.Program
4、s that Certify Usage: Not applicable: this document is not a lesson learned, but rather one of the Reliability Preferred Practices that were moved to the LLIS. Since the narrative is not based upon an event, there is no issue of a “recurrence“ to control.Center to Contact for Information: JPLImpleme
5、ntation Method: This lesson learned is based on Reliability Practice number PD-ED-1212, from NASA Technical Memorandum 4322A, Reliability Preferred Practices for Design and Test. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Benefit: The practice o
6、f designing and analyzing electronic circuits for worst case environments and part variations maximizes the probability of mission success by assuring that all assemblies meet their mission electrical performance requirements at all times. Implementation Method: Worst Case Analysis (WCA) evaluates f
7、actors that may cause hardware to degrade or perform out-of-specification based upon a variety of input factors. For electronic circuits, the analysis generally includes the effects of temperature, initial tolerances, aging, radiation, and other effects as appropriate. The analyst derives part param
8、eter variations for the environments and life of a specific mission and combines them with the initial tolerances of the parts as procured to produce a worst case part variation database for each mission or project. Applying classical circuit analysis techniques, the analyst can determine if each ci
9、rcuit and each assembly meets its specified performance attributes over the most extreme but realizable combinations of part variation sources. Technical Rationale: Classical reliability practice is generally associated with minimizing catastrophic failures of parts. Of equal importance, however, is
10、 assuring that the desired essential mission controls and scientific measurements are made with the intended accuracy, fidelity, and stability. To this end, a uniform, disciplined, systematic approach to performance design verification is essential. Uniformity is achieved by use of a common part var
11、iation database by all analysts on a specific project. Discipline is achieved by a common analysis containing qualitative and quantitative circuit performance attributes which are traceable to the assembly, subsystem, and system requirements. Also required is a stated or implied level of statistical
12、 confidence which results from the use of either an EVA (Extreme Value Analysis) or an RSS (Root Sum Squared) approach to the circuit performance variation at some statistical level, usually 2 or 3. Another statistical approach is the Monte Carlo (MC) method of repeated trials with randomly selected
13、 combinations of part variations. Table 1 compares the relative merits of these three approaches and summarizes the differences in input and output formats and information content. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Table of Comparison o
14、f Advantages/DisadvantegesIt is generally recommended that the EVA be applied as the required method for non-serviceable hardware because of its extreme conservatism. The RSS and MC methods are considered approximately equal to each other if utilized at the same statistical level. Both methods can b
15、e safely employed for serviceable or recalibrateable equipment, but are considered the lowest level of confidence allowable for non serviceable (i.e. satellites, spacecraft, etc.) hardware. Both the RSS and MC methods should be accepted only as a formal waiver to the EVA process. Any circuit which d
16、oes not meet its attributes at 3 extremes cannot be considered high reliability in the functional sense. To achieve the project benefits from performing a WCA, the commitment must be mission wide to prevent any “weak links“ in the performance chain. For critical circuitry, preliminary analyses may b
17、e required to validate a conceptual design approach at Preliminary Design Review (PDR). For maximum benefit, the WCA is typically conducted concurrently with detailed design and completed Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-in advance of,
18、 and in support of, the Critical Design Review (CDR). Impact of Nonpractice: Early designs of electronic circuits were either empirical or considered initial part tolerances only. This proved to be inadequate in that equipment often failed to remain within specifications at extreme temperatures or o
19、ver prolonged life. More disciplined approaches followed, but analysts functioning independently were inconsistent in their part variation assumptions. The number of test and field failures fell, but still remained intolerably high, and large systems suffered from inconsistent risk levels. Since WCA
20、 first came into practice around 1965, its systematic use by a competent circuit designer has been found to increase the probability of the output design passing all WCA criteria. The absence of a structured WCA activity jeopardizes the long term integrity of the initial design. The probability of f
21、ailing a hardware qualification test and subsequently needing design modifications is greatly increased. Using inherited hardware designs in environments which are different from the original adds an additional degree of jeopardy if there is no documented WCA from which to extrapolate. Reference: 1.
22、 JPL Publication D 5703, “Reliability Analysis Handbook“, July 1990. Impact of Non-Practice: The absence of a structured WCA activity jeopardizes the long term integrity of the initial design. The probability of failing a hardware qualification test and of subsequently needing design modifications i
23、s greatly increased.Related Practices: N/A Additional Info: a71 Project: variousProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2007-10-30a71 Approval Name: ghendersona71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
