1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-09a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Electrical, Electronic and Electromechanical (EEE) Parts Derating Practice: Practice: Derate applied stress levels for electrical, electronic, and el
2、ectromechanical (EEE) part characteristics and parameters with respect to the maximum stress level ratings of the part. The allowed stress levels are established as the maximum levels in circuit applications.Programs that Certify Usage: This practice has been used on all Goddard Space Flight Center
3、(GSFC) flight programs.Center to Contact for Information: GSFCImplementation Method: This Lessons Learned is based on Reliability Practice No. PD-ED-1201; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.EEE parts derating can be established as either de
4、sign policies or from reliability requirements. In general, NASA has taken the approach of establishing derating policies that cover all applications of the various part types in space flight equipment. These policies are available in MIL-STD-975, “NASA Standard Parts List.“ Table 1 provides typical
5、 derating factors from that document. If derating is to be determined from a reliability requirement, the reference document is MIL-HDBK- 217, “Reliability Prediction of Electronic Equipment.“ MIL-HDBK-217 contains the information necessary to quantitatively estimate the effects of stress levels on
6、reliability.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Table 1. Typical Part Derating Guidelines PART TYPE RECOMMENDED DERATING LEVELCapacitors Max. of 60% of rated voltageResistors Max. of 60% of rated powerSemiconductor DevicesMax. of 50% of r
7、ated power Max. of 75% of rated voltage Max. junction temperature of 110CMicrocircuitsMax. supply voltage of 80% of rated voltage Max. of 75% of rated power Max. junction temperature of 100Inductive DevicesMax. of 50% of rated voltage Max. of 60% of rated temperatureRelays and Connectors Max. of 50%
8、 of rated currentNOTE: Maximum junction temperature levels should not be exceeded at any time or during any ground, test, or flight exposure. Thermal design characteristics should preclude exceeding the stated temperature levels.Technical Rationale:The reliability of a EEE part is directly related t
9、o the stresses caused by the application, including both the environment and the circuit operation. MIL-HDBK-217 contains specific part failure rate models for a wide variety of part types. The models include factors for calculating the effects of various stresses on the failure rate and thus on par
10、t reliability. The types of factors include (for example): environment, quality levels, voltage, frequency, and temperature. Given the extensive tables of factors in MIL-HDBK-217, one can formulate reliability predictions for piece parts.As shown in Figure 1, the plot of piece part failures versus a
11、n application stress level such as temperature, voltage, or current indicates decreasing failure rates for lower levels of stress. Therefore, a parts reliability in an application can be increased by decreasing the maximum allowed stress levels from the absolute maximum for which a part is rated.Pro
12、vided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Impact of Non-Practice: Derating lowers the probability of failures occurring during assembly, test, and flight. Decreasing mechanical, thermal, and electrical stresses lowers the p
13、ossibility of degradation or catastrophic failure. Derating policy documents such as those prepared by NASA and DoD, and generally required in their contracts, allow the designer to avoid lengthy and involved calculations by mandating the derating of specific characteristics and parameters.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-03-09a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
