Single Event Latch-Up 4 New Space : SEL 4 NS

The Galactic Cosmic Rays environment is characterized by an Integral LET ( Linear Energy Transfert) Spectrum ( see figure on the right). Destructive Single Event (SEL, SEB, ..) sensitivity is determined by two parameters: LETthreshold in MeV.cm²/mg, and Saturated SEL Cross Section (XSsat) in cm². Based on published data, XSsat is between 1E-7 and 1E-4 cm².device. Here, we will consider a 1E-6 cm² device XSsat part. Three LETth values are analyzed:

Case 1 : LETth < 15 MeV.cm²/mg . It is strongly recommended to avoid using these parts, due to SEL sensitivity to protons

Case 2 : 15 MeV.cm²/mg < LETth < 30 MeV.cm²/mg . May Be used depending on SEL failure rate calculated.

Case 3 : LETth > 60 MeV.cm²/mg . Use it !

SEL Failure Rates are calculated for this 3 remarkable LETthreshold:

SEL LET_threshold = 15 MeV.cm²/mg  / LEO-MEO-GEO

SEL LET_threshold = 15 MeV.cm²/mg / LEO

SEL LET_threshold = 30 MeV.cm²/mg  / LEO-MEO-GEO

SEL LET_threshold = 30 MeV.cm²/mg &  / LEO-MEO-GEO

SEL LET_threshold = 60 MeV.cm²/mg  / LEO-MEO-GEO

SEL LET_threshold = 60 MeV.cm²/mg  / LEO

FIT DEFINITION :

The term FIT (failure in time) is defined as a failure rate of 1 per billion hours. A component having a failure rate of 1 FIT is equivalent to having an MTBF of 1 billion hours. Most components have failure rates measured in 100's and 1000's of FITs. For components, such as transistors and ICs, the manufacturer will test a large lot over a period of time to determine the failure rate. If 1000 components are tested for 1000 hours, then that is considered to be equivalent to 1,000,000 hours of test time. There are standard formulas that convert the number of failures in a given test time to MTBF for a selected confidence level. For a system of components, one method of predicting the MTBF is to add the failure rates of each component and then taking the reciprocal. For example, if one component has a failure rate of 100 FITs, another 200 FITs and another 300 FITs, then the total failure rate is 600 FITs and the MTBF is 1.67 million hours. For military systems, the failure rates of each component can be found in MIL-HDBK-217. This document includes formulas to account for environmental and usage conditions such as temperature, shock, fixed or mobile equipment, etc. In initial stages of a design, these calculations are useful in determining the overall reliability of a design(to compare with the specified requirement) and which components are most significant in terms of the system reliability so that design changes can be made if deemed necessary.