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12/4/2012Digital System ReliabilityFaculty: Anoop Mathew, HoD ECECourse: ES 10 105 A /VL 11 105 A - Electronic System design

IntroductionReliability of a digital system is the likelihood that it works correctly when you need it. Mathematically, Reliability is a real number between 0 and 1; that is, at any time 0 R(t) 1.

Typical reliability function

Reliability

N identical copies of the system. Let WN(t) denote the number of them that would still be working at time t.

Reliability of new components may be estimated or extrapolated from experience with something similar. A components reliability is typically described by a single number, the failure rate.

Parameters associated with ReliabilityMean Time between Failures (MTBF)Mean Time to Failure (MTTF)Availability

Mean Time between FailuresIn the early stages of operation failures could be highFailures decrease as weak components fail quickly and are replacedLate in its life, components may begin to wear out or corrosion may take its toll, and the failure rate may start to rise again.The reciprocal of failure rate during the constant period is known as the mean time between failures (MTBF). MTBF is generally quoted in hours, while failure rate is quoted in faults per hour.

Mean Time to Failure (MTTF)MTBF measures equipment reliability on the assumption that it is repaired on each failure and put back into service.For components which are not repairable, their reliability is quoted as mean time to failure (MTTF).MTTF can be calculated statistically by observing a sample from a batch of components and recording each ones working life, this procedure known as life testing. The MTTF for this batch is then given by the mean of the lifetimes.

Availability (A)Availability is given by the ratio of up-time, (U) during which the system is switched on and working, to total operating time. A = U/(U + D) A =MTBF/(MTBF x MTTR)Availability can also be interpreted as a probability that at any given instant the system will be found to be working.

Reliability Vs Cost

Design for ReliabilityThe goal of any circuit designer is to reduce the failure rate of their design to the minimum achievable within cost constraints.

The factors that help to meet DFRuse effective thermal management to minimize temperature rise;de-rate susceptible components as far as possible;specify high reliability or quality assured components;specify stress screening or burn-in tests;keep circuits simple, use the minimum number of components; anduse redundancy techniques at the component level.

TemperatureHigh temperature is the biggest enemy of all electronic components and measures to keep it down are vital. Temperature rise accelerates component breakdown because chemical reactions occurring within the component.The rate of reaction is determined by the Arrhenius equation, = K.e(-E/kT) where, - a measure of failure rateK - a constant depending on the component type.E - reactions activation energyk - Boltzmanns constantT - Absolute temperature

De-RatingThere is a very significant improvement to be gained by operating a component well within its nominal rating. Capacitor life tests show that as the maximum working voltage is approached, the failure rate increases as the fifth power of the voltage. Semiconductor devices are normally rated for power, current, voltage and de-rating on all of these will improve failure rate.

Usage of High Reliability ComponentsIt is worth spending extra to specify and purchase parts with a guaranteed reliability specification at the front end of production.

Stress Screening and Burn-inThis phrase refers to testing the components under some type of stress, typically at elevated temperature, under vibration or humidity and with maximum rated voltage applied, for a given period. This practice is also called burning-in. Another common test is a repeated temperature cycle between the extremes of the permitted temperature range, which exposes failures due to poor bonding or other mechanical faults.

SimplicityThe failure rate of an electronic assembly is roughly equal to the sum of the failure rates of all its components.Reducing the number of components will reduce the overall failure rate.

RedundancyRedundancy is employed at the system level by connecting the outputs of two or more subsystems together such that if one fails, the others will continue to keep the system working. A typical example might be several power supplies, each connected to the same power distribution rail (via isolating diodes) and each capable of supplying the full load.

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Thank YouQueries ???Mail to: anoopmathew@ieee.org