The Circuit Designer's Companion - diagramas.diagram...

308 The Circuit Designer’s CompanionMean time to failureMTBF measures equipment reliability on the assumption that it is repaired on eachfailure and put back into service. For components which are not repairable, theirreliability is quoted as mean time to failure (MTTF). This can be calculated statisticallyby observing a sample from a batch of components and recording each one’s workinglife, a procedure known as life testing. The MTTF for this batch is then given by themean of the lifetimes.AvailabilitySystem users need to know for what proportion of time their system will be availableto them. This figure is given by the ratio of “up-time”, during which the system isswitched on and working, to total operating time. The difference between the two is the“down-time” during which the system is faulty and/or under repair. ThusA = [U/(U + D)]Availability can also be related to the MTBF figure and the mean time to repair(MTTR) figure byA = [MTBF/(MTBF + MTTR)]The availability of a particular system can be monitored by logging its operatingdata, and this can be used to validate calculated MTBF and MTTR figures. It can alsobe interpreted as a probability that at any given instant the system will be found to beworking.9.4.2 The cost of reliabilityReliability does not come for free. Design and development costs escalate as moreeffort is put into assuring it, and component costs increase if high performance isrequired of them. For instance, it would be quite possible to improve the reliability of,say, an audio power amplifier by using massively over-rated output transistors, butthese would add considerably to the selling cost of the amplifier. On the other hand, ifthe selling cost were reduced by specifying under-rated transistors, the users would findtheir total operating costs mounting since the output transistors would have to bereplaced more frequently. Thus there is a general trend of decreasing operating or “lifecycle”costs and increasing unit costs, as the designed-in reliability of a given systemincreases. This leads to the notion of an “optimum” reliability figure in terms of cost fora system. Figure 9.8 illustrates this trend. The criterion of good design is then toapproach this optimum as closely as possible.Of course, this argument only applies when the cost of unreliability is measured instrictly economic terms. Safety-critical systems, such as nuclear or chemical processplant controllers, railway signalling or flight-critical avionics, must instead meet adefined reliability standard and the design criterion then becomes one of assuring thislevel of reliability, with cost being a secondary factor.9.4.3 Design for reliabilityThe goal of any circuit designer is to reduce the failure rate of their design to theminimum achievable within cost constraints. The factors which help in meeting thisgoal are• use effective thermal management to minimise temperature rise