ReliabilityDemonstrationMethods
Whendemonstratingthereliabilityofaproduct,theexactfailuretimecanneverbeknow.Notwithstandingthisfact,wearerequiredtohypothesizeregardingourconfidenceintheintegrityofournumbers.TheChi2distributionisoftenusedinthiscasebecauseitisaprobabilitydistributionthatrelatestheobservedtotheexpectedvalues.
TherelationshipbetweenfailurerateandtheChi2distributionisasfollows:
lamda=X^2(Alpha,Bate)/2*time*Af
Where
alpha=((100–CL)/100CL(ConfidenceLevel)=60%,90%.
Bate=Degreesoffreedom=2*numberoffailures+2
Af=Accelerationfactor,asdeterminedbyoneofthemethodsdiscussedearlier
Whenestimatingfailurerates,Vicorusesaccelerationfactorstodetermineusefailure
ratesbasedonlabfailurerates.
AcceleratedLifeTesting
Theequationbelowisusedtomodelaccelerationduetotemperatureandisreferredtoas
theArrheniusequation.TheArrheniusequationrelateshowincreasedtemperature
acceleratestheageofaproductascomparedtoitsnormaloperatingtemperature.
Af=e^{(Ea/k)*(1/Tu-1/Tt)}
Af=accelerationfactor
Ea=activationenergyinelectron-volts(eV)
k=Boltzmann’sconstant(k=8.617x10-5eV/Tk)
Tk=Kelvin
Tu=referencejunctiontemperature,indegreesKelvin(K=C+273)
Tt=junctiontemperatureduringtest,indegreesKelvin
e=2.71828(baseofthenaturallogarithms)
MTBF=1/lamda |