One dependent champaign that confuses specifiers of mightiness supplies is comparison MTBF numbers from dissimilar manufacturers, who oftentimes occupation dissimilar standards for calculating the release of hours. There are many good written articles going inward to dandy exceptional available on the cyberspace on the calculation of MTBF, but this weblog article volition endeavour to simplify things.
MTBF (Mean Time Between Failures) is the hateful fourth dimension betwixt successive failures, too alone actually applies to a purpose that volition live repaired too returned to service. So if the up-time of the mightiness render was a yr inward each instance below, too therefore the MTBF would live ½ x (1 yr + 1 year).
A depression terms mightiness render volition belike non live repaired too if it is nether warranty, it volition usually live replaced. In this case, the numbers to aspect for would live MTTF (Mean Time To Failure), but that figure is non widely stated. Usually life testing of a large release (to cutting the testify fourth dimension down) of mightiness supplies is used to calculate that.
The MTBF release is oftentimes idea to live the minimum (guaranteed) fourth dimension earlier a failure; that is sure enough non the case! Reliability “R” is based on the probability that a slice of equipment, inward our instance a mightiness supply, volition piece of work satisfactorily for a given fourth dimension catamenia “t” (based on specified weather – for instance ambient temperature too output load).
For random failures, the probability that a mightiness render volition endure to its calculated MTBF is simply 37%, no thing what the MTBF release is:
R(t) = e –t / MTBF = e-1 = 0.368 (when t = the MTBF number)
To complicate things further, a multifariousness of methods are used to calculate MTBF.
Prediction Standard | Applications | Disadvantages |
MIL-HDBK-217F | Provides failure charge per unit of measurement information too stress models for parts count too parts stress predictions. It besides provides models for many environments ranging from solid soil benign to projectile launch | Hasn’t been updated since 1995, gives higher failure rates of commercial parts than is seen inward actual production life |
Telcordia SR332 | Gives iii prediction methods based on parts count, lab testing too champaign life | Narrow ambient temperature range |
RCR-9102 | Produced past times JEITA - Nihon Electronics too Information Technology Industries Association. In each update constituent failure rates (FIT) accept been changed, peculiarly fans | Issued inward 1994, based on MIL-HDBK-217F |
RCR-9102A | Issued inward 2000, based on MIL-HDBK-217F (Notice 2) Includes SMT parts & pcbs | |
RCR-9102B | Issued inward 2006 |
Usually for commercial mightiness supplies, the figures are calculated at 25oC, solid soil benign or fixed
Taking TDK-Lambda’s RWS150-B serial every bit an example, the calculated numbers are every bit follows:
RCR-9102 444,089 hours
RCR-9102B 218,172 hours
Telcordia 2,235,743 hours Ta=25℃
Telcordia 1,063,230 hours Ta=40℃
It tin live seen from the inward a higher house numbers, that in that location is a 10-fold divergence betwixt RCR-9102B too Telcordia, too to a greater extent than than a two flexure divergence betwixt RCR-9102 and RCR-9102B. Several customers accept asked why our newer products calculated using the JEITA method appeared to live less reliable than older products, but did non know the meaning touching on of the updated, harsher standard.
Engineers should live to a greater extent than concerned most electrolytic capacitor too fan life (if used) every bit these are the typical failure modes. Many manufacturers are showing expected capacitor lifetimes inward their reliability reports. Below are the plots for the RWS150B, which was designed for long capacitor life. As a note, to a greater extent than or less manufacturers exhibit like plots, but acre inward modest impress that the convection cooled mightiness supplies had external forced air cooling applied.
Nice post..
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