NEWS // MOUSER NEWS // FEATURES // NEW PRODUCTS
How to Improve Power Supply MTBF and Reliability
P
ower supply reliability is important – no one wants their production line, measurement instrument, communications system or electronic product to stop working prematurely due to a failure. How is reliability
defined, what methods can designers use to improve reliability and how do you ensure that a selected product will meet expectations?
Defining MTBF and MTTF The MTBF (Mean Time Between Failures) is a parameter that is widely used for determining the reliability of a power supply, but it is also often misunderstood and misused as a determining factor. A manufacturer’s datasheet quoting an MTBF figure of 300,000 hours does not indicate that the power supply will last that long before failing. Operating 24 hours a day, 365 days represents 8,760 hours operating time in a year. 300,000 hours is over 34 years and very few pieces of equipment installed in 1982 are still operating reliably.
MTBF is the time elapsed between two successive
failures, and is a substantially different concept to MTTF (Mean Time To Failure). MTTF is a statistical parameter referring to the time elapsed from the start of operation and the first failure. It assumes that the mathematical model of reliability is the exponential function, which would seem sensible following real-world examples. The MTTF value represents the time where 63% of the samples under test will have suffered their first failure. This is a number often used to estimate the number of units needed to be stocked for replacements. As this sounds
a more reasonable method of determining how long a power supply will operate, why then is the MTBF number referred to so frequently? To explain
January 2017
this, one needs to examine figure 1, representing the pattern of the “failure rate” over a period of time. The bathtub curve represents the typical development
of failures over time. There are three distinct zones: Infant mortality; Constant (random) failure; and Wear-out failure. In the constant failure rate zone, failures occur in a random order – making it difficult to predict which failure mode will occur - but the rate of failures is predictable. It is in this zone where the mathematical models for MTBF and MTTF converge, a reason why the MTBF number is more widely considered. Unfortunately, this does not assume that a power supply will be reliable. A power supply with an MTBF of 80,000 hours (approximately 9 years) does not mean that it will last for an average of 80,000 hours. When two units are considered, the nine years becomes 9/2 = 4.5 years, or 9/4 = 2.25 years for four units. When life testing is conducted, the probability of a
failure can be calculated. If we test 100 power supplies for one year (8,760 hrs) and during this time ten fail. The estimate of MTBF is the total number of device hours (876,000) divided by the number of failures, which is 87,600 hours / failure. The probability that a power supply will survive to
its calculated MTBF is 36.8%, no matter what the MTBF number is. R(t) = e –t / MTBF = e-1 (when t = MTBF). A system made up of “n” separate parts is defined
statistically as a series system; this is common practice for a power supply that is made up of multiple components. The system is defined as functioning when all the parts are working, and will be in a state of failure when just one single part turns out to be defective. Each individual
Figure 1: The typical bathtub curve.
www.electronicscomponentsworld.com /
www.electronicsproductionworld.com
part making up the system is independent of the others, whether for reliability or for its
EIU 21
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30
