# Learn (or review) the difference between MTBF and lifetime

## There has been confusion in understanding the difference between mean time between failures (MTBF) and lifetime. A product might have an MTBF of 500,000 hours, but a lifetime expectancy of 20,000 hours. So why is there such a large discrepancy? Puls LP says the answer is easy if you understand...

09/24/2008

St. Charles, IL – There has been confusion in understanding the difference between mean time between failures (MTBF) and lifetime. A product might have an MTBF of 500,000 hours, but a lifetime expectancy of 20,000 hours. So why is there such a large discrepancy? Puls LP says the answer is easy if you understand the difference between the terms, because one does not have anything to do with the other.

 Puls explains the difference between MTBF and lifetime.

MTBF represents the statistical approximation of how long a number of units should operate before a failure can be expected. It is expressed in hours and does not represent how long the unit will last. There are many ways of calculating MTBF. Use calculations based on models such as SN 29500, MIL HDBK-217 or Belcore; use field failures, or Field MTBF; or use laboratory testing, or demonstrated MTBF. For instance to test 10,000 units for 1000 hours with 10 failures, the MTBF would be 1 million hours. This does not suggest the unit will operate for 114 years. A better representation would be if 500 units operate at the same time, a failure could be expected every 2,000 hours, or 83 days.
Unlike the hours from the MTBF calculations, lifetime indicates operating hours expected under normal operating conditions. It is the period of time between starting to use the device and the beginning of the wear-out phase. This is determined by the life expectancy of components used in assembly of the unit. The weakest component with the shortest life expectancy determines the life of the whole product. For power supplies, electrolytic capacitors have the shortest lifetime expectancy.
To understand MTBF versus lifetime, think of a product going through three phases over its lifetime. In the first, the failure rate is high and is referred to as the “infant mortality” phase. In the second, the failure rate is low and fairly constant. In the third, the failure rate begins to increase and is called the “wear out” phase. The complete graph is the “bathtub curve” because it looks like one. MTBF is a way of determining how many spare parts you might need to support 500 units, but a poor guide on when those parts should be changed. A unit that operates eight hours a day will last three times longer than a device operating around the clock. However, MTBF is the same because both units receive the same number of hours in service.
Many factors determine reliability. Low failure rate and long life are two. A good quality process control and a high degree of automation during production can lower the defect rate and improve reliability. A rugged design using high-quality components can improve reliability system-wide. Environmental conditions such as vibration and temperature can play a major role in defect rate and reliability. For power supplies, heat is the enemy and can shorten the life of electrolytic capacitors dramatically. The industry rule states that every 10 °C increase in temperature reduces the life of the capacitor by half.
As that relates to products, for instance, Puls uses large-diameter, high-quality capacitors, allowing the Dimension series to have a rated life of at least 50,000 hours. The older Puls SilverLine Series use capacitors rated with a longer life than competitors with current product, the company says.

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