MTBF, Service Life and Warranty: What Each One Promises

Reliability figures are among the most misread numbers in a power-supply datasheet, because three unrelated quantities all look like a lifetime.

MTBF is a failure rate, not a lifespan

Mean Time Between Failures describes the random failure rate during a product’s useful life, once early failures are past and before wear-out begins. A figure of 700,000 hours does not mean a unit runs for eighty years. It means that across a large population in the stated conditions, you would expect roughly one failure per 700,000 unit-hours of operation. Deploy a thousand units and you should plan for occasional failures from the first year.

Always check how it was calculated. Figures derived from different prediction standards, or quoted at different ambient temperatures, are not comparable — and MTBF falls sharply as temperature rises.

Service life is the wear-out limit

This is usually set by the electrolytic capacitors, and increasingly by the fan. Capacitor life roughly halves for every 10 °C rise in operating temperature, which is why the same supply may be quoted at five years in one condition and considerably longer in a cooler one. For equipment expected to run for a decade, service life matters far more than MTBF.

Warranty is a commercial term

• It reflects the manufacturer’s confidence and their commercial position — not a technical prediction.
• It is normally void outside the stated operating conditions, so derating protects your cover as well as your kit.

The practical takeaway is the same in all three cases: run the supply cooler and derated, and every number improves. Ask us for the reliability data on anything you are specifying.