MTBF and MTTF: Fundamentals for Power Supply Selection

When selecting power supplies, you’ll come across two fundamental parameters: mean time between failures (MTBF) and mean time to failure (MTTF). Although these different but related terms typically appear in the reliability section of a power supply manufacturer’s datasheet, they should be used judiciously.

For instance, MTBF is often misunderstood as being an indicator of how long a power supply will last. However, this metric is actually based on consecutive failures, derived from field data, over the device’s functional lifetime. It is also dependent on the failure rates of the supply’s internal components and environmental stressors. MTBF is useful for inferring the supply’s overall reliability, not as a predictor of a device’s lifetime.

Individual manufacturers present MTBF and MTTF figures differently on their datasheets and predicate that information using various standards and test methodologies. When determining whether a power supply will perform reliably in its intended application, buyers should have a basic understanding of MTBF and MTTF and the tests vendors use to establish these metrics. Here is a quick overview of the fundamentals to consider when making a power supply selection:

• Mean-time-between-failures: A statistical average of the amount of time between failures for a device in the field. Prediction guides exist to help power supply manufacturers calculate MTBF. MIL-HDBK-217F and Telcordia SR/TR-322 (Bellcore) are the most accepted guides among those summarized below:

• MIL-HDBK-217F: The Reliability Prediction of Electronic Equipment in the U.S. Military Handbook. MIL-HDBK-217F — also common in commercial areas — provides failure rate and stress factors for components used in electronic systems as well as application-specific stresses.

• Telcordia SR/TR-332 (Bellcore): Bellcore took MIL-HDBK-217 and modified it for commercial applications, emphasizing parts count, lab test, field test and burn-in test data to predict reliability.

• IEC 61709:2017: This guide emphasizes environmental factors to forecast reliability.

• 217Plus: 2015: Based on MIL-HDBK-217, Quanterion Solutions developed the methodology using “enhanced approaches to account for environments, for quality, and for cycling effects on reliability”* for government and industry.

• Others: 299C (Chinese standard), RCR-9102 (Japanese standard).

These guides and methodologies place different emphases on various stress and environmental factors, so be sure to ask the power supply manufacturer how it calculates MTBF. Knowing which prediction method was used can influence your confidence in a supply’s MTBF figure.

• Mean-time-to-failure: An average amount of time that the device is expected to perform in the field. It applies to non-repairable devices, so consider the power supply’s end product. If you expect it to have a short service life or operate a limited amount of times before replacement, MTTF may be a useful reference. It may also be suitable for critical applications in which failure is not an option.

Anyone interested in power supply reliability should have an understanding of MTBF and MTTF. Polytron Devices publishes MTBF information in its datasheets (under “Physical Specifications”) as well as product pages on www.polytrondevices.com. Since military testing is more stringent, we typically base our MTBF figures on MIL-HDBK-217F, but other MIL standards or guides may be used depending on the product. The datasheet may also include the test conditions such as ambient temperature and whether the device was tested under full load. Our technical staff can answer any concerns you may have about how our power products are tested for reliability.

About Internal Components

Engineers should also ask the power supply vendor for reliability information pertinent to the unit’s internal components. Note the electrolytic capacitors because they are often the first internal component to fail. Finally, keep the anticipated thermal conditions in mind, too.

When selecting your power supply, consider MTBF or MTTF an initial clue to its reliability. Find out how the manufacturer calculated the statistic, based its prediction methodology, and tested the unit and under what conditions. Also, learn as much as you can about the reliability of the internal components. The more information you obtain about these and other factors, the better you’ll be able to decide which power supply suits your application.

To learn more, get in touch with our technical staff.