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MTBF and MTTF: Fundamentals for Power Supply Selection

When evaluating power supplies, you’ll come across two important 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:

  • 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.

For more information about Polytron power supplies, visit www.polytrondevices.com

Power Supply Showcase Covers Industrial, Medical and Railway Products


Polytron Devices just published its 2018-2019 Power Supply Showcase. This informative reference provides design engineers with an overview of Polytron DC-DC and AC-DC converters and power supplies for industrial, medical and railway environments. Updated with new products and information, the showcase offers detailed descriptions and specifications like inputs, efficiencies, dimensions and packaging along with regulatory approvals and compliance data. It also divides the products by application area and product type for at-a-glance scanning. You can download Polytron 2018-2019 Power Supply Showcase here, or call 973-345-5885 to receive your print copy.

 

 

 

How Long Can It Go? An Introduction to Power Supply Reliability Information

Engineers looking for a power supply have to sort through countless choices. There are countless available models to evaluate, each with a product description or datasheet that touts important characteristics relating to the supply’s performance and operating features. The description may even include pricing information. But when you take a deeper dive into the product information, you’ll see acronyms like MTBF and MTTF. They may not mean much if they go into machines that will only see sporadic use. But, when your device is expected to run 24/7 or go into, for example, a life-saving medical device, reliability should be very high on a designer’s list of selection criteria. If a power supply fails, it can disable a system and also cause major damage to critical equipment.

This blog series will review basic power supply reliability concepts, including mean time between failure (MTBF) and mean time to failure (MTTF). Both terms are deceptively similar yet very different. Moreover, power supply manufacturers use different standards and methodologies to calculate reliability ratings. We’ll also discuss power supplies with internal electrolytic capacitors and how a cap’s lifetime can be just as important as MTBF and MTTF when predicting system reliability. This series intends to help foster a better understanding of the information available to buyers looking to select a power supply that will meet the reliability requirements that their application demands.

Stay tuned for Part Two of this series, and be sure to sign up for our newsletter to receive updates by email.

 

White Paper Demystifies Today’s Complex Power Supply Data Sheets

The operating and safety specifications for power supplies have become more complex, adding to the length, level of detail and complexity of their data sheets. Our latest white paper, Interpreting Complex Data Sheet Specifications for Power Supplies, provides an overview of the extensive technical information that today’s power supply designers must examine as they review product data sheets.


The white paper covers important concepts such as interchangeability considerations, thermal management, and the mechanical drawings that show the relationships between specifications. It also helps engineers sift through the growing number of mounting and remote placement options so they can make efficient use of space in ever-shrinking end products. And since specific industries like medical and railways have special requirements, readers will find a section explaining safety and certification information. While data sheets are becoming longer, the information they provide is now much richer. This white paper will help customers apply that additional knowledge to make well-informed purchasing decisions.

For more information, download our white paper, Interpreting Complex Data Sheet Specifications for Power Supplies.

 

 

 

One MOPP or Two? Select a Medical Power Supply That Prevents Shock

Medical equipment designers have to not only make sure their device performs as intended but also do so without compromising the safety of the patient or operator. Electrical shock is an important safety hazard in medical equipment, and design requirements that address it are strict by necessity. To mitigate shock and other safety risks, power supplies must comply to International Electrotechnical Commission (IEC) 60601-1 3rd edition medical equipment standards pertaining to isolation voltage, leakage current and creepage/clearance distances. This blog will review two of the many safety categories to remember when selecting a power supply for medical devices.

Isolation and Insulation Requirements

Patients can be potentially vulnerable to electric shock when they come in contact with, or come in close proximity to certain medical devices. IEC 60601-1 3rd edition standards for isolation between circuits within the medical power supplies include AC input, internal high-voltage stages and DC output. Isolation is accomplished using either double insulation or reinforced insulation. (Shock prevention in Class I electrical equipment only calls for basic insulation and uses protective earth grounding methods.)

Within IEC 60601-1 3rd edition, isolation protection can be classified as MOOP (means of operator protection), which means the device doesn't have to come into contact with a patient, or MOPP (means of patient protection) in which contact is required. For medical power supplies, the insulation criteria for MOOP or MOPP protection are: one layer of insulation at 240V AC requires a test voltage of 1500 V ac; MOOP requires 2.5 mm of creepage, and MOPP requires 4 mm of creepage.

However, advances in power supply technology allow for an even higher grade of safety. When choosing a medical power supply, consider manufacturers that offer two means of patient protection for devices requiring MOOP or MOPP safety. 2MOPP classified power sources at 240 V AC require a test voltage 4000 V AC and 8 mm of creepage. Supplies that meet these highly stringent 2MOPP requirements not only offer optimal patient shock protection but also help ease the end product approval process in a broad range of medical products.

Medical device designers should work closely with the power supply manufacturer to determine the shock protection they need while also considering other safety and performance factors. Not all power supplies adhere to the values spelled out in IEC 60601-1 3rd edition. Polytron's medical power supplies meet or exceed IEC 60601-1 requirements and offer 2MOPP protection. When combined with their very low leakage current characteristics, they can be an ideal choice for medical devices that contact operators or patients.

For more information, download our white paper, Selecting Power Supplies for Medical Equipment Designs.