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​Internal Capacitors May Decide Your Power Supply’s Reliability

Internal-Capacitors-transDesigners commonly look at the MTBF (mean time between failure) ratings of power supplies to make sure they will operate reliably in an intended application. Although it can be a useful indicator, MTBF doesn’t give you the entire story about a supply’s reliability.

For instance, MTBF does not predict the power supply’s lifetime. It is the total functional life divided by the number of failures. But that expected time between failures can be longer than the life expectancy of the power supply’s internal components.

Electrolytic capacitors are usually the first components to break down inside a power supply. If the capacitor cannot reliably store energy as needed, the power supply’s reliability suffers. Several conditions can cause a capacitor to fail, so consult with your power supply manufacturer to determine how the capacitor will handle the rigors of the application environment.

Why Capacitors Fail

Some of the common reasons why capacitors fail or wear out include:

  • Voltage rating. Applying a higher voltage than the capacitor’s voltage rating can cause catastrophic failure.
  • Ripple currents. Extreme ripple currents can heat up the capacitor and dry out the electrolyte.
  • Heat. Hot operating conditions shorten the capacitor’s life. Or, the circuit board can transmit heat to cause the electrolyte to vaporize.
  • Short or open circuits. Short circuits can occur between the electrodes, and mounting errors can cause open circuits.
  • ESR. A capacitor with a higher equivalent series resistance (ESR) is less able to handle high ripple currents. High temperatures can raise a capacitor’s ESR.
  • Reduced capacitance. A capacitor’s performance can decline over time. As ESR increases, the capacitor heats up and dries the dielectric.
  • Storage life. If you expect your power supply to be inactive for long periods, remember that electrolytic capacitors have a limited storage life.
  • Other. Chemical leakage (which leads to corrosion), high leakage current, cold temperatures, capacitor size and many other factors affect the life of an electrolytic capacitor.

Internal Capacitors May Decide Your Power Supply’s Reliability

MTBF is not the only reliability metric to look at when selecting your power supply. Internal components like electrolytic capacitors have limited lifetimes, so be sure to take them into account. Capacitor lifetimes can be determined by a host of both operating and environmental factors, especially when it comes to thermal conditions.

Although it is desirable to look for built-in capacitors that offer long lifetimes in a wide range of operating environments, not all capacitors are equal: Better-quality capacitors use better-quality electrolytes. Also, be sure to get as much information as possible about the built-in electrolytic capacitor from your power supply manufacturer in order to determine whether it is appropriate for the intended application. Polytron Devices can provide the technical information, expertise and testing pertaining to internal electrolytic capacitors to help you select a power supply that will meet your expectations in the field.

For more information about Polytron power supplies, visit

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 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

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.