Power
How long can it go? An introduction to power supply reliability
Designers and buyers looking to specify a power supply for a new product have to sort through countless choices. But a deeper look into the product information reveals acronyms like MTBF and MTTF. In this article, Wiebe Hart, Gresham Power Electronics reviews basic power supply reliability concepts including mean time between failure (MTBF) and mean time to failure (MTTF)
T
hese terms may not be relevant if the selected power supply goes into systems that will only see sporadic use. But when the end product is expected to run 24/7 or be installed into a life-saving medical device for example, reliability should be very high on a designer’s list of selection criteria. If a power supply fails, it can disable a system, cause major damage to critical equipment and involve costly downtime and repair.
MTBF and MTTF: Fundamentals for power supply selection When evaluating power supplies, designers and buyers will 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. 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 summarised below. MIL-HDBK-217F: The Reliability
Prediction of Electronic Equipment in the U.S. Military Handbook. MIL-HDBK-217F — also commonly used 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, emphasising parts count, lab test, field test and burn-in test data to predict reliability. IEC 61709:2017: This guide emphasises
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.
Internal components When selecting a power supply, consider MTBF or MTTF an initial guide to its reliability. Engineers should also ask the power supply
vendor for reliability information pertinent to the unit’s internal components. Take note of the electrolytic capacitors because they are often the first internal component to fail. Finally, keep the anticipated thermal conditions in mind, too.
28 November 2018 Components in Electronics A - Infant mortality, B - Useful like, C - Wear out
Designers commonly look at the MTBF 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 reveal 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. The failure rate of the equipment depends on the following factors: Complexity – Keep things simple, because what isn’t there can’t fail but, conversely, what isn’t there can cause a failure. A complicated or difficult specification will invariably result in reduced reliability. This is not due to the shortcomings of the design staff, but to the resultant component count. Every component used will contribute to the equipment's unreliability. Stress – For electronic equipment, the
most prominent stresses are temperature, voltage, vibration and temperature rise due to current. The effect of each of these stresses on each of the components must be considered. Great care and attention to detail is necessary to reduce thermal stresses as far as possible. The layout has to be such that heat-generating components are kept away from other components and are adequately cooled. The importance of these provisions cannot be overstressed since the failure
rate of the components will double for a 10ºC increase in temperature. Decreasing the size of a unit without increasing its efficiency will make it hotter, and therefore less reliable. Generic – Generic reliability (also known
as inherent reliability) refers to the fact that, for example, film capacitors are more reliable than electrolytic capacitors, wire wrap connections more reliable than soldered ones, fixed resistors more reliable than potentiometers. Components have to be carefully selected to avoid the types with high generic failure rates. Quite often there is a cost trade-off, as more reliable components can be more expensive.
Assessment – This is the most useful and accurate way of predicting the failure rate. A number of units are put on life test, at an elevated temperature, and so the stresses and the environment are controlled.
MTBF is not the only
reliability metric to look at when selecting a 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. Gresham Power Electronics can provide the technical information, expertise and testing relevant to internal electrolytic capacitors to help you select a power supply that will meet your expectations in the field.
www.greshampower.com
www.cieonline.co.uk
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