POWER ELECTRONICS FEATURE Going back to basics…


Arun Ananthampalayam, Product Marketing Engineer at CUI Inc. discusses reliability considerations in power supplies


T


he importance of reliability can be demonstrated using an anecdote Arun


Ananthampalayam, Product Marketing Engineer at CUI Inc. advises was once told by a friend back in 2008: “When working for a major IC firm, my colleague received a shipment of new and somewhat problematic desktop PCs, which had started to crash within months. The IT department was rolled in to fix the assumed operating system gremlins and/or viruses but after much investigation it was eventually revealed that the problem was caused by substandard bulk capacitors in the ac-dc power supply.” Continuing, Ananthampalayam, added: “So, while power supplies may not have the glamour, nor get the attention that processors and displays receive, they are just as vital to system operation.”


PREDICTING THE POWER SUPPLY'S EXPECTED LIFE First, a few definitions: •Reliability, R(t): The probability that a power supply will still be operational after a given time


• Failure rate, : The proportion of units that fail in a given time, note, there is a high failure rate in the burn-in and wear- out phases of the cycle (figure 1).


• MTTF, 1/: The mean time to failure. MTBF (mean time between failures) is also commonly used in place of MTTF and is useful for equipment that will be repaired and then returned to service. A supply's reliability is a function of


multiple factors: a solid, conservative design with adequate margins, quality components with suitable ratings, thermal considerations with necessary derating, and a consistent manufacturing process.To calculate the probability of a component not failing after a given time the following formula is used: R(t) = e-t Three methods can be used to calculate


failure rates, prediction (during design), assessment (during manufacturing) and observation (during service life). Prediction uses a standard database of


component failure rates and expected life, typically MIL-HDBK-217 for military and commercial applications or Telcordia for telecom applications. Using the prediction method means several, often incorrect assumptions need to


temperature, with carefully controlled and increased stress factors. Observation in the field is also possible, but


this is more difficult as it is impossible to control all of the conditions a supply has been subjected to and therefore more difficult to undertake reliable causation analysis.


IMPROVING POWER SUPPLY RELIABILITY THROUGH DESIGN Obviously, the paper design and topology should be robust and cautious. This should take into account the effects of load and line transients, as well as noise. The designer should also carefully determine the required minimum/maximum values of component parameters to ensure reliable operation, as well as those for critical second- and third-tier parameters,


/ ELECTRONICS Figure 2:


Curve showing the probability that a component is still operational over time


including less-publicised factors in the magnetic components, such as temperature coefficient of some values. SPICE (simulation program with integrated circuit emphasis) or similar modeling of the design is essential, using realistic models of the components and PC boards and tracks, to verify both static and dynamic performance. The choice of components must be done


with conservative bias, with extra margin in both initial and long-term values for many of their specification values. Furthermore, the layout must accommodate the fact that most supplies are dealing with significant current flows, on the order of 10, 20 or more amps. The next critical step is selection of


be made. But it is the least time consuming method and by applying it consistently across different designs, it can indicate the relative reliability of topologies and design approaches, rather than absolute reliability. Assessment is the most accurate way of


predicting failure rate, but requires greater time and resources. This method subjects a suitable number of final units to an accelerated life test at elevated


Figure 1:


The bathtub curve, failure rate plotted against time with the three life-cycle phases: infant mortality, useful life and wear-out


specific components. As it’s nearly impossible to distinguish a poorly made or counterfeit unit, vendor credibility is key. Furthermore, components must be compatible with the manufacturing process. Even the basic soldering process used in


supply construction is an area for consideration. While the common reflow- soldering temperature profiles are well established, the regulatory mandate for lead-free (Pb-free) components and solder also means that a different reflow soldering profile is needed and all components used must also be qualified to perform to specification after this higher reflow temperature and soak time. Reliable supply design is not a guessing


game. A reliable supply requires suitable design and analysis, components, manufacture process, test, and installation. No single step will ensure a reliable supply, although there are many ways to decrease the supply's reliability. When a vendor analyses the supply's


expected reliability, it is important to be consistent in databases, models, environmental conditions, and manufacturing in order to yield meaningful results that can be compared across different power supplies and implementations. There are a number of additional things a power supply vendor can do to increase reliability, including using components that are inherently more reliable, and using them well below their rated specification. If we look at temperature, a component rated for reliable operation at 85C will have a significantly improved lifespan if used at 55C - typically, a component's life doubles for every 10C decrease in temperature.


CUI Inc. www.cui.com esales@cui.com


ELECTRONICS | JULY/AUGUST 2015 19


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