New-Tech Europe | April 2016 | Digital edition

Back to basics - Reliability considerations in power supplies

By CUI Inc

ntroduction The importance of reliability

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equivalent and MTBF is the more commonly used in the power industry. 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 reliability - the probability of a component not failing after a given time - the following formula is used: For example, the probability that a component with an intrinsic failure rate of 10-6 failures per hour wouldn’t fail after 100,000 hours is 90.5%, after 500,000 this decreases to 60.6%, and after 1 million hours of use this decreases to 36.7%. Going through the mathematics can reveal interesting realities. First, the failures for a constant failure rate are

processors and displays receive, they are just as vital to system operation. Here we look at reliability in power supplies, how it’s measured and how it can be improved.

can be best demonstrated using an anecdote I was told by a friend back in 2008; when working for a major IC firm from San Francisco, he had received a shipment of new and somewhat problematic desktop PCs. Within months these PCs had started to crash, with the IT department being rolled in to fix the assumed operating system gremlins and / or viruses that were affecting these new computers – to no effect. After much investigation, and with many a stripped down PC, it was eventually revealed that the problem was caused by substandard bulk capacitors in the ac-dc power supply. These had deteriorated in use, and were causing the supply rails to be out of regulation, producing the random crashes. The episode highlights that, while power supplies may not have the glamour, nor get the attention that

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, λ: 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 – see figure 1 MTTF, 1/λ: 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. MTTF is technically more correct mathematically, but the two terms are (except for a few situations)

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