Wednesday, 13 February 2013

Design for life - Part 1

Design for life

It's interesting. Being involved with switch mode power supply design for over 40 years, you learn what works and what doesn't. What once seemed like quite complex theory becomes second nature and you instinctively have a feel for what is required to make a design work.

However, working in the lab as a prototype is one thing, working faultlessly for the next 20 years is quite another.

It's feedback that the design engineer seldom has the opportunity to benefit from. You rarely see the end application, let alone the condition of the components after years of operation in an industrial environment.

This is probably of little concern to the glut of far eastern manufacturers with products at throw-away prices, but there are sill plenty of applications where long term reliability and build quality are paramount, and this remains a stronghold of UK design and manufacturing. Lets face it, if you are manufacturing in the UK and you are not focusing on quality - you are dead in the water.

Of course, there are a host of general parameters that effect long term reliability of a power supply. Fundamental circuit design, component selection, mechanical construction, assembly process, storage and handling all play a big role. However these tend to be well appreciated at the design and manufacturing stages.

Looking at things from the service return side gives the engineer an entirely new perspective. It allows a unique appreciation of what, in practice causes power supplies to fail in the field. And it's not always obvious.

Part 1 — Electrolytic capacitors

The drying out of wet electrolytic capacitors is perhaps one the most widely recognised causes of age related failure, and it is certainly prevalent. Modern demands for ever decreasing sizes can result in thinner dielectric materials and less volume of electrolyte. Although the loss of electrolyte is by some means the natural wear out mechanism, it can be slowed considerably by reducing the core operating temperature of the capacitor. Locating caps away from other high dissipation components is one obvious example, but the core temperature is also very much influenced by the ripple current flowing through the ESR (equivalent series resistance), namely the electrolyte.
A typical 105°C rated capacitor has a ripple current typical in a 105°C ambient, giving a core temperature of approx 115°C. The specified load life under these conditions can be as low as 1,000 hours (42days), although in practice most caps will continue to operate for longer than this, albeit with reduced capacitance and or higher ESR.

Most practical applications do not subject passive components to more than 50°C, so it can be tempting to increase the ripple current above the rated maximum. This is not recommended because the temperature rise is proportional to the square of the ripple current multiplied by the ESR. Because ESR increases with time, end of life failure will occur sooner, faster than for a cap operating at 105°C and maximum rated ripple current.

Vented capacitors at end of life failure

Output capacitors on small ‘flyback' power supplies, operating in the discontinuous current mode are especially vulnerable to early failure due to the large ripple currents inherent in this topology, so they need specifying carefully. By comparison, continuous current flyback and ‘forward' converters have typically a 20% peak to peak current ripple compared to the 100% of the discontinuous mode flyback converter.
Conversely, small (approximately 6 x 12mm) electrolytic caps commonly used in power supply control circuitry can cause problems in high local ambient temperatures, even when run at very small ripple currents. These capacitors are often used in conjunction with a high resistance connected to a HT rail to provide a start up supply to the control circuit. Due to the very small amount of electrolyte they contain, they can dry out before any other component fails and prevent the power supply starting at turn on due to high impedance or current leakage. Often this can go completely unnoticed until the first mains blackout and subsequent restart attempt.

Careful electrolytic capacitor selection is becoming increasingly important as more and more far-eastern manufactured components enter the market and it is important to pay a good deal of attention to the detailed specification of such components. Cutting costs by using inferior capacitors is rarely money well saved when it results in a dramatic reduction in service life, potentially high warranty costs and a blemished reputation.

Better cooling, larger capacitors or solid electrolyte capacitors are alternative solutions. Niobium solid electrolyte caps are a cap alternative to tantalum caps, which are becoming more expensive as tantalum reserves diminish.

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