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Power Supplies I Design Techniques


Design for life


Paul Horner looks at some practical design techniques to address the need for ultra reliable power supplies in the field


H


aving been involved with switch mode power supply design for 40 years, you learn what works and what doesn’t. Devices that work in the lab are one thing, but working faultlessly for the next 20 years is quite another. Design engineers


rarely receive feedback or see the end application, let alone the condition of the components after years of operation. For manufacturers with products at throw-away prices that is of little concern,


but in the UK where long term reliability and build quality are paramount it can be critical.


When it comes to power supplies there are a host of general parameters that


effect long term reliability. Fundamental circuit design, component selection, mechanical construction, storage and handling all play a big role. These tend to be well appreciated at the design and manufacturing stages but looking at things from the service return side, the engineer gets an entirely new perspective which allows a unique appreciation of what can cause power supplies to fail in the field.


Electrolytic capacitors The drying out of wet electrolytic capacitors is recognised as a major cause of age related failure. The demand for ever decreasing can sizes result in thinner dielectric materials and less volume of electrolyte. The loss of electrolyte can be slowed by reducing the core operating temperature of the capacitor, so locate caps away from other high dissipation components. However, the core temperature is also 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 rating 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, although most caps will continue to operate for longer.


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 as 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 than for a cap operating at 105°C and maximum rated ripple current. 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. By comparison,


Vented caps at end-of-life failure 28 July/Aust 2011 Components in Electronics www.cieonline.co.uk


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