Thursday, 9 May 2013

Design for Life - Part 4

Last time we looked at Power MOSFETs and uncovered some surprising factors which have a big impact on long term reliability. This week we look at Optocouplers and see why this innocent little component constitutes such a high failure risk for power supplies and what you can do about it.

Optocoupler Ageing

Most designers have a good appreciation of electrolytic capacitor ageing, but we also see many age related failures due to optocouplers. Generally this manifests itself as a reduction in the effective current transfer ratio (CTR) over 'me. This doesn’t sound too serious until you recognise that optos are commonly used to enable the converter stage of a power supply across a primary to secondary isolation barrier.

A degraded opto can and o2en does render the entire power supply inoperable and as such can be considered a high failure risk. The primary piece parts of an optocoupler are a photo-detector IC and an infrared emi5ng LED (typically Gallium Arsenide). Experimental analysis has shown that the LED is the only portion of the optocoupler that has a significant impact on life, with light output degradation leading to a decrease in CTR.

Furthermore, it is the actual current through the LED which is by far the most dominant factor. For longest possible service life therefore, it is desirable to allow at least 50% margin for a reduction in CTR over 'me and to drive the LED at as low a current as possible for the required CTR.

Next _me we will look at Spike & Surge—the primary natural cause of catastrophic psu failure in the field.

Advance Product Services Ltd
Paul Horner is Managing Director at Advance Product Services Ltd


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