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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