5 Aging and Reliability

It is a well-known phenomenon that the actual light output of LEDs degrades over time. Degradation of light output affects many optocoupler device parameters and most of them are usually not mentioned in data sheet. Current transfer ratio (CTR) is one such parameter where aging can be clearly seen. An example of CTR degradation as a function of test time is shown in the application note by Toshiba titled Basic Characteristics and Application Circuit Design of Transistor Couplers.

At some point in optocouplers life, CTR falls to a level at which the device fails to operate normally, leading to poor reliability (high FIT rate and low MTBF). Also, digital isolators isolation and control circuits are very well trimmed, minimizing their performance variation due to aging. Aging is also already considered as part of device minimum and maximum specifications in the data sheet. The very well-controlled manufacturing process of digital isolators also achieves very high reliability (low FIT rate and high MTBF).