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Opto Challenges

Optocouplers are inherently limited. At the core of an optocoupler is an LED which has large output variation over input current, temperature and age. These variations reduce the operating performance of the devices over their lifetime which causes additional design complexity for system engineers and reduces the life expectancy. A major concern with optocouplers is the long term reliability. Because of the aging effects, optocouplers are often specified to last up to ten years. This is a serious problem for modern industrial system designers who want to provide twenty plus year warranties to their customers. Speed is also a limiting specification on optocouplers due to the slow switching properties of the LED. Another serious concern is the common mode transient immunity. Poor immunity is caused by the inherently high parasitic capacitance of the optocoupler architecture and can lead to incorrect data being passed across the isolator. Finally, the internal LED on the optocoupler requires a high driver current to turn it on. This limits how low the power requirement of an optocoupler can possibly be. These limitations on optocouplers can be addressed in various ways; however, improving one of the parameters invariably degrades another aspect of the optocoupler. For instance, the CMTI can be improved by driving the LED with a higher current. But then the power consumption is higher and the lifetime of the device is reduced.

PTM Published on: 2012-10-24