Most optocouplers spend most of thier time isolating mains voltages.They are reliable doing this and they are cheap and simple.How would things go if the same garden variety opto was isolating a DC potential ? A work associate did a HVDC supply in a previous life and found opto failure despite using the opto well below its ratings .He said that this was due to electromigration,he started life as a chemical engineer .Is this true ? Does this mean that optos should be derated for DC isolation? What would be a reasonable derating factor? Should one not use optos period for HVDC ? Would isolating a negative voltage be better than isolating a positive voltage due to electrochemical reasoning ?
Electronic – Optoisolator Long term Reliability at HVDC
hvdcopto-isolatoroptoelectronics
Best Answer
If reliability is a problem get a part with an MBTF listed in the datasheet, that means that the supplier built it with longevity in mind and tested it. Generally ic manufacturers must live up to the specs in the datasheet (read the legalese printed on most suppliers datasheets). Once you have a part that is guaranteed to work for a long time, make sure it doesn't get damaged by running it outside of spec, overvoltage, ESD and that it has the proper soldering time at the PCB assembly house. If your really interested in reliability, then derate it and make sure that the parts specs are over-preforming in every way. LED's generally have a lifetime of 100k hours, I'm not saying that this applies to optoisolators but the physics are the same. Maybe you could build the design so its running condition runs with the LED off.