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Optocoupler Triac dv/dt notes

Opto-isolator/ optocoupler for electric isolation in IC design

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I came across while reading the opto-isolator technique used for electric isolation in embedded systems/ Integrated Circuits design that ” Opto-isolator is not recommended for electric isolation in power applications IC’s mainly due to it’s dV/dt sensitivity”. So I would like to know:

1)How dV/dt sensitivity affects isolation?

2)Is dV/dt sensitivity the only reason to avoid optocoupler especially in power application IC/embedded systems?

3)Are there other limitations in optocoupler which compel one to chose other isolation techniques?

Looking forward to your suggestions and guidance. Thanks.integrated-circuitopto-isolatoroptoelectronicsShareEditFollowFlagasked 4 hours agoBam_Khel311 bronze badge

  • 1What do you mean by “dV/dt sensitivity“?. Can you please give us the link of your reading? – tlfong01 3 hours ago   
  • 1″dV/dt sensitivity” Explanation: There is always a capacitance between input (LED) and output (transistor, driver …) of an optoisolator. According to I = C × dv/dt a high dv/dt will create a proportionally high displacement current from input to output. – Bam_Khel 3 hours ago
  • Many thanks for your explanation. I must confess that I have only a superficial knowledge on optocoupler. Perhaps I should google to learn more and come back to you if I think I can help. I randomly googled the following optocoupler datasheet as the first case study. Please let me know if you have any comments or suggestions. Cheers. Reference: VO4257, VO4258 Optocoupler, Phototriac Output, High dV/dt, Low Input Current – Vishay Semi docs.rs-online.com/4f24/0900766b80e541d3.pdf. – tlfong01 1 hour ago   
  • I skimmed the datasheet and now have a vague picture of what is going on. I have only 50% chance of giving a correct answer. – tlfong01 50 mins ago   
  • #Bam_Khel, I have the feeling that your question is sort of an assignment, for which I usually only give hints or indirect answers for you to complete your own homework, this way you learn deeper and remember harder. – tlfong01 32 mins ago   

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Question

The OP read the following advice:

“Opto-isolator is not recommended for electric isolation in power applications IC’s mainly due to it’s dV/dt sensitivity”

He would like to know the following:

  1. How dV/dt sensitivity affects isolation?
  2. Is dV/dt sensitivity the only reason to avoid optocoupler especially in power application IC/embedded systems?
  3. Are there other limitations in optocoupler which compel one to chose other isolation techniques?

Answer

  1. I am trying to give answer by referring to the Vishay 4257 optocoupler. I have summarized important points in the appendices below.
  2. I skimmed my summarizes and found that there is only one point I should dig deeper to know more to answer the OP’s question. The other things like high input sensitivity, high isolation voltage etc are irrelevant to the OP’s question.Now the only important point in the datasheet is repeated below:The … phototriac uses dV/dt clamp resulting in a static dV/dt of greater than 5 kV/µs.
  3. / to continue, …

References

1 VO4257, VO4258 Optocoupler, Phototriac Output, High dV/dt, Low Input Current – Vishay Semi

2 Triac – Wikipedia


Appendices

Appendix A – VO4257 Datasheet Summary

VO4257 Datasheet Summary

Appendix B – VO4257 Electrical Characteristics

VO4257 Elect Characteristics

ShareEditDeleteFlagedited 17 secs agoanswered 40 mins agotlfong011,45211 gold badge55 silver badges99 bronze badgesAdd a comment

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