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ISO7421: Safety Analysis of ISO74xx

Wayne Liu
Expert 8605 points
Part Number: ISO7421
Other Parts Discussed in Thread: ISO7721

Hi,

My railway customer wants to do the safety analysis, two questions:

1. What is the possibility of input and output short? In what conditions the short will happen?

2. If the isolation voltage exceed the spec, what will happen?

Thank you very much!

  • 0
    TI__Intellectual 2855 points
    Hi Wayne Liu,

    If the isolator barrier is subjected to voltages beyond the isolators’ rated limits, the isolation barrier breaks, creating a short circuit
    between input and output.

    If customer requires higher isolation rating, I would recommend to promote ISO7721 reinforce isolator.

    you may find more information on isolator failure modes in below white paper
    www.ti.com/.../slyy081.pdf

    Regards
    Tejas
  • 0 in reply to Tejas Hommaradi
    TI__Expert 8605 points

    Hi Tejas,

    Thank you so much for your help.

    Few further questions:

    1. If the barrier broke, does the digital communication still work properly except no isolation function? The system needs to know the barrier has been broken and the system is in danger because there will be no isolation. If the communication still works, is there any way for the system to know the barrier has been broken?

    2. In the failure modes AN, it states

      In TI reinforced isolators that use series-capacitor 

    isolation, this damage is limited to one capacitor.
    That leaves the other capacitor intact, causing these
    isolators to “fail open,” thus, preserving
    basic isolation.

     

    Same question as question 1, in this case, If the barrier broke, does the digital communication still work properly? Is the ISO77xx and ISO784x serails are all series-capacitor isolations?

    Thanks.

  • 0 in reply to Wayne Liu
    TI__Intellectual 2855 points
    Hi Wayne Liu,

    Below are the answers to your question,

    1) In case of basic isolation, on barrier brake down, the communication may or may not work. It totally depends on the type of stress the isolator was subjected to. So it is always suggested to select the isolator carefully based on the application. Applications that require higher isolation rating or if it is safety critical application, we recommend customers to use Reincforced Isolators.

    2) In case of Reinforced Isolation (TI reinforced isolators that use series-capacitor ), the isolation damage is limited to one capacitor and we have observed that the circuit on the over stress side fails even before the capacitor is damaged and hence the communication will not work. As the other capacitor is intact the system will still be protected with basic isolation.


    Regards
    Tejas