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SN65HVD82: How to handle ESD protection for isolated RS-485 circuit in DC system ?

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Replies: 3

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Part Number: SN65HVD82

Hello~

My product is powered by external AC/DC power so there are two wires (+/-) in the input.

There is a isolated RS-485 circuit in the product like below :

The question is :

My system is DC system and no Earth Ground in it.

How will I protect Data Line A/B even if I put TVS on them?  What should TVS be connected to ?

Thanks in advance.

  • Hello CC,

    The TVS can be referenced to the same ground that is used by the RS-485 transceiver. It would be good to still include the "RHV" and "CHV" components shown on the above schematic, since these will prevent charge accumulation on the isolated ground.

    Is the AC/DC converter used for the primary-side power supply isolated as well, or is it referenced to earth ground? Does the system have a conductive chassis, or is it fully insulated?

    Max
  • In reply to Max Robertson:

    Hello Max~

    I draw a simple system architecture for reference, Actually there is no earth ground in the product.

    So I don't know how to connect TVS.

    TVS connect to floating RS-485 Common seems useless because there is no path for ESD surge to go back earth ground.

  • In reply to CC Lai:

    CC,
    You could add cap between isolation barrier to create a current path during ESD events. This note http://www.ti.com/lit/an/slyt725/slyt725.pdf detailed ESD results with 100pF cap. You can find how the parasitic cap is made by PCB layers in this guideline http://www.ti.com/lit/an/slla368a/slla368a.pdf.
    Regards,
    Hao

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