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SN65HVD1781 and IEC 61000-4-2 compliance

Pablo Liberman
Prodigy 130 points
Other Parts Discussed in Thread: SN65HVD1781

Hi,

I'm using a RS485 transceiver (SN65HVD1781) which is +/-70V fault-protected.
Our device must meet the IEC 61000-4-2 standard so we thought about adding a TVS protection.
For example, a SM712 TVS (+12V/-7V), but I pressume that it would burn if an overvoltage fault occur.

What TVS I would have to choose to maintain the fault protection?

Thanks,
Pablo

  • 0
    TI__Mastermind 28265 points

    The HVD1781 was designed for operating in 24V DC-bus environment. The 24V are nominal and can rise up to 30V and sometimes even up to 35V. In this case you would need a TVS with 36V stand-off (or breakdown voltage) which usually has a clamping voltage in the range of some 60V. In the event of a DC-short to the RS-485 data lines, the HVD1781 would still withstand the high clamping voltage of the TVS.

    Any TVS would fry if a long term overvoltage occurs due to the overcurrent detection (or GCI) taking too long to switch off the DC-bus supply. There are many high-power TVS diodes available (from 1.5kW up to 30kW) but their power rating is based on a 10/1000us transient maximum. This forces the over-current detection to be faster than 1ms.

    If you cannot guarantee this time, then a two stage protection might be in order where you use a gas-discharge tube in combination with a TVS. Before I go further into detail I would need to know what your overvoltage constraints or worries are.

    Otherwise, if your anticipated fault conditions are far below from the scenario I described above, then a SM712 will be just fine.

    Regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    Prodigy 130 points

    Thanks Thomas,
    We had thought the same thing. A TVS with 36V stand-off voltage would be a good solution.

    Best regards,
    Pablo

  • 0 in reply to Pablo Liberman
    TI__Mastermind 28265 points

    Pablo,

    do you need anything else or can I close this post?

    Thanks, Thomas.