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ISO3082: Input protection against 24V

franziskus
Expert 8209 points
Part Number: ISO3082
Other Parts Discussed in Thread: ISOW7841, SN65HVD1781A-Q1, ISO7741

Team,

this app note shows protection for lines A and B up to 19V using PSM712.

Can you recommend a solution that can protect up to 24V?

Thanks!

  • 0
    TI__Expert 6195 points

    Hello,

    I'm not sure that I totally understand your question. If you are asking for a TVS to protect the ISO3082 up to 24V, this TVS will do. 

    To clarify, the TVS device clamps at 19V allowing the system with the ISO3082 in it to withstand voltages much higher than 19V.

    The clamping voltage limits the voltage across the protected circuit (in this example, the ISO3082). So the TVS limits the voltage across the ISO3082 bus pins to 19V as to not violate absolute maximum on the device (which could cause damage to the device). 

    I could be misinterpreting your question. If so please correct me and I'll be happy to further support you. 

    Let me know if that helped or you need me to clarify anything!

    Best regards, 

    Dan

  • 0 in reply to Dan Kisling
    Prodigy 25 points
    Hello,

    I'm interested in a solution for protecting the lines against continuous 24V, not just transient voltage.
    Is this TVS suitable?

    Thank you.

    Best regards,
    Radu
  • 0 in reply to Radu Corb
    TI__Expert 6195 points

    Hi Radu, Franziskus,

    As 24V violates the absolute max spec of the ISO3082, I do not think this TVS solution is suitable for continuous 24V. 

    Here are a few options to consider for 24V continuous solutions:

    You could add series resistors to the A/B lines to drop the voltage before it reaches the part in addition to the TVS. The big downside I see with this solution is you will constantly be drawing power and you'll have to size the resistors appropriately. 

    Another option to consider is using a fault-protected RS485 transceiver and digital isolator. For example, the SN65HVD1781A-Q1 has an abs max of +/-70V which would give you plenty of margin to 24V. You could pair this with the ISOW7841 to get isolation and an isolated power supply to power the RS485 transceiver. If you already have an isolated power supply you could use the ISO7741. The downside to this solution is the multiple parts will take up more board space. 

    Perhaps we should revisit the 24V continuous specification. Could you give me some insight into how you came to this specification? I may be able to give you a better recommendation if I understand the system specifications better. 

    Best regards, 

    Dan

  • 0 in reply to Dan Kisling
    Prodigy 25 points

    Hi Dan,

    24V continuous voltage is specified in case of wiring mistake.

    I would try a discrete solution, because of cost..

    Thank you.

    Best regards,

    Radu

  • 0 in reply to Radu Corb
    TI__Expert 6195 points

    This discussion was moved over to email and phone conversation, so I have taken a moment to summarize the results and discussion below:

    To protect the system against a 24V incorrect wiring event, a solution involving PTC fuses and TVS diodes was chosen. Protection devices are required because 24V violates the abs max spec of the ISO3082 bus pins (-9, +14V).

    TVS Diode Selection

    The TVS will protect the ISO3082 for short duration events (<1second). In the event of incorrect wiring, the TVS will be the first device to protect the ISO3082. The TVS is very fast, so it clamps as soon as the overvoltage event happens. The TVS is chosen to stay not violate abs max spec of +14V. The key parameter here is Maximum clamping voltage at Ipp. For example, the SMAJ8.0A maximum clamping voltage at Ipp of 29.4A is 13.6V which fits inside the abs max spec with a margin of 0.4V. Note that if the current on the bus was found to be less than Ipp, one could determine a different maximum clamping voltage. For example, if the maximum current during an incorrect wiring event was found to be 20A, a device with a maximum clamping voltage at 14.4V at 27.8V could also work (even though 14.4V > 14V), as the actual clamping voltage would be reduced (to a voltage < 14V). To determine the clamping voltage at a specific max current one would have to refer to the IV characteristic of the specific TVS. The TVS must also be chosen so it does not interfere with normal operation. 

    PTC Fuse Selection

    The PTC fuse will protect the bus for longer duration events (>1 seconds). When the PTC fuse is subjected to current beyond the rated trip current, the temperature of the PTC is increased to the point that causes the material inside the device to increase resistance and therefore impede current flow. This will restrict the current into the TVS and allow it to survive during long incorrect wiring events (otherwise the TVS would burn up as it cannot dissipate that much power). The PTC will automatically reset after the wiring mistake is removed (and the temperature of the PTC fuse reduces). In normal operation the PTC fuse has very low resistance (<2 ohms) so it does not affect the bus significantly.

    Dan