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SN65HVD1785: RS-485 Robust bus

Part Number: SN65HVD1785

Hi

I m trying to design a robust circuit using SN65HVD1785

circuit power supply 12-48 V DC

during a test using 40V DC power supply, I deliberately mis-wired BUS A (PIN6) to V+ (40V ) , but the IC failed ,

I did the the same test several times with new ICs but result was the same, IC fails

I need my circuit never be failed at mis-wiring situation, do I missed anything in my design?

  • Hello Macit,

    You may want to add a TVS diode on the A and B signal lines for additional protection. Just the connection of the A or B line to +40 V will not damage the HVD1785 device, but if this connection is made through a cable then there may be some voltage spikes that occur when the short-circuit is connected/disconnected or when the transceiver output toggles (since these conditions result in abrupt current changes through the inductance of the cabling). I suspect these may be what is resulting in damage to the transceiver, although you could verify this further by monitoring the A and B lines on an oscilloscope when the short-circuit occurs. (If you do this, be careful to make sure that your probes or scope inputs are able to withstand high voltages. A resistive attenuator may be useful in protecting the measurement equipment.)

    Here is an article that further details how to choose an appropriate TVS diode based on application/transceiver specifications:

    e2e.ti.com/.../how-to-choose-a-tvs-diode-for-rs-232-rs-485-and-can-based-on-voltage-ratings

    I hope this helps, and please let me know if you have any further questions.

    Regards,
    Max
  • In reply to Max Robertson:

    hi Max,

    thank you for reply and complete clarification.
    I understand the cause of failure now, but I expected embedded TVS inside SN65HVD186 !
    I will try to use TVS diodes, and share the result here.

    regards
  • In reply to Macit Seyedin:

    Hi Macit,

    You are right that there is some level of transient protection built in to SN65HVD1785, but it may not be sufficient for the device to survive all stresses that could occur in an application. The transceiver is rated for human body model ESD strikes up to 16 kV (on the A and B pins), but the transient overvoltages that arise from a short-circuit could be more severe.

    Please keep us updated on whether or not the issue persists after adding additional TVS protection, and if so we can help further brainstorm on other likely causes of failure (and corresponding solutions).

    Regards,
    Max
  • In reply to Max Robertson:

    Hi

    I added two TVS diodes one between A and GND , other between B and GND
    and repeated my test (mis-wiring 40V-55V ) to both A and B,
    for iteration about 100 times, no problem occured yet ! seems problem solved.

    thanks
    regards
  • In reply to Macit Seyedin:

    Macit,

    That's great to hear - thanks for the update! Just let us know if you experience any problems in the future.

    Max
  • In reply to Max Robertson:

    Hi Max

    my circuit almost finished. 

    as I mentioned before, there are 2 TVS diodes (SMAJ48CA) 1 : between bus A and GND , 2: bus B and GND

    I made 10 sample devices, all 10 devices are common connected to data bus, and did some robustness forced tests as below:

    1) mis-wire connetion:

    all devices survive from all possible combination of mis-wiring 55V, about hundred iterations.

    the result is as I expected, and pass the test

    2) ESD test

    I did the test with ESD gun 20 pulse +6000V and 20 pulse -6000V to both A and B of bus respect to GND

    but some devices' ICs (sn65HVD1786) fail after ESD test, 

    I did the experiment several times and in some tests 1 IC , some times 2ICs or 3 ICs fail both A and B some times only A or B

    fail mostly occures in devices near the test point, all TVS diodes survive after ESD

    I also repeated my test using different IC sn65HVD1781but the result is same:

    they fail after ESD shock,

    I have no idea why this happen, according datasheet both sn65HVD181 and sn65HVD1786 are ESD protected up to +-16000 volts, but they fail in my 6000V test

  • In reply to Macit Seyedin:

    Hi Macit,

    That is strange, I would have expected the TVS diodes to clamp the ESD transients such that the transceiver would remain unaffected. Just to clarify, what type of ESD are you applying (HBM, CDM, MM, IEC, etc.)? Is it a direct discharge to the A and B signal lines?

    A potential solution may be to insert some small series resistance (5-10 Ohms) between the transceiver pins and the TVS diode. This helps to limit currents that may flow into the transceiver if the TVS's peak clamping voltage during the ESD strike still exceeds the ratings of the transceiver. Another option might have been to reduce the breakdown voltage of the TVS, but in your case this would likely result in the TVS diodes breaking when conducting the mis-wiring test.

    Are there any other components installed on the RS-485 signal lines besides the TVS diodes?

    Regards,
    Max
  • In reply to Max Robertson:

    hi Max

    my previous tests were IEC-04 (8kv) direct discharge from 7 mm , I saw the arc from tip of gun to data line

    I also did new test with IEC-01 (2kv) from 1mm  - arc rarely can be seen - after 50 shots,  B line damaged again, first nearest node of 10 nodes, 

    are there other pin compatible alternatives like sn65HVD1781sn65HVD1786

    changing PCB design is difficult, maybe my last choice !

  • In reply to Macit Seyedin:

    Macit,

    Thanks for the info. There can be a lot of variables in this kind of testing, so sorry I have a lot of questions. From the screenshot it looks like this is a test per IEC 61000-4-2, and so I assume you are using 150 pF / 330 Ohm for a discharge network in the gun - let me know if this is not the case. The SN65HVD178x devices are not rated for this type of stress (which tends to be stricter than just HBM ESD), but with the SMAJ48VCA diodes I would not expect any issues, especially at a relatively low level like 2 kV.

    Where exactly are you discharging the gun? Is it on the chassis/casing of your system, or is there a connector that you are striking (and if so, are you striking the A and B pins specifically)? Since the A and B lines are protected by the TVS I was wondering if maybe the overvoltage transients are coupling through and stressing other pins (like VCC, D, DE/RE, etc.).

    It may be helpful if we reviewed a fuller schematic and PCB layout to see if there are any potential issues. Should I send you an email so we can share information privately?

    We could also try to get the damaged units back from you and perform some failure analysis. That may give us some better idea which portion of the IC is damaged and what type of damage it is.

    To answer your question on pin-to-pin alternatives - we do have several RS-485 transceivers that are rated for very high levels of ESD immunity, but none of them at the moment also have the ability to withstand +/- 70 V on the bus pins like this family does.

    Max
  • In reply to Max Robertson:

    Ok Max

    thank  you very much for spend time and caring about my project

    my test setup is 10 circuits connected (soldered) each other using 10 cm 0,34mm2 cables V+, V-, D+, D-

    there is a 15cm male 4x0,34mm2 socket soldered to first circuit as input

    I connect another 15 cm female socket to  input, other side of female soket is 4x0,34mm2 free cables (V+)(V-)(D+)(D-)

    gun GND is connected to (V-)

    I strike (V+)(D+)(D-) 

    no problem on (V+) side , there is TVS diode and TPS54360, solved in other thread recommended by John Tucker

    https://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/t/627295?tisearch=e2e-quicksearch&keymatch=TPS54360:%20short%20circuit%20fail%20at%2045V 

    so A, B are striked through 15 cm female + 15 cm male soket

    TVS diodes are  on circuit very close to A, B of IC,

    I should say TVS diodes in my current PCB are soldered manually , they were carefully soldered and checked before test

    but maybe considered as fail cause

    my new PCB will arrive soon,

    let me redo my test again with new revision PCB to acheive more precise facts to solve the problem

    Regards

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