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Original question:

ISO7241C: ESD protection for isolator

DAC7760: ESD protection methods for systems with isolated supplies.

John Barrett
Prodigy 170 points
Part Number: DAC7760
Other Parts Discussed in Thread: TIPD153

Hello,

We have completed a design for a power meter that uses isolated supplies. We have just performed ESD pre-compliance testing. The ESD simulator strikes are damaging our analog O/Ps. The analog O/P are implemented using a TI DAC7760 device.

We followed the instructions on the TI app note TIDU011 and TIDU189 and added the appropriate protection devices. However the devices are still getting damaged. We have some ideas on what's going on i.e. the ESD discharge path is back to the digital ground that is supplied by a 24V supply that has Y capacitors back to Earth. The discharge path must be from our analog ground across the isolation barrier to this Dgnd. The discharge must somehow be inducing damaging voltages on the DAC7760 and damaging the device.

We did add gas discharge tubes between the two grounds in an attempt to divert the discharge path but we still damaged devices. We really need to get this resolved as soon as possible so we would like your opinion and help on the counter measures that we can take before we do any more trial and error testing and damaging any more boards in the process.  

We can send block diagrams, schematics  and PCB layout data and any other information required.

Thanks in advance for your help.

  • 0
    TI__Mastermind 46290 points

    Hi John,

    Is it safe to assume this is the same IEC61000-4 ESD test setup? Is there a specific configuration which is leading to the failure (i.e. air vs contact). What voltage level are you applying to the design? I am just trying to assess how 1:1 this is to the reference design. I know that the IEC standard is very common for factory or process automation applications, but I am not so sure what is common for power meters.

    I think a schematic would be helpful. I understand that you have looked into our reference designs, but none-the-less looking over the schematic for details would be nice. I understand if you do not want to share the whole schematic, but I would ask to at least be able to see the surrounding components on the DAC outputs and the DAC power supply.

    Our tests, as you probably deduced from at least the TIPD153 reference design, were largely targeted to demonstrate immunity on the component level where our expertise resides. As it concerns whole-system design with your complete setup to Earth-ground we begin to differ a bit and this begins to leave my immediate expertise, but we can work together to try to understand what is happening.

    One last question - when the units are damaged what symptoms do you observe? For example, is the VOUT damaged but IOUT continues to function or vice-versa? Another example, perhaps the reference is still active but the outputs are disabled or perhaps the power supplies themselves are collapsing. This could help give clues on where to focus in a schematic review.

  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Kevin,

    Yes we are testing to IEC61000-4. We can get failures with air and contact discharges.

    We have seen various failure types:

    1) Devices appear to be operational but are getting excessively hot

    2) Iout operational but Vout stuck at +15V.

    Power supplies always OK.

    Hopefully we can send you schematics etc. directly.

    Thanks again for your help with this. it is greatly appreciated.

    best regards,

    JB.

  • 0 in reply to John Barrett
    TI__Mastermind 46290 points

    For future readers - this conversation has been taken offline. If this thread is still unlocked upon resolution I will update it with any non-confidential content possible concerning the resolution.

    Thanks!