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ISOW7821: ESD Discharge Test Withstand

Stefan Olejniczak57
Prodigy 140 points
Part Number: ISOW7821

Dear Ladies and Gentlemen,

In my new design the device must withstand an ESD discharge test as specified in EN 61000-4-2 Level 4. This means that the capacitor of 330 pF of the test generator is charged to 8 kV and discharged via 330 Ohms into GND of the isolated side. If no stitching capacitance is used this means that the isolated GND charges to 8 kV against non-isolated GND. Unfortunately I cannot find specs in the datasheet directly related to this test.

My questions are now:

1. Will the device pass this test without isolation barrier breakdown without any additional measures?

2. If yes, which of the specs would guarantee this (V_IOTM production test, V_IOSM qualification test)?

3. If not, which ESD bypass components (i.e. suppressors) do You recommend between isolated and non-isolated GND? Gas discharge tube, TVS, varistor?

4. Since the specs are equal, can I assume Your answers are valid for the ISOW784x as well?

Thanks and best regards!

  • 0
    TI__Genius 17100 points

    Stefan,

    Based on internal tests the ISOW7821 is expected to pass 8kV ESD charge/discharge tests. This is not currently specified in the datasheet.

    Although EN 61000-4-2 Level 4 is a system level spec, the ISOW782x and ISO784x are all expected to pass this test without external components since they are the same device with only different channel configurations. If redundancy is desired, a y-capacitor with low ESL and ESR which meets isolation creepage and clearance constraints may be used. This capacitor works by effectively shorting the two ground planes at very high frequencies.

    I hope this is helpful! If you would like additional suggestions for external components or methods to optimize your system, please follow up! Welcome to the E2E forums :)


    Thank you for posting,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 140 points

    Hi Manuel,

    thanks for your prompt reply. This is good news, although I recommend to specify ESD in a future datasheet revision.

    Regarding your y-capacitor recommendation: Although this appears in design notes, I'm wondering if it helps for the worse case. Imagine the tester applies the 8 kV ESD pulse several consecutive times to the DUT's isolated side (he definitely does!), then the y-capacitor will charge step by step up to 8 kV (assuming the isolation resistance is somewhere in the Tera-ohm range). Depending on leakage it will take quite an amount of time, until this voltage declines. Furthermore a user who touches the so charged interface will be exposed to a slight electrical shock.

    Therefore my idea is to crowbar the ESD pulse from isolated to non-isolated GND by an GDT (gas discharge tube). This discharges the isolation barrier capacitance (a few pF) and does not significantly increase isolation capacitance. An alternative would be to limit the cross-barrier voltage by a TVS diode, but this adds leakage and capacitance.

    How do you think about this?

    Thanks and best regards, Stefan

  • 0 in reply to Stefan Olejniczak57
    TI__Genius 17100 points
    Stefan,

    The Y-capacitor will behave depending on the ramp rate of each 8kV ESD test. When the voltage rises very quickly, the capacitor is shorted and does not store any charge. I will follow up on the comment about users experiencing electrical shock.

    A TVS diode can be used to protect the IC as well, but if it is not desired then gas discharge tubes do offer a strong case. I have not seen these in many uses, but if there's a schematic of this idea you can share I'd like to provide thorough feedback.

    I will await your response.


    Thank you,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    TI__Genius 17100 points
    Hi Stefan,

    I investigated gas discharge tubes further, and although Y-capacitors are our recommendation for device protection from fast transients, gas discharge tubes are a valid solution. They do not store energy so high enough potential differences are discharged all at once, preventing shock from touching a could-be charged component.

    Would you like feedback specific to your application? Are there additional comments or questions you have?

    Please feel free to reply to this thread in either case! Thank you.


    Thank you for your time,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    TI__Genius 17100 points
    Hello Stefan,

    Since it has been a couple weeks since your last response this thread will be closed out. To reopen it, simply reply with a comment here and we can continue the conversation. Otherwise you may ask a new/related question using the red and yellow buttons in the top right corner of this window!


    Have a great weekend,
    Manuel Chavez