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ISO7841: V_IOWM compared with ISOW7841, power supply recommendation

Brian Bienvenu
Prodigy 40 points
Part Number: ISO7841
Other Parts Discussed in Thread: ISOW7841,

I see the V_IOWM rating for the ISOW7841 (3/1 data + integrated power) and ISO7841 (3/1 data, no power) is very different:

  • ISOW7841 DWE (>8mm clearance/creepage):
    • 1000V RMS or
    • 1414V DC
    • Figure 42 1000V RMS for 1184 years
  • ISO7841 DW (>8mm clearance/creepage):
    • 1500V RMS or
    • 2121V DC
    • Figure 1: 1500V RMS for 135 years

  • ISO7841 DWW (>14.5mm clearance/creepage):
    • 2000V RMS or
    • 2828V DC
    • Figure 2: 2000V RMS for 34 years

I have an application where I need to measure 1V on a shunt resistor on top of a 1600V DC rail, which might go as high as 1850V without load.

From my interpretation, it seems like the actual basic insulation lifetime for all of these parts is exactly the same, it's just that a different "optimal" point on the insulation lifetime projection was selected for the datasheet. I understand that the wider DWW part, with higher creepage and clearance, allows for a higher safe working voltage (so, it rides further down the lifetime projection, with only 34 years lifetime, instead of 134 years).

However, I don't understand why the ISO7841DW is rated above the ISOW7841DWE. Are they not operating on the same lifetime projection curve?

Separately, what power supply is recommended for an ISO7841DWW implementation which actually requires the high V_IOWM rating?

Thanks,
Brian

  • 0
    TI__Mastermind 49415 points

    Hi Brian,

    Thank you for reaching out to us with your questions.

    Your observation is correct. ISOW78xx family is different from ISO78xx from the fact that ISOW78xx integrates a DC/DC converter as well. Since the DC/DC converter has an isolation power transformer, the life of the transformer affects the overall performance of the device. The transformer is the weak link in the device and hence, it decides the isolation ratings. That's how the isolation specs of ISOW78xx and ISO78xx are quite different.

    The isolated supply that we recommend to be used with digital isolator is SN650x based solution. SN6505BB should meet your requirement. Since it uses external transformer, isolation specs will be driven solely by the transformer.

    Let me know if you have any questions, thank you.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 40 points

    Hi Koteshwar,

    Thanks for your prompt reply. Is there a app-note/discussion of the ISOW784x's DC/DC isolation power transformer breakdown over lifetime anywhere? 

    Thanks,
    Brian

  • 0 in reply to Brian Bienvenu
    TI__Mastermind 49415 points

    Hi Brian,

    We do not have a separate App Note / document on ISOW784x DC/DC converter (excluding data channels) lifetime. The TDDB qualification is done on the whole device and hence the data is only available for whole of ISOW784x. The datasheet of ISOW784x has this TDDB plot provided, as you have already noticed.

    Since the DC/DC converter in ISOW784x is the weak link, the life of ISOW DC/DC is expected to end before the data channels. This means that if we were to plot only the DC/DC converter, it should ideally look like the ISOW784x whole device TDDB plot. Hence, you could equally use the TDDB plot to be ISOW DC/DC converter plot.

    Let me know if this answers your question, thank you.

    Regards,
    Koteshwar Rao