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ISO1211: I tried to add a high voltage protector to ISO1211

David Chance
Genius 4760 points
Part Number: ISO1211

I added a high voltage protector (http://ww1.microchip.com/downloads/en/AppNotes/AN-D66.pdf to the input of ISO1211, hoping to limit the input current to less than 10mA to prevent accidental high voltage applied to the inputs, but it doesn't work as I expected

When bypassing the protection circuit, ISO1211 works fine when I apply 30V to it (it outputs 1), but with the protection circuit, ISO1211 outputs 0 when I apply 30V to the input.

Anything I missed here?

Note: Please ignore the inaccurate symbol for the N-Channel Depletion-mode MOSFET

Thanks for any pointer!

  • 0
    TI__Expert 6955 points

    David,

    What is the reading on the sense pin when you apply 30V to the input of the bidirectional current limiter circuit?

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    Genius 4760 points
    When I remove the connection between the protection circuit and ISO1211, and apply 30V to the inputs, the output of the protection circuit reads 30V (the IN on ISO1211 is around 0V at this point), but once I reconnect the protection circuit and ISO1211, the IN on ISO1211 reads around 3V. I tried to short the 500 Ohm resistor between the two MOSFET, making the protection circuit ON all the time, the result is the same.
  • 0 in reply to David Chance
    TI__Expert 6955 points
    David,

    It appears that the protection circuit is causing too large of a voltage drop across it once connected to the ISO121x. The Rds on of the MOSFETs you are using are 850ohms typical which may be contributing to this. Have you considered any alternatives to mitigating high voltage exposure? Maybe TVS diodes or Varistors?

    Respectfully,
    Lucas
  • 0 in reply to Lucas Schulte9
    Genius 4760 points
    The spec sheet of ISO1211 doesn't spell much detail of its IN & SENSE.

    If we take the high voltage protection circuit as 1.7K (850x2), based on the voltage drop (30->3V), the equivalent impedance of ISO1211's IN/SENSE is less than 200 Ohm. This thought doesn't seem very comfortable when it needs to deal with 24-60V inputs

    Thanks!
  • 0 in reply to David Chance
    TI__Expert 6955 points

    I apologize for my delayed response here. The ISO1211 already has a built-in current limit (2.25mA). So the external circuit would never hit its current limit, and allow high voltage (>60V) to be applied to the ISO1211, hence damaging it.

    Looking at the D/S of the Depletion MOSFET used, the current limit circuit will not work (FETs don’t have enough drive).Even if the series 500 Ohm resistor is shorted, the current at VGS=0 is ~1.6mA < 2.25mA needed for the ISO121x to work reliably. When 500 Ohm is present, for a 2.25mA current to be established in the input path, one of the FETS would have to see -2V VGS, at which point the FET is well and truly off.

    Respectfully,

    Lucas