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ISO1211: Surge protection and SIL diagnostics

Part Number: ISO1211

Hi,

I'm working on a industrial input/output combination which needs to be certified for SIL level 2. For this I want to be able to test if my input is still working correctly. To do so I have the following circuit;

Now I have 2 questions regarding this:

  1. I want to verify the correct working of my input (for diagnostics), therefor I am switching the FGnd with a MOSFET to logic GND, should there be any decoupling between these?
  2. For my input protection I'm using a Pi filter, a freewheel and TVS diode, however I'm not sure if the chosen TVS diode is the needed size (possibly too big), any hints on how I can calculate the correct specs for this, as in the ISO1211 datasheet much smaller TVS diodes are recommended.
  • Hi J-J,

    Welcome to TI E2E forum!

    Thanks for sharing the schematic and details related to your questions. ISO1211 is usually used to isolate a field digital input in industrial applications and if there is no isolation requirement, then you might not be making the best use of ISO1211. I see that the schematic has GND node on both left and right side of ISO1211 indicating that isolation isn't needed in your application. Could you please confirm this?

    I want to verify the correct working of my input (for diagnostics), therefor I am switching the FGnd with a MOSFET to logic GND, should there be any decoupling between these?

    This is not a problem for device operation but I am not quite sure if this is going to benefit you in anyway. Shorting FGND pin to GND1 will only short the two internal GNDs in ISO1211 thereby compromising isolation. But it doesn't make the input signal (from right side) bypass the device and go to the output (the left side). It still will pass through the device internal circuit the device behavior is still going to be the same. I am assuming that you meant to bypass the device by shorting FGND and GND1 but that is not going to happen.

    For my input protection I'm using a Pi filter, a freewheel and TVS diode, however I'm not sure if the chosen TVS diode is the needed size (possibly too big), any hints on how I can calculate the correct specs for this, as in the ISO1211 datasheet much smaller TVS diodes are recommended.

    The TVS diode is chosen such that its max clamping voltage doesn't exceed device's max voltage rating when it is connected directly to device pins. The diode chosen seems to be perfect to be connecting between SENSE pin and FGND.
    We can talk more about this once find out if isolation/ISO1211 is really needed for your application. Thanks.


    Regards,
    Koteshwar Rao

  • Thank you for your reply.

    I think I need to clarify a bit more about the application;

    This is an input/output combination where I can use it as a high-side output (2A up to 32V) or a (sinking) input for input type 1 and 3 (according to EN 61131-2).

    Due to the nature of the input/output combination I cannot take advantage of the isolation (configured as output there needs to be a path to GND/GND1). 

    For SIL diagnostics I wish to check whether the input and output working so when used as output I can read the input to see if it switches. When used as input the normal state would be high and I cannot control this, this is where the MOSFET is used for, to pull the input low for a small amount of time to verify the correct working of the input.

    To be fair, I'm using the ISO1211 for the voltage conversion and current limiter which I need for the Type 1 and 3 input and not for isolation which I cannot benefit from due to the input/output combination.

    About the TVS diode, I need to place it at the input/output pin as it needs to protect both the high-side switch and input. But I'm not sure if this diode is over dimensioned.

  • Hi J-J,

    Thanks for further inputs.

    Please refer to the application diagram shown in the first page of ISO1211 datasheet, copying below for your quick reference. As you can see, the input signal from the field is referenced to FGND pin of device and the SENSE/IN pins are going to sense input voltage correctly only when FGND is connected to the return/reference terminal of the sensor/switch input from the field.

    In your application, I am assuming DIO is referenced to the system ground GND and hence, FGND should be permanently connected to GND for the device work. Thus, there is no need of using MOSFET Q1.

    Connect both FGND (pin 6) and GND1 (pin 4) to system ground GND and the device should work fine in all conditions. This does short remove isolation barrier between field and logic side but that shouldn't be a problem as you do not require isolation and you are only using ISO1211 for its circuit.

    About the TVS diode, I need to place it at the input/output pin as it needs to protect both the high-side switch and input. But I'm not sure if this diode is over dimensioned.

    Since the BSP772T is lower voltage rated than ISO1211, I would ideally choose TVS diode with a max clamping voltage that is less than the absolute maximum voltage of Vbb or OUT of BSP772T which seems to be 40V from the datasheet. SMCJ24A should be a good option.

    I hope this helps you with all the information you were looking for. Let me know if you have any further questions. If you have a question on a different topic or another device, please consider posting a new question to help others find answers by searching. Thanks.


    Regards,
    Koteshwar Rao

  • When I connect FGND to GND1 I cannot pull the signal low for diagnostics, this is why I'm aiming to use the MOSFET to be able to pull the signal low and know the input is still able to switch. Or am I missing something in my reasoning?

  • Hi J-J,

    I am not quite sure if I completely understand what you are trying to explain.

    Did you mean to say that you could not make DIO or IN/SENSE pin LOW?
    ISO1211 operates in a way that when voltage is applied to IN/SENSE pin and is above its VIH threshold then it sends a HIGH to the other side ISO1211. When no voltage is applied or applied voltage is less than VIL threshold of ISO1211, then the output of ISO1211 defaults to LOW. So you don't necessarily have to make OUT pin of BSP772T go to 0V, just turning OFF BSP772T by making its IN pin LOW is enough to make ISO1211 output go to LOW.

    I do not see any reason why connecting FGND to GND1 should cause any issue. I would expect BSP772T to still be able to make DIO input ON and OFF.

    If you still see any concerns, please do share more details why you think it is not going to work. Thanks.


    Regards,
    Koteshwar Rao

  • Hopefully this makes thing a bit more clear; the circuit has 2 functions, it can function as an output of 32V 2A and it can be an industrial (PLC) input according to a type 1 or 3 input (sensor).

    I want to be able to tell of the high side switch is working correctly and to do so I'm using the ISO1211 to verify it is able to switch;

    High side tests:
    When ON, ISO1211 reads high, toggle the high side switch 1ms to verify it is working properly (ISO1211 reads low)
    When OFF, ISO1211 reads low, toggle the high side switch 1ms to verify it is working properly (ISO1211 reads high)

    When used as industrial input it is connected as a sinking device and therefor DIO is connected to VCC with a switch/sensor:
    Input tests:
    When sensor is closed, ISO1211 reads high, toggle MOSFET (pull low signal) 1ms to verify it is working properly (ISO1211 reads low)(this is the situation where the MOSFET is needed for)
    When sensor is open, ISO1211 reads low, toggle the high side switch 1ms to verify it is working properly (ISO1211 reads high)

    That's the reasoning why I need to switch/disconnect the FGND, does that make sense?

  • Hi J-J,

    Thanks for sharing further clarification, this is helpful to understand.

    I think I understood why you are using MOSFET Q1 between FGND and system GND. I see you are using Q1 to disable / enable (disconnect / connect) the field side of ISO1211 to verify if the output on the logic / MCU side (VCC1 side) of ISO1211 is legit. This is perfectly fine and device should work fine accordingly. Thanks.


    Regards,
    Koteshwar Rao