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BQ79731-Q1: Insulation Detection feature of BQ79731-Q1

Nishant Kumar
Prodigy 132 points

Part Number: BQ79731-Q1

Tool/software:

Hello All,
I want to implement isolation monitoring method using BQ79731-Q1. I am referring to datasheet and EVM board document to understand how to implement. 


1. In this datasheet it is mentioned to control SW1, During ON and During OFF condition of SW1.
And solve two equations for Risop and Rison. What about SW2 and SW3. Can someone explain this in detail?

2. And I am using TIDA - 010232 as reference, for isolation monitoring. The method mentioned in TIDA - 010232 cannot be used using this IC? 

Thanks, and Regards
Nishant Kumar

  • 0
    TI__Expert 6030 points

    Hi Nishant,

    Sorry for the delayed reply.

    What are you planning on using SW2 and SW3 for? You only need SW1 to measure insolation resistance. SW2 and SW3 can be used however you would like, they are there to offer more options, weather that is redundancy, or isolation measurement at a different point.

    TIDA - 010232 is a separate system, you can use it to measure isolation resistance or you can use BQ79731.

    Thank you,

    Zachary

  • 0 in reply to Zachary Balov
    Prodigy 132 points

    Hi Zachary, Sorry for the late reply.
    Thanks for your response. 
    In the use case mentioned in BQ79731-Q1, what is the use of SW3 and SW2.

    1. To isolate chassis ground SW2 is used?
    2. In this use case SW3 is permanently connected?
    3. How is redundancy maintained in BQ79731-Q1?
    4. What equations we will get if using BQ79731-Q1 method?
    5. Why TPSI2140QDQRWQDQ1 is used in SW2 position and at SW1 and SW4 positions only MOSFETs are used?
    6. Our system is designed for 96V system. So, how to select the resistor values R1, R2, R3 and R4?



    If possible, can you explain the difference between BQ79731-Q1 method and TIDA - 010232?


  • 0 in reply to Nishant Kumar
    TI__Expert 6030 points

    HI Nishant,

    Sorry, I don't fully understand the line of questioning. Are you trying to copy this example without understanding its purpose? However, I fully understand your confusion. The drawing includes SW2 and SW3 as if they have a purpose in this design, but it seems that they don't; SW2 will always remain open and SW3 will always remain closed in this example.

    1) Please reference the FMEDA documentation in mysecure to understand our general redundancy implementations on BQ79731. My comment was made in reference to the other 2 SW pins. They exist to offer customers options if, for example, they would like to implement redundancy in their custom solutions. 

    2) The equations are derived simply from 2 states. SW1: 0; SW2; 0; SW3; 1 and SW1:1; SW2:0; and SW3:1. With these 2 states, you can easily create a simple system of equations to calculate your unknowns. Why this was not included in the DS I am unsure. 

    3) Can you please let me know where you are referencing this schematic from?

    4) Unfortunately, I am not an expert on the TIDA solution so I am unable to answer this question fully.

    This part of the datasheet is understandable confusing so please let me know if you need further explanation.

    Thank you,

    Zachary