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Original question:

Reverse polarity protection - proper TVS location

Reverse polarity protection - proper TVS type and location

Stanislav Dzurila
Prodigy 10 points

Hello to all, 

I have a question, what is needed, what will work and what shouldn't be done.

My question is regarding correct choosing of components and their order. This is for automotive, and that means it must undergone all of the automotive certifications.

My main concern is regarding surge, transient and reverse polarity protection. 

"Case 1" order:  Input (13,5V) --> bi-dir TVS --> Schottky diode --> buck converter (12V --> 5V, 3V8, etc.)

"Case 2" order:  Input (13,5V) --> Schottky diode --> uni-dir TVS --> buck converter (12V --> 5V, 3V8, etc.)

Is "Case 2" usable in your oppinion?  During testing there are all the starting pulses, reverse polarity, transient pulses (ISO 7637-2, ISO 16750-2, etc.). 

Thank you in advance.

 

  • 0
    Prodigy 10 points

    Or also "Case 3" order:  Input (13,5V) --> Schottky diode --> bi-dir TVS --> buck converter (12V --> 5V, 3V8, etc.)

    I also want to avoid negative voltages / transients during reverse polarity on the board because of some sensitive circuits on the PCB. 

    Also one must thing about over-engineered design because of a mass-producion.

     

    Case 1:

    Case 1

    Case 2:

    Case 2

    Case 3:

    Case 3

  • 0 in reply to Stanislav Dzurila
    TI__Expert 5290 points

    Hi Stanislav,

    The TVS should be placed before the Schottky diode, as to prevent the surge current from flowing through the Schottky. If you are worried about reverse polarity protection, I would use a bi-directional TVS, the same way you have listed in "Case 1".

    Please let me know if you have any questions!

    Best Regards,

    Josh Prushing

  • 0 in reply to Josh Prushing
    Prodigy 10 points

    Hi Josh,

    Thank you for your helpful reply.

    I have another question: we have produced also some peaces in the "Case 2" configuration, and it has passed all the tests. My question is, how bad is this solution? What do you think please? 

    Thank you in advance.

    Best regards,

    Stanislav Dzurila 

  • +1 in reply to Stanislav Dzurila
    TI__Expert 5290 points

    Hi Stanislav,

    Case 2 may be viable for some scenarios, but the intent of putting the TVS diode first is to send any surge current to ground as soon as possible. Having this surge current flow through the Schottky poses a threat for potential damage depending on the ratings of the Schottky diode. The TVS is designed to handle these intense events, and will offer protection for any downstream circuitry that may not be able to withstand the high voltage/surge events that can occur.

    Best Regards,

    Josh Prushing