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Thread: High side switches reverse battery protection

  1. #1
    Beginner Beginner suly is on a distinguished road
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    High side switches reverse battery protection

    Hi everybody,

    I see that the Infineon smart high side switches provide battery reverse protection, by connecting a 150 ohm resistor in GND as a current limiter.
    Just as a reference see the note at the bottom of the first page in the ITS711 datasheet.

    http://goo.gl/UYabFM

    Nothing is stated about the power dissipation of this resistor, but, from the diagram at page 9 of the same datasheet, it appears to be

    (Vbb - Vtvs) ^2 / 150

    This is 5.68W, with 30V reverse battery voltage and an estimated 0.8V drop on the internal TVS diode, and then quite a large value.


    Is my estimation correct?

    Many thanks in advance for any comment.


    Marco

  2. #2
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    Luxun is on a distinguished road
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    Hi Marco,

    I think the internal zener diode is in blocking direction, how do you come up with 0.8 V drop voltage?

    Luxun

  3. #3
    Beginner Beginner suly is on a distinguished road
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    Hi Luxun,

    the internal zener diode is conducting because it is forward biased, i.e. with plus on anode and minus on cathode and then, in this situation, it acts as a normal diode.

    Please see attachment.



    Marco
    ?????
    Last edited by suly; Dec 22nd, 2015 at 08:27 AM.

  4. #4
    Beginner Beginner suly is on a distinguished road
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    It would be nice to have some more comments ...


    Marco

  5. #5
    b123
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    Hi,

    I think you can use a schottky diode instead of 150 ohm resistor for this purpose.

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    Hi b123,

    first of all, thanks for your comment.

    Your porposal seems to me very appropriate, but what makes me puzzled is why Infineon suggests the use of a resistor that ends up to be unreasonabling huge.
    What do you think?

    Marco

  7. #7
    Beginner Beginner suly is on a distinguished road
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    Hi Damon,

    I agree with you, but why Infineon reccomends to use a 150 ohm resistor in a lot of datasheets?


    Marco

  8. #8
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    Click image for larger version

Name:	RGND.PNG
Views:	0
Size:	53.1 KB
ID:	2228

    My guess is that this zener diode in the ITS711 datasheet (here in the picture VZ2) should explain the overvoltage protection. Regarding the reverse power protection there are only the path of the output with the load, where the load has to limit the current, and the path of GND to the Status pin ST, where RGND has to limit the current. Do you have a sample on hand to check this and measure the current into the pin GND with R=150Ohms connected?
    The views expressed here are my personal opinions, have not been reviewed or authorized by Infineon and do not necessarily represent the views of Infineon.

  9. #9
    Beginner Beginner suly is on a distinguished road
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    Hi Schulz,

    tanks for your comment


    Currently I don't have a chip to test but, from the schematic that you posted, we can see that, in the event of battery reverse, the main path that causes power dissipation on the board, is through VZ2 and Rgnd.
    In this situation VZ2 is conducting because it is forwarded biased, and then it acts as a normal diode with a drop voltage that we can estimate of about 0.7 1 V.

    So Rgnd is the only current limiting element, and it has to dissipate several watts.


    Marco
    Last edited by suly; Apr 5th, 2016 at 11:44 AM.

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