H-bridge protection

Discussion in 'General Electronics Chat' started by Dritech, Nov 13, 2013.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi,

    I am going to use two BTS7960 in an H-bridge configuration. Since this IC use the same grounding for the power and the logic, is there a way to avoid noise or voltage leak to ground from interfering with voltage references or from damaging any sensitive ICs??
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    1,786
    Make separate runs back to the power supply for the logic ground and the power return. That way, whatever noise there might be has to go all the way to the supply in order to affect anything. Allocate more copper(wider and/or thicker traces) to the power return to lower the resistance and inductance.
     
    Last edited: Nov 13, 2013
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. Can you explain further please? I didn't really understand what you meant by that.
     
  4. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Also, will a Ferrite bead help to reduce distortion/noise?
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    The concept is straightforward. The ground pin on your chip should go directly to the location on the board where the power supply comes in. The ground return from the load should also go directly to the location on the board where the power supply comes in. These two traces should touch each other at EXACTLY one point. The point where the power supply comes into the board and NOWHERE else.

    Any noise on the ground return from the load has to "go past" the power supply before it gets to the ground pin of the chip. Similarly, and noise on the ground pin of the chip has to "go past" the power supply on it's way to the load.

    The naive way of doing the layout would connect the pins together at the chip and then run a trace to the place where the power supply enters the board. In this case noise on one ground is immediately coupled to the other.
     
    Last edited: Nov 13, 2013
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  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Can a Ferrite bead also help to reduce distortion/noise?
     
  7. TheComet

    Member

    Mar 11, 2013
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    12
    Usually it will, yes, but it depends on what frequency you're planning to drive your H-bridge at. Ferrite beads only start being effective at frequencies over 10 MHz.

    The most effective solution in your case would be an LC filter. Something around 100uF and 10uH begins filtering frequencies above 5kHz. You can combine that with a ferrite bead and another capacitor if you're worried about higher frequencies. Also make sure to use an electrolytic capacitor and a ceramic capacitor in parallel, as electrolytic capacitors are poor at filtering high frequencies.

    [​IMG]

    TheComet
     
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  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
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    Thanks a lot for the reply. So I connect the LC filter to the H-bridge power pin or to the logic pins?
     
  9. TheComet

    Member

    Mar 11, 2013
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    12
    The power pin. Logic pins don't need filtering.
     
  10. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi again, I was reading your post again and noticed that this is not possible for my application since the power grounding and the logic grounding of the BTS7960 are internally connected. Hence, I cannot separate both groundings. Am I right in saying that?
     
  11. TheComet

    Member

    Mar 11, 2013
    88
    12
    In that case, you can also connect the power ground to the same ground as the logic ground. Although this might sound like a bad thing, as long as you filter the power (+V) lead with something you'll be fine.
     
  12. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. I will be using PWM to control the motor speed. Can the LC filter effect the performance of the PWM?

    Also, where would be the best option to connect the PWM, to the H-bridge enable logic pins or with the H-bridge power pin? (please refer to attached diagram)

    Thanks in advance.
     
  13. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Can someone help me determine the values for the IS and SR resistors with the BTS7960 IC please?

    Any help will be appreciated.
     
  14. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi again,

    I decided to use the BTN7970 instead of the BTS7960. The datasheet recommend to use 10Kohms resistors with the IN and INH pins (logic pins). Since I am going to use opto isolators to switch the logic inputs (please see attached diagram), do I have to leave the resistor values at 10Kohms?

    If I change the resistor values to 1Kohms and make the opto isolator resistors at 10Kohms, will it work?

    Thanks in advance.
     
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