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Common Mode choke and grounding

Discussion in 'Electronic Design' started by AJ, Aug 18, 2005.

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  1. AJ

    AJ Guest

    Hi,

    I am looking for opinions on grounding techniques, particularly in
    automotive circuits. I have designed a circuit that uses both digital and
    analogue circuits and I was planning to connect these grounds at a single
    point to attempt to keep digital noise away form the analogue section. I am
    also using a switch mode power supply operating at 300KHz so I was thinking
    of using a common mode choke on the input to help EMC. I was wondering the
    best way to connect this, I am a bit confused......

    I connect positive and ground to one side of the choke so on the other side
    I will have a filtered positive and a filtered ground. Does this then give
    me a 3rd ground? I was thinking about connecting the unfiltered ground to
    the Chassis and running a ring around the outside of my PCB on both layers.
    I was then going to connect the digital ground the filtered output after the
    choke and connect the analogue ground like described above Some circuits I
    have seen appear to have the ground connected together before and after the
    choke so I cant see what the pint is of filtering the ground in the first
    place in that example..

    I hope I have been clear on what it is that I am trying to do and really
    appreciate any feedback

    Best Regards

    AJ
     
  2. It gives you a ground that has instantaneous voltage variations from
    the ground on the other side of the choke.
    I agree. A simple inductor in the positive side of the supply would
    make more sense.
    I think it is a good idea to draw a schematic of your circuit and all
    its external connections, with each wire drawn as an inductor. Then
    try to visualize (or actually simulate) what external sources of noise
    and internal sources of current variation (like those produced by that
    switcher) will do to the voltage across those inductors. If the DC
    currents are significant, or the circuit has large DC sensitivities,
    include resistance in each connection, so you can think about where
    the DC drops will be, also.

    Just throwing filter components or ground planes at a circuit without
    having a specific purpose for each (based on circuit analysis) is
    unlikely to provide anything like an optimal use of the available
    choices. Every resistive drop and inductive voltage needs to be
    accounted and compensated for or the compromise consciously chosen.
     
  3. Yes: to be kept separate from unfiltered ground, otherwise the CM choke
    is effectively short-circuited (in BOTH legs).
    Correct, and connect it to nothing else.
    No, keep it OFF the board entirely. It would act as an antenna, spraying
    the smog that you CM choke filtered out back into your signal circuits.
    Not described clearly enough for me. Connect it to the filtered ground,
    the same as for the digital ground.
    See above. It can't be right to do that, AFAICS.
     
  4. Ken Smith

    Ken Smith Guest

    So far so good but what are the I/O connections on this device. The wires
    entering and leaving the device are where the trouble comes in. You
    really need to consider this before you go any further.
    Imagine your switcher being put inside a box. All the connections of the
    supply go through one single hole in the side of the box. Each wire has a
    capacitor just inside the box that RF bypasses it to the box. Each wire
    then passes through a core and has a second bypass capacitor outside the
    box. Even the ground wires of the supply are handled this way. The box
    has a ground wire attached to it near the opening and no other connections
    to it.

    If you do this, there is no way for RF created inside the box to get
    outside the box. This concept is what you are trying to duplicate with
    the common mode chokes etc.
    If anything, you want to run the filtered ground to the rings around the
    PCB. Breaking the ring and putting a resistor in the break can be used to
    make the ring lossy if you want to be extreme about things.
     
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