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Ringing from switch-mode PSU poluting power rails

Discussion in 'Electronic Basics' started by Daniel Kelly \(AKA Jack\), Oct 15, 2004.

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  1. Hi,

    I have a switch-mode powersupply unit that's injecting a high-frequency
    ringing onto the input power rails (which is from a battery). Does anyone
    have any ideas how to stop this? Would a simple diode and capacitor work?
    Or an inducer? In which case, how do I chose the value of the inducer?

    The DC-to-DC converter is:

    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1831

    Thanks a lot,
    Jack
     
  2. Most likely the ringing comes from radiation. Too long powerlines that are
    not properly decoupled or long and/or thin traces in the power circuit. The
    coil itself also may be a radiation source. You may have to redesign the
    layout. Keep the traces around chip and coil thick and short. The same for
    the other components. You may need some shielding or a ground plane. Keep
    the input lines away from the powersupply.

    I see no use for a diode in the power supply input. A decoupling capacitor
    is absolutely necessary as shown in the example circuit. If that capacitor
    is good there is no use for a coil. A good decoupling requires a low ESR
    elco of some tens uF (or larger, depending on the current) parallel to a
    ceramic capacitor of let's say 10nF.

    Main rule: Prevent the disturbing "signal" to escape from the circuit.

    petrus bitbyter
     
  3. Hi,

    Thanks loads for your ideas. Yes, I originally thought it was radiation.
    But then I cut the PCB in half, separating the powersupply and the mic
    pre-amp. This didn't solve the problem. Then I noticed that if I powered
    the switchmode PSU from a separate battery then the intereference went away.
    I've also done some tests with an oscilloscope and the PSU is DEFINITELY
    injecting noise into the battery.

    Here's a quick box diagram showing the circuits:


    Battery---+---------+
    | |
    PSU Amp
    |
    Camera
     
  4. Brian Howie

    Brian Howie Guest

    You might be getting ground/supply bounce on common return/suppy lines.
    Use a star point technique for both ground and supplies to minimise this.
    Make sure the prime supply ac impedance is low too. Put big C's at the
    SMPS ,Mic and star point. Use ground plane at the SMPS.

    If that doesn't work try one of these

    http://www.animationlibrary.com/Animation11/Creatures_and_Cartoons/Fairy_Tal
    e/Magic_wand.gif

    Brian

    --
    Brian Howie
    BAE SYSTEMS Avionics Limited
    Sensor Systems Division
    Crewe Toll Phase II, 1st Floor,
    Edinburgh EH5 2XS
    Phone +44 (0)131 343 8769
    FAX +44 (0)131 343 8941
    Email
     
  5. This circuit makes noise when the internal switch disconnects the
    LXP,LXN terminal from ground. This allows the inductor to snap
    positive, forward bias the output diode and dump the inductor current
    into the output capacitor. So the ground terminal of the output
    capacitor has fast rising current coming out of it. If this capacitor
    is not connected very closely to the bottom of the input capacitor and
    to the ground terminal of the chip (where the internal switch suddenly
    turns off the current), this current path switch produces voltage drop
    between these three points on the ground line. This voltage tends to
    leak out into the rest of the system.

    If you already have these three points closely connected, then you may
    have to add another small inductance in series with the input supply
    line. I would first try a bead on lead or other small inductance with
    a low DC resistance. It has to be rated to not saturate at the
    maximum current that will pass through it. Increasing the value or
    paralleling a couple caps at the input side may help. also.
     

  6. So the battery and its lines are prime suspect. You have to decouple the
    battery and the amp the same way you have to decouple the PSU. Then you have
    to use separate power lines for PSU and Amp directly from the decoupling
    capacitors of the battery so the powerline parts used by both simultaniously
    are as short as possible. That should be enough. But, as you know, "should
    be" is not always enough. You can try a coil in the powerline to the PSU, as
    close to it as possible. You can also try ferrite rings in the power lines.

    To rule out radiation further you can experiment packing camera, PSU and
    the powerlines to it in alufoil. (Pack it in a plastic bag first to prevent
    shorts.)

    (As an aside: I'm not interested in discussions about top posting. IMHO it's
    a free choice for everyone. I just brought down your responce for my own
    conveniance.)

    petrus bitbyter
     
  7. Mac

    Mac Guest

    I don't have tons of experience with this kind of problem, but I'll share
    my views anyway.

    For this configuration, it seems to me that adding another inductor
    between the battery and the power supply might not be necessary, since the
    current is already being drawn through an inductor.

    So, try a bigger (or lower ESR) capacitor to ground. Put the capacitor
    closer to the output inductor, and keep the ground lead short and fat. The
    chip itself undoubtedly produces current spikes when it turns its
    internal FET's on, so try putting a high frequency cap as close as
    possible to the chip (again, keep any leads as short as possible).

    If all else fails, put an inductor in series with the power supply line.
    Experiment with different values. You could always start by putting in
    another one of the main inductors, since you probably have them lying
    around. If that works, and if you want, you can try lower values until you
    find the smallest value that will work.

    Good luck.

    --Mac
     
  8. Thanks a lot, everyone!

    All these ideas have helped... but I'm still getting noise reaching the amp.
    Urg! So now I'm using two separate batteries! Not the most elegant
    solution but it works.

    Jack
     
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