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SMPS design EMC conducted noise problem

Discussion in 'Electronic Design' started by [email protected], Jun 3, 2007.

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  1. Guest


    My first SMPS (switching mode power supply) design's conducted signal
    noise, major one is the 260KHz, which is the SMPS switching frequency,
    is over the CISPB standard by 2.48dB.

    How do I suppres the 260KHz signal down which is conducted back to the
    power supply line?

    The chip I used is the National LM2672-12, for a 12V DC output. The
    current consumption of the whole circuit is about 120mA-130mA. Power
    supply in is 24VAC.

    There is a 220uF SMD electrolytic capacitor and a 100nF SMD ceramic
    capacitor served as the input cap. The 220uF cap also serves as the
    filtering capacitor after a half bridge rectifier/diode.

    The output cap I used is the same 220uF SMD electrolytic capacitor.
    Its impedance is 180mOhm @20degC 100KHz. Datasheet link for this cap
    There is another 100nF capacitor in parallel with the 220uF output

    There is a gound plane on the top of the PCB, but not very big, and
    its shape has changed to a very uncommon one to allow other wider
    pads. The ground plane shape is like a "O", but with a small open at
    the left bottom corner.

  2. MooseFET

    MooseFET Guest

    I take it from this that you don't have any impedance between the
    bridge and the switcher. You typically need a small impedance in that
    path so that the impedance going back out to the power lines is
    greater than the impedance of the input filter. If you can't
    rearrange things so that an existing impedance does this, adding an
    inductor here is likely to help.

    This sounds very bad. Is this only a 2 sided PCB?

    You need to make all the ground currents in a switcher get to the
    common point with a low enoigh impedance that it doesn't find another
    path shuch as out th eoutput side wires and back in on the mains
  3. Guest

    Many thanks for your answer.
    It is a 2 sided PCB. Would a 4-layer PCB help?

  4. legg

    legg Guest

    I'm unfamiliar with a CISPB standard.

    CISPR standards, set by the "Comite International Special des
    Perturbations Radioelectriques" include references to Class B
    information technology equipment in the CISPR22 standard. Is this what
    you are testing to, using the Line Impedance Stabilization Network
    (LISN) of CISPR11?

    If so, then conducted emissions will see a dierential mode source
    impedance of 50 ohms, and a common mode source impedance of roughly
    half that. I'm unsure how this unit of yours can be tested without a
    wall-wart source, as conducted emission limits are only intended to
    apply to the common AC line ports (commonly 120 or 240 VAC).

    Class B levels require interference currents to be reduced to the
    microamp range. Simple KVL and KCL impedance calculations suggest that
    increasing the impedance of the source (ie the power supply) will be
    required to inhibit switching current flow in the input wiring, during
    input rectifier conduction periods, if the wall-wart doesn't produce
    this impedance transformation.

    At this frequency, the ground plane has little effect on conducted
    interference of a single converter operating from a two-wire input,
    unless an inappropriate connection is made to the safety earth ground
    plane in the test site. The input current to the buck regulator is
    discontinuous and pulsating with a strong fundamental, which must be
    supplied solely by your local capacitors.

    Perhaps you could clarify both your reference to the standards and
    your test set-up.

  5. MooseFET

    MooseFET Guest

    Sometimes you have to go 4 layers. If you can't make a good ground
    with only double sided, it is worth going to 4 layers.

    Before going to the added cost, you can try filling the other side of
    the PCB with ground traces and adding a bunch of vias to hook the top
    and bottom ground connections together. Basically, you do it like

    Assuming most of the top side traces run north-south and most of the
    bottom side traces run east-west these will also be the directions of
    the gaps in the ground. On both sides, there will be grounds between
    the traces. You add the vias as needed to hook the grounds to the
    grounds to shorten the current paths.
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