SMPS noise reduction

Discussion in 'The Projects Forum' started by witssq, Jan 29, 2012.

  1. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    The commercial SMPS 5V/1A is purchased. It have 140mV ripple, and then
    I reduced ripple 40mV using pi LC filter as attached schematic.

    But as you see on the scope view 2.jpg attached, about 330KHz transient(impulse) voltage exists. I would like to eliminate transient 40mV to make to 10mV ripple.

    Could you anybody help me?




    PS.
    1.jpg - No filter component
    2.jpg - After filter applied
     
    • SMPSNoise.pdf
      File size:
      37.5 KB
      Views:
      199
    • 1.jpg
      1.jpg
      File size:
      219.4 KB
      Views:
      208
    • 2.jpg
      2.jpg
      File size:
      220.2 KB
      Views:
      181
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Maybe some ferite beads or the classical noise supressing ferrite toroids could help?
     
  3. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    As I may know, the ferrit bead is used for high frequency above 1Mhz.
    If it is true, It may vbe useless. How do you think about?
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    This is the world of EMI, I spent about 25 years fighting it. There are no magic fixes. It radiates so easily that the wires inside a power supply always pick some of it up. You have to do the mechanical design to contain the E MI radiating components and use filtering into th "clean areas". One solution:

    Build a steel box with banana plugs on one end to plug into the front of your P/S and jacks on the front. Inside the box, put some good bypass caps and series L filters. You can reduce the ripple based on how good you build the filter.
     
  5. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    Thanks for your response.

    Please advise me whether or not Twin-t notch filer could eliminate the above transiend transient.
    If possible, target frequecy become 330KHz because that the transiend interval is 3.03ms. is this right?

    The calculator/schematic is linked as http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm

    Regards
    SunSung Hwang
     
    Last edited: Jan 30, 2012
  6. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    No the target frequency is the frequency in the transient itself . From you picture. I would say it has period around .2 usec roughly estimated or about 5 MHz. It is important to use both caps and inductors that are suited for such high frequency
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    The frequency at which the noise repeats is not what you have the problem with: look at the noise waveform. The sharply rising edges have components that go into the hundreds of Mega Hertz. That's whay they are so hard to kill, the radiate into just about any short wire they can find.

    Use a low pass filter. As stated, you must use caps that are good at mega hertz ranges like ceramic caps and possibly polyester film caps. Aluminum electrolytics and tantalums will have very little effect since the noise frequencies are above their resonant frequency.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Just from looking on the picture I would say that the fundamental of the noise is about 10-20MHz. Just try the ferrite beads and see if it helps. Also try measuring the noise with faster timebase and higher bandwidth probes.
     
  9. witssq

    Thread Starter Active Member

    Mar 29, 2009
    48
    0
    It seems to be about 15-20Mhz. I applied ferrit bead, Noise is attenuated
    from 120mV pk to pk, 80mV pkpk. but I need 5-10mV. I seems more to
    do.

    I will test with another smps, then return here.


    Thank you all.
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,013
    3,233
    The capacitor and ground connections must be as short as possible.

    You might try a feed-through type capacitor or filter. They minimize the effect of wire inductance which is a problem with high frequency noise.
     
Loading...