How to select bypadd capacitor to filter 1.4MHz?

Discussion in 'General Electronics Chat' started by tprofits, Mar 26, 2013.

  1. tprofits

    Thread Starter New Member

    Mar 25, 2013
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    I have a SMPS that has a 1.4MHz transient I'm trying to shunt to ground. How do I select the correct capacitor to filter 1.4MHz?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Capacitors filter a broad range of frequencies with their impedance inversely proportional to frequency and capacitor size. You want a capacitor with a low ESR and inductance, such as ceramic types, perhaps in parallel with a larger electrolytic capacitor.

    Such transient noise from an SMPS is very hard to filter and may require a inductor along with the capacitor or perhaps a feedthrough filter or common-mode choke. Without knowing your configuration, it's difficult to make better recommendations.

    Where is the noise appearing?
     
  3. tprofits

    Thread Starter New Member

    Mar 25, 2013
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    I have the following images. One is a screen shot of the transient. The second is the circuit. I'm thinking a capacitor in parallel with the 68uF might help. But, I'm not sure how to calculate the correct capacitor?

    [​IMG]
    [​IMG]
     
  4. crutschow

    Expert

    Mar 14, 2008
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    There is no "correct" value. You just need a very low impedance capacitor to high frequencies which is typically provided by small ceramic types of 0.1μF. The problem generally is the inductance of the connections to the capacitor. Even a mm of wire has a significant inductive impedance to high frequencies, thus the best way to connect the capacitor is a surface-mount type connected directly to the ground plane with the trace being filtered running through (in and out of) the other capacitor pad.

    Alternately some type of feed-through capacitor connected directly to a grounded chassis can help, or an added inductor in series with the output with another capacitor to ground.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Putting an inductor in series with a capacitor is not a good idea. This is a resonant circuit.
    100μH in series with 68μF will resonate at 2kHz.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I bet that is a part of a buck switching converter.
     
  7. tprofits

    Thread Starter New Member

    Mar 25, 2013
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    crutschow - I was using 4" leads of wire soldered to my PCB. I'm going to add a 0.1μF cap in series with the 68μF (with a short lead wire). I'll record the results. Thanks!
     
  8. kubeek

    AAC Fanatic!

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    4 inch wires between the transistor and the output filter? That will hardly ever work well.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    As long as there is no source of 2kHz to excite the resonant circuit, it shouldn't be a problem.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    4" wires are a no-no.
    And you want the 0.1μF cap in parallel not in series.
    How short is "short"?
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    Yes, sorry about my post. I noticed all SMPS do the same thing.
     
  12. tprofits

    Thread Starter New Member

    Mar 25, 2013
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    Thanks for the assistance! I tried a 0.22μF, then a 0.1μF and then a 0.01μF ceramic capacitor in parallel with the 68μF capacitor. It didn't fix the transient. Would you mind double checking my scope calculation. Am I targeting in on the right frequency (1.33MHz)? The capacitor wires measured 4.5mm.

    [​IMG]
     
  13. kubeek

    AAC Fanatic!

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    Can you post a picture of your setup and the whole schematic?
     
  14. tprofits

    Thread Starter New Member

    Mar 25, 2013
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    Here is the schematic. Does this help with any ideas on how to reduce the transient or the ripple amplitude?
    [​IMG]
     
  15. kubeek

    AAC Fanatic!

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    How long are the ground and power traces to the diode, induictor and output cap?
    Can you post a picture of the physical layout?
     
  16. tprofits

    Thread Starter New Member

    Mar 25, 2013
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    Here is the PCB.

    [​IMG]
     
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