pitfalls of series capacitors in power converters

Discussion in 'Power Electronics' started by ak52, Sep 19, 2016.

  1. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Hello Everyone,

    As the title says i would like to know any critical pitfalls and disadvantages of series capacitors,on high power converters.

    For example in a buck,if i require an input decoupling capacitor rated for at least 200v or an output LC filter capacitor:
    Would it be better to get a big bulk caps,which are rated for 500v and above - usually these are expensive.
    Or
    Connect lower rated capacitors in series to meet the required voltage,I am concerned about the ESR and other losses in this case.?

    All capacitors are electrolitic.

    Thanks in advance,
    AK
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Back in the day (yes, I'm old) a high value resistor was placed in parallel with each of the series capacitors. The effect was to attempt to prevent a capacitor having a high leakage resistance from seeing a higher voltage than those with lower leakage resistance. Folks with more modern know how may have a different opinion.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Without the resistors mentioned above, the applied DC will distribute itself according to the actual capacitance values. With low cost caps that have a +80%/-20% tolerance, it is very possible for your 500 V to be distributed 400V / 100 V instead of 250 / 250.

    ak
     
    cmartinez likes this.
  4. dannyf

    Well-Known Member

    Sep 13, 2015
    1,830
    366
    The issue here is leakage and resulting voltage distribution. You can get into a situation where the capacitor with the least leakage will take on more voltage than it can handle, leading to successive failure.

    Generally this can be cured with balancing resistors but they defeat the purpose of the capacitor.
     
  5. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Hmm..i see ,so it would be better to go with a single high voltage capacitor?

    Also would this problem be limited if i use ceramic caps..
     
    Last edited: Sep 20, 2016
  6. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,574
    2,545
    One way to limit the problem would be using tighter tolerance capacitors. But that would be a rather expensive solution.
    Another option would be using lower value capacitors with much higher voltage rating each. But again, price would be an issue.

    It's best to simply use a single capacitor with the proper voltage rating.
     
    #12 likes this.
  7. #12

    Expert

    Nov 30, 2010
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