Capacitor values on switching buck boost power supply?

Discussion in 'General Electronics Chat' started by TheLaw, Dec 24, 2010.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Hello,

    -----Short Version-----

    Does the capacitance value of the primary side capacitor and the secondary side capacitors of a switching power supply matter?

    Can I use capacitors with higher capacitances instead?


    -----Long Version------

    I was going to build an AC>DC Power supply, but after quarreling with the thought of death by electrocution, I decided to do something a little less potentially hazardous:
    http://www.ladyada.net/make/mintyboost/process.html

    Essentially the idea is to use a buck boost type topology to be able to charge 5V USB devices. So, it is pretty straightforward, BUT, when I build an electronics project, I like to put more focus on performance/quality than on being economic. Of course, I won't be throwing in a $25 tantalum capacitor, but rather above average components.

    So regarding the component selection. I've read the datasheet thoroughly, and for the main smoothing/input cap, it seems that the value does not have to be exact. It says anywhere from 10uF to 100uF is acceptable. I was curious as to what would happen if say I used a 220uF cap in its place. It should theoretically reduce ripple, but would it have any negative effects?

    Switching power supplies are more picky about components than linear, I have determined. But does the value of that capacitor matter?

    In addition to that, what effect does the value of the output capacitor have? The more farads the merrier? Or not?

    Inductors, I have learned, are generally not to be played with. It seems that their values must be very precise. So that remains the same.

    I just got a bulk shipment of components in from Mouser and I'm hoping to actually get a project going after quite some time. (I've spent more time trying to pick out a project than actually building them!)

    Merry Christmas/Happy Holidays.

    Thanks a lot.
     
    Last edited: Dec 24, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    220uF might be OK, but the most important thing is that it have a low ESR. As capacitors get larger in size physically, they typically have greater parasitics (leakage current, inductance, resistance) and that would tend to negate the advantage of greater capacitance.

    You'll frequently see large and small caps used in parallel; the large cap is for the low-frequency transients, and the small cap for the high frequency transients.
     
  3. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    They would both be the same type of capacitor? ie both electrolytic?

    Thanks.

    Say I got a nice low ESR polymer that was 220uF...Should that be better than a 100uF capacitor with the same ESR?

    Thanks again.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You'll usually see something like a 10uF-470uF aluminum electrolytic in parallel with a 0.1uF ceramic or metalized poly cap.

    If they have the same ESR, then sure.
    Of course, you could use two 100uF low-ESR in parallel; then the parallel ESR would be cut in half.
     
  5. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    The transient response of the converter, and the closed loop stability may be impacted. You may want to assess the sensitivity of loop response to output capacitance (C + ESR).

    Yes, the ripple level may change. The converter footprint may change. The inrush current may increase. You probably don't have a voltage rail sequencing concern.
    Do you need extra capacitance for a reason?

    You mention ripple - is that your driving force, because if it is then I suggest you firstly assess the many other reasons for ripple generation and management in a switchmode than just output capacitance size.

    Ciao, Tim
     
  6. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Thanks for the responses.

    The only reason I am asking is because there wasn't too much information about capacitor selection in the MAX756 datasheet and thus I was a little confused.

    I haven't ever worked with switching supplies and from what I understand, they have a different set of needs when compared to a linear supply.

    It was mostly out of curiosity.

    So if you used two low ESR 100uF caps in parallel, then the total ESR would only be the ESR of one cap? So if CapX's ESR = 5 and CapY's ESR = 5, if they were in parallel, the rough ESR total would be ~5?

    And then for even better performance, I could parallel a ceramic cap (CapZ) next to the two already in parallel.

    Would the addition of a polypropolene cap do anything beneficial? Or are too many caps bad for transient response?

    Hopefully I got this right. Thanks.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No; the ESR would be /2 = 2.5.

    You could do that.

    Metalized poly caps are good; better than ceramic. Small caps help with the high frequency transients.
     
  8. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Should I use a ceramic AND poly? Or just poly? And to filter different frequencies of transients, does the physical size of the capacitor or the value of its capacitance effect what frequency it filters?

    Thanks.
     
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