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Boost converter - ripple current

  1. Oct 20, 2013 #1
    Whilst looking at the possible parts for a boost converter I came across references to making sure that the capacitor could handle the "ripple current".

    I looked a bit further and can see 2 possibilities.
    1) The ripple in the capacitor current itself ie the difference between the minimum and maximum capacitor current.
    2) The other possibility is that there is a ripple current called delta i,L which is mainly affected by the size of the inductor for given input voltage and duty cycle.

    I found a capacitor which lists its "ripple current" rating as 0.8A/800 mA. Does this mean it can handle a ripple of 0.8A in the inductor current or can its own current only vary by +/- 0.8A?

    The source of my confusion may be misunderstanding a page here - http://www.analog.com/library/analogdialogue/archives/45-09/boost.html

    "The current ripple, shown as ΔIL ...The average inductor current flows into the load, while the ripple current flows into the output capacitor"
     
    Last edited: Oct 20, 2013
  2. jcsd
  3. Oct 20, 2013 #2

    Baluncore

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    Science Advisor

    The ripple current is the average current flowing in the capacitor leads.

    If the load current is continuous and a converter produced bursts of higher current, then the ripple current is the average of the absolute tidal current.

    Say output current is 10A and current pulses from converter are 40A, (for 25% of the cycle), then output capacitor ripple current is ((40A – 10A) * 0.25 ) + (0.75 * 10A) = 7.5A + 7.5A = 15A (Note that the two terms have the same value).

    Capacitor ESR is also important in analysis because it produces step changes in capacitor voltage, rather than the ramp due to charge and discharge. The voltage ripple should include the ESR step component.
     
  4. Oct 20, 2013 #3
    Thank you very much. It was a little more complicated than I thought but your explanation was very helpful.
     
  5. Oct 20, 2013 #4

    meBigGuy

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    Gold Member

    The ripple current in the capacitor is also important because it causes power dissipation. When they say a capacitor is rated for 800ma ripple current it means it can handle the power dissipation associated with that current and it's effective series resistance.
    For example, in this data sheet there is a 100uF low esr capacitor with 1 ohm ESR and is rated at 100ma ripple current at 125C. The capacitors with lower ESR have higher ripple current ratings.
    http://www.mouser.com/ds/2/315/ABA0000CE101-183196.pdf

    If you exceed the ripple current rating the capacitor will degrade and fail. Choosing the proper input and output capacitors is an important part of switching converter design.
     
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