How ESR works in Buck Converter

Discussion in 'General Electronics Chat' started by Guinness, Nov 24, 2010.

  1. Guinness

    Thread Starter Member

    Dec 31, 2009
    I have read up a lot on how buck converters work over the last week, I understand everything I have read and all the equations to work out the parts values.

    But the only thing I can't seem to grasp is what the Capacitor ESR is all about?
    What it does to the circuit and how to work out what value ESR I need?

    If someone could enlighten me then I can start to build my first sample.

  2. tom66

    Senior Member

    May 9, 2009
    ESR is equivalent series resistance. ESR is usually undesired; it is a parasitic property of the capacitors' construction. Higher ESR leads to higher internal temperatures in the capacitor (shortening lifespan), lower efficiency and poorer transient response.

    For a buck regulator, find a capacitor with a low ESR. Low would generally be less than 0.5 ohms. Ceramic capacitors are ideal, but they are not available in very large capacitances (max around 47µF) so electrolytics or tantalums must be used.

    There is one exception, though it's not for buck regulators: LDO's. Low dropout regulators often require a capacitor with an ESR in a certain range, for example 0.5 < ESR < 5 ohms.
  3. Guinness

    Thread Starter Member

    Dec 31, 2009
    I will make sure to stay below 0.5 ohms and lower if possible then.

    Cheers for making it nice and simple for me to understand. Shame other places can't just make it that simple. Spent 2 days just searching the internet for a answer like the one you gave.

    Time to go build

    Thanks again
  4. mik3

    Senior Member

    Feb 4, 2008
    Choose an ESR value which gives you an output ripple voltage less than the desired. Usually the output ripple voltage due to capacitance is satisfied if you find the capacitor with the calculated ESR.
  5. timrobbins

    Senior Member

    Aug 29, 2009
    The cap ESR also influences the transfer function of the converter, and hence the stability response. If control response is on your radar then you will need to cross-check the influence of the ESR level you may be choosing.