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Thread: Is leakage current of capacitor important?

  1. #11
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    Quote Originally Posted by Jon Gerow View Post
    Leakage current of a capacitor is not in direct proportion to the leak current measured of a power supply. Don't confuse the two.

    That is why I put the word ground in

    "Now for medical grade power supplies leakage to ground is an issue."

    do I know what I am taking about? I thinketh not.

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    Leakage current is left as a footnote only for datasheets on which it can affort to be a footnote because its importance is directly determined by the application.

    Mentioning medical power supplies is a great move as it illustrates exactly that point.

    That said, since ashiekh is after knowledge, let's put everything on the table and then sort it out.

    Leakage current is a broad term generally used to identify and signify an unwanted migration of electric charge.
    It can be considered internally or externally to the capacitor, i.e. there are, in general, two types of leakage occurring.

    The first one is due to the dielectric not being an ideal dielectric; meaning it allows some charge mobility and thus recombination.
    The net result is that the capacitor plates/electrodes will be in partial contact through an electrically semi-porous insulator, which leads to charge recombination and thus a drop in the stored energy. This makes the capacitor less suitable for long-term storage, but for applications such as switched-mode power supplies, you generally don't care as the effect is slow and miniscule compared to the operational parameters of the capacitor and what's expected of it.

    The second leakage mode happens due to the capacitor being a part of a complex circuit, which provides alternate paths to the stored electric charge to move to. This one is a big deal for medical supplies as well as for precision measurement tools and for high-quality audio circuits, because it can lead to loss of isolation between different circuits and/or coupling. This means parasitic signals, false positives or unwanted bridging of logical/data and power circuits.

    The leakage current figure given in most datasheets is actually a sum effect of the two leakage modes, and as such is not always directly usable. It is, usually, the worst-case scenario for both applications (energy storage and isolation/decoupling), but can be an overly conservative estimate, depending on the dielectric characteristics and the operational parameters of the circuit the cap will be in.

    For PC (and most other SM) PSUs, leakage current won't matter much. Not even for the primary capacitor, as leakage current is at worst an order of magnitude below the expected/useful current coming out of the cap, usually more than two orders of magnitude below for high quality parts.
    Careful what you wish for... You just might get it.

  3. #13
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    As I understand it, leakage current for a new (or long unused) electrolytic starts off large as the dielectric reforms, then reduces.

    But it is good to have it confirmed that one does not need to worry over leakage current in non-medical power supplies.

    https://www.mouser.com/datasheet/2/3...041-947478.pdf
    talks about millionths of an amp (after 2 mins heal time)
    Last edited by ashiekh; 3 Weeks Ago at 03:01 PM.

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