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I would like to use solid polymer electrolytics for decoupling in my designs instead of standard electrolytics for their longer lifespan and substantially lower ESR for marginally increased cost

When choosing decoupling capacitors its frequently mentioned that the lower ESR has a greater tendency to cause ringing so its advised to use medium/high ESR capacitors.

How would a circuit/PCB be designed to avoid power supply ringing with Low ESR capacitors without artificially adding ESR with series resistor as this defeats the purpose of using low ESR parts?

Very low ESR ceramic decoupling caps close to the load is standard practice, how is ringing not an issue there?

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  • \$\begingroup\$ if you're worried about ringing, you can add a higher capacitor in parallel with your low esr capacitors which will damp the ringing. \$\endgroup\$
    – BeB00
    Commented May 8, 2020 at 20:52
  • \$\begingroup\$ really any ceramic capacitor, including X7R. You can certainly have ringing with X7R, and this ringing can actually destroy the device if used in a high current area, for example the input to a buck or boost converter. \$\endgroup\$
    – BeB00
    Commented May 8, 2020 at 20:56
  • \$\begingroup\$ You can use shunt dampling like BeB00 said or you can add a series resistor just in case. Often a very small series resistor will completely stabilize an otherwise under-damped LC circuit. Ringing is always caused by an interaction between a capacitor and some inductance. Sometimes the inductance is a stray inductance from a trace or wire. \$\endgroup\$
    – user57037
    Commented May 8, 2020 at 21:18
  • \$\begingroup\$ Also the snubber circuit or high esr cap in parallel that everyone suggests... this provides high impedance for low frequency and low impedance for high frequency, shunting the ringing to ground.... but isnt that pretty much what a low value capacitor would do aswell? yet a low value cap in parallel with high value was mentioned to potentially cause ringing \$\endgroup\$
    – Jay
    Commented May 8, 2020 at 22:34

2 Answers 2

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The main problem with ringing occurs when capacitors of very different size are used. For instance, I often see schematics with 1nF, 10nF and 100nF in parallel. The stray inductance of the 100nF can resonate with the 1nF capacitance.

In most of our designs, this is avoided by using a large number of same size capacitors on any on line, usually 10nF to 100nF. If a larger 'bulk' capacitor is needed, say 33uF, then a tant or alli is used rather than ceramic. If a very high frequency small RF capacitor is needed, say 10pF for use at GHz, then it will be decoupled from the main line by a ferrite bead or a few ohm resistor anyway for decoupling reasons.

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  • \$\begingroup\$ I see frequently mentioned trace ESR/ESL in relation to this subject and its really confusing. The ringing occurs from CLC network formed due to inductance between the 2 caps, yet at the same time some trace length is said to help ''damp'' the resonance by adding some ESR which also adds some ESL. you add ESL wth a ferrite bead to decouple 10pf cap from main line, I thought that is exactly what is unwanted like the stray inductance of 100nF cap? \$\endgroup\$
    – Jay
    Commented May 8, 2020 at 21:35
  • \$\begingroup\$ What difference does it make whether we use different values or several same value? I am not sure what ringing means here. \$\endgroup\$
    – gyuunyuu
    Commented Jun 29, 2023 at 14:28
  • \$\begingroup\$ @gyuunyuu Ringing => a resonance with a high Q LC circuit. Same size capacitors have same size Ls and Cs, which tends to reduce the available Q. When two capacitors of very different sizes are used, the large capacitor will generally have a large inductance, which can create a high Q resonance with the small capacitor. \$\endgroup\$
    – Neil_UK
    Commented Jun 29, 2023 at 16:56
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Define "low". Because there is low like an X7R capacitor which generally causes no problems, and C0G/NP0 which his very low and should not be used for decoupling for this reason (also expensive and overkill).

When choosing decoupling capacitors its frequently mentioned that the lower ESR has a greater tendency to cause ringing so its advised to use medium/high ESR capacitors.

You never mentioned whether the context in which you saw this. I have only ever seen it mentioned for ceramic caps which are used for high frequency decoupling. I haven't read about it being a problem and I think it's because they are used for lower frequencies.

Spend less money and use a cheaper polymer cap that isn't so low in ESR. But if you insist on spending X amount of money and want polymer longevity but your ESR is too low, then going to a higher voltage rating tends to increase ESR (as well as probably increasing longevity).

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  • \$\begingroup\$ As mentioned in my question comment, X7R can certainly result in destructive ringing in a case where electrolytic wouldn't \$\endgroup\$
    – BeB00
    Commented May 8, 2020 at 20:58
  • \$\begingroup\$ this is what I read on diy audio forums. Im interested in what conditions cause ringing, X7r is generally ok, what is an example of when it wouldnt be? \$\endgroup\$
    – Jay
    Commented May 8, 2020 at 21:10
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    \$\begingroup\$ A classic one is between the power cable of a device and the first buck converter, where the interaction between the inductance of the cable and the low esr capacitor can cause ringing >50% higher than the supply voltage, which can explode the buck converter. One way to solve this is a parallel electrolytic or ceramic + resistor \$\endgroup\$
    – BeB00
    Commented May 8, 2020 at 21:16
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    \$\begingroup\$ @BeB00 Maybe that's why. I tend to use only electrolytics as the decoupling capacitor at the end of a long power cable. \$\endgroup\$
    – DKNguyen
    Commented May 8, 2020 at 21:19
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    \$\begingroup\$ Yes, this is correct. Depending on what you're working on, a series resistor or PTC may not be acceptable due to the power loss. In these situations, a parallel electrolytic is a good solution, as mentioned in that app note. \$\endgroup\$
    – BeB00
    Commented May 8, 2020 at 21:23

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