Electrolytic capacitor as decoupling capacitor for high frequency applications? - Page 1
Forum section: electronics / beginners

author: VanillaIceCream
author_rank: Contributor
timestamp: November 21, 2021, 11:15:20 am
content: Can I use an electrolytic capacitor as decoupling capacitor in parallel to my Power supply (with a blocking diode) to lower the peak current and peak power consumption of my PSU if it powers an amplifier or switching circuit with a switching frequency of +100Khz for example?Would that somehow negatively affect the electrolytic capacitor?My circuit is just is a simple switch with one transistor.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 21, 2021, 01:46:14 pm
content: Yes, of cause electrolityc capacitor will work at 100 kHz. Just choose a right (optimal) value and type. I suggest low-ESR type, but a general type will do some work too.You may start with 1000 uF value.

author: Manul
author_rank: Super Contributor
timestamp: November 21, 2021, 02:04:54 pm
content: Decoupling is a big topic actually, huge can of worms. Switching circuit usually implies a broad frequency spectrum, not just the fundamental. There could be frequencies well into tens or even hundreds of Mhz. Electrolytic capacitor will be good up to a few Mhz typically, after that their parasitic inductance becomes significant and they act less and less as a capacitor. Nothing bad will happen, but the bypassing is not ideal. So the solution is to help the electrolytic capacitor by pairing it with a ceramic or other high frequency, low inductance capacitor. It is typicaly placed near the point where current switching happens, for example IC power pin (so the loop area is as small as possible).In higher power applications you should watch out for maximum ripple current of the capacitor. High ripple currents cause power dissipation in electrolytic capacitor and temperature increase. Again, parallel ceramic capacitor might help if the ripple is high frequency. If it is quite low frequency, you can parallel multiple electrolytic capacitors for increased ripple handling.Quote from: VanillaIceCream on November 21, 2021, 11:15:20 amNot sure why you need a diode. It is good idea (sometimes a must) to include bulk capacitor in your circuits, as well as some smaller ceramic ones. It provides circuit with low impedance power, reduces loop area. And yes, it will also reduce ripple current seen by PSU.

author: bob91343
author_rank: Super Contributor
timestamp: November 21, 2021, 06:22:18 pm
content: I have measured a number of electrolytic capacitors and most of them are worthless above 100 kHz, if even that high. If you want to use one, at least put it in parallel with a good high frequency capacitor, say ceramic.

author: Manul
author_rank: Super Contributor
timestamp: November 22, 2021, 01:56:29 am
content: Quote from: bob91343 on November 21, 2021, 06:22:18 pmI'm not sure why your measurements were so pesimistic or what parameter you were measuring, but 100khz is a typical frequency at which aluminum electrolytic capacitors are specified in datasheets and they have quite low impedance, which is what matters in bypassing application. And in my experience most good quality capacitors have a useful, low enough impedance up to a few Mhz. Although, I never tried how cheap or no-name brands perform. Some capacitors are supposed to be general purpose, high ESR and/or ESL by design, not sure about these too, but they are not used for serious bypassing (at least, they should not), just bulk energy storage or signal coupling, so their high frequency performace does not matter much.

author: bob91343
author_rank: Super Contributor
timestamp: November 22, 2021, 02:57:04 am
content: Manul, I did not do a scientific study. I just grabbed several electrolytics from my junkbox and tested them. Self resonant frequencies were low, and impedances generally soared above 100 kHz. Perhaps I didn't get a fair sample but those were my results.

author: VanillaIceCream
author_rank: Contributor
timestamp: November 22, 2021, 02:01:39 pm
content: Quote from: Manul on November 22, 2021, 01:56:29 amThanks.I simulated it in LTSpice with a BJT Switch with a 100Khz Signal, 20V VCC and 1-100mA at RLoad, I used a 680uF low ESR capacitor with 50nH parasitic inductance for the absolute worst case scenario and the output signal and everything is fine.I originally used the diode because I thought that I need something like a backflow protection or something like that.But I just realized that the capacitor doesnt really work at all in the circuit in the simulation if there is no diode in front of it at VCC.Is a diode in front of the capacitor really always required for it to work and provide the circuit temporarily with extra power like in this example or am I getting something wrong?I've watched many videos about this topic and and never found something about the need for diodes anywhere.

author: Manul
author_rank: Super Contributor
timestamp: November 22, 2021, 03:36:33 pm
content: Quote from: VanillaIceCream on November 22, 2021, 02:01:39 pmNo, it is not needed. Note, that in simulation you used voltage source which is zero impedance, ideal source, no matter how much current you take from it, voltage stays precicely same. So without a diode, capacitor for sure does nothing, because it is directly in parallel with ideal voltage source and the impedance is already zero because of ideal voltage source. It is a bit hard to put into words, but hope it is clear. If you want to model a non-ideal voltage source, add a small series resistor, like 0.5 or 1 Ohm, also you can add some series inductance. That would be similar to real world supply.

author: elektrolitr
author_rank: Contributor
timestamp: November 22, 2021, 04:22:28 pm
content: In your simulation, the voltage source is ideal (no parasitics) and thus the voltage is absolutely stable, hence the capacitor does nothing. Try adding some internal resistace and inductance to the voltage source, and you'll see the difference.

author: The Electrician
author_rank: Frequent Contributor
timestamp: November 22, 2021, 05:01:29 pm
content: Quote from: bob91343 on November 22, 2021, 02:57:04 amTell us some details about your measurements. How many uF were the capacitors you measured? Were they radial lead or axial lead? How did you measure the SRF? What typical values did you get for SRF?

author: bob91343
author_rank: Super Contributor
timestamp: November 22, 2021, 05:59:58 pm
content: I didn't document my results, and I did the test about a year ago. My test setup was the nanoVNA. I did a Smith chart on the part and moved the marker around to find things like impedances and resonances. The parts themselves are back in the junkbox, no idea which ones they are.As a sanity check I also tested some ceramic and plastic parts, which were far better.As I said above, this was not a scientific analysis, just a seat of the pants test for my own edification. As the saying goes, your mileage may vary.

author: The Electrician
author_rank: Frequent Contributor
timestamp: November 22, 2021, 08:22:55 pm
content: Quote from: Manul on November 22, 2021, 01:56:29 amHere is the spec sheet for a very low ESR electrolytic. I have some of the 1800 uF, 16 Volt parts, and I'll show some measurements on one:https://www.nichicon.co.jp/english/products/pdf/e-hz.pdfHere is a sweep of impedance magnitude (green) and ESR (yellow) from 100 Hz to 5 MHz. The scale is logarithmic with .001 at the bottom and 1 at the top. The self resonant frequency is at about 45 kHz where the impedance curve reaches a minimum and just touches the ESR curve. There is a marker (B) at 101.41 kHz which is essentially 100 kHz. In the upper right corner can be seen the value of |Z| and ESR (Rs). The measured |Z| is 8.136 milliohms but the spec sheet says it should be a maximum of 6.5 milliohms. Part of the reason for the discrepancy is that I'm measuring in a room at 13 degrees C. ESR (and |Z| near the SRF) of an electrolytic varies quite a bit with temperature; I can lower the measured ESR noticeably by just squeezing the cap between finger and thumb as it sits on the fixture, raising its temperature:Here's another sweep showing the ESL. The impedance of the cap is capacitive below the SRF and the curve is off the bottom of the screen. Above the SRF, the impedance becomes inductive and a curve showing ESL appears on the image. Above 100 kHz it's nearly constant and at the upper right of the image we see that its value is 7.3 nH. So the capacitor looks like 1800 uF in series with an inductor of 7.3 nH. This is still useful as a capacitor up to several megahertz:]

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 22, 2021, 10:34:23 pm
content: Quote from: bob91343 on November 21, 2021, 06:22:18 pmLow-ESR (and polymer one too) electrolytes - exist.CPU VRM typical frequency is 200-500 kHz.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 22, 2021, 10:35:46 pm
content: Quote from: bob91343 on November 22, 2021, 02:57:04 amIt doesn't mean the capacitor doesn't work above self-resonant frequency.

author: bob91343
author_rank: Super Contributor
timestamp: November 23, 2021, 12:37:57 am
content: Good work guys, and a sensible discussion. I learned a few things and I am always up for that.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 23, 2021, 08:36:30 am
content: Quote from: bob91343 on November 21, 2021, 06:22:18 pmEven if you measure correctly, the results are meaningless if you don't understand how to interpret them.A decoupling capacitor is not "worthless" above SRF. This assumption is just wrong. SRF is not a magical limit. The load is only interested in having low enough impedance, and what is low enough at each frequency, depends on the load. Impedance onlystartsto rise after exceeding SRF.Electrolytic capacitors are physically bigger than, say, 0603 chip capacitors, setting a limit how low ESL can be. This is the root cause for the low SRF, and also sets the actual limit of operation (which will be beyond SRF, but how much, depends on the circuit).Electrolytics and small SMD MLCCs make a great combo because the high ESR of the elcaps dampen the possible LC ringing caused by the very low ESR of MLCCs.