Ripple Current and ESR

It means that it withstands a high ripple current.

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What's to "withstand"? Electroytics are designed to smooth that out, after all.

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** Ripple current is simply the RMS value of the current flowing into and o ut of a capacitor as it charges and discharges - something it must do in or der to smooth rectified AC.

The current's magnitude depends on the p-p ripple voltage and the value of the cap and is somewhat greater than the DC load current.

Ripple current heats the cap according to "I squared R" where R is the actu al ESR at the frequency and temperature of operation. So a low ESR cap has a higher ripple current rating.

However, I have not come across a electro cap where operation at 100Hz and with the usual 10% to 15% ripple voltage exceeds its ripple current rating.

When an electro cap is used on the OUTPUT of a SMPS the rating becomes more important as the ripple voltage at high frequencies can be small while the current is large. Luckily the ESR of a electro caps improves at high frequ encies and also when the temperature rises.

Electro cap life is normally a function of operating temperature, halving f or each 10 degrees C rise above room temp. It is purely due to the liquid e lectrolyte escaping from the cap as vapour - a few molecules at a time.

Manufacturers use about 3 time more electrolyte than needed inside electros so the cap will have a long life without change is ESR or capacitance unti l nearly all the electrolyte is gone.

.... Phil

the capacitor is not perfect - it also has internal resistance (and inductance). As ripple current flows in and out of the capacitor it causes heat via the internal resistance. To much heat and the service life is short. Way too much heat and it vents messy crap all over the place.

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Very nice explanations!

ESR is an apparent series resistance inside the capacitor. Ripple current is the capacitor charging and discharging from the rectifier pulses of current and the load current in between the rectifier current.

The ripple current flows through the apparent series resistance and produces heat. heat accelerates the processes that cause ESR to increase.

Low ESR is good, but its things like SMPSUs that need the more expensive parts.

Very often you can improve matters by putting metalized foil or MLCC capacitors in parallel with electrolytics.

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** Fraid it does the opposite.

ESR is almost entirely due to the conductivity of the electrolyte and THAT improves as it get hotter. At max rated temp ( ie 85C or 105C ) the ESR falls by a factor of 4 or 5 compared to room temp.

** No you can't.

.... Phil

Thanks, Ian. I think I've got it now. Great explanation.

Right...so in such an application as SMPSs you would seek caps with the lowest ESRs and the highest ripple current capability?

Would specifically audio-type electrolytics fit the bill here, I wonder?

What for? Flattening off spikes and transients?

Has Australia run out of lithium again?!!!

ESR improves temporarily with temperature rise - heat accelerates the process of increasing ESR by evaporating the "conductive" electrolyte and/or forcing it out of the seal.

Perhaps you should tell the manufacturers who waste millions padding electrolytics with ceramic capacitors.

Well, no. The SMPS has an input filter, at a few hundred volts bias, that only sees an amp or so, as well as a 3.3V output filter, that sees near a hundred amps of ripple. If you put the low-ESR cap at the input filter, it just means that your ON/OFF switch makes a fat spark every time you apply power. Switch life, and RF radiated interference, are your new problems.

and, if you DON'T put a low-ESR cap at the output filter, the ten milliohms of ESR, which feeds that hundred amp load, can have up to 1V of effect (i.e. your regulation suffers).

If the ESR of the output caps and capacitance of the output caps are both in the 'good enough' range, go on to the next problem. One 10 milliohm capacitor won't do, but fifteen in parallel is OK. The CPU that takes all that power, has a dozen power input pins, anyhow: there's wiring resistance to consider, so a microohm ESR for the capacitor doesn't help much.

Only partly - MLCC capacitors are among the lowest ESR, but most non electrolytic are not too bad. They present a lower impedance to fast transients and take some of the heat off the electrolytic.

Its quite possible that very expensive low ESR electrolytics are being offered to audiophools - but I wouldn't take their word for it without reading the data sheet.

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** Audiophool bollocks.

...... Phil

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** Really, what PSU is that? SMPSs come in all sizes.

** Horse poo.

SMPSs always have an inrush surge limiter fitted.

..... Phil

duces

** Fraid it does the opposite."

That depends on the exact parameter you are attacking.

It is true that a lytic will have higher ESR at lower temperatures, but whe n exposed over time to higher temperatures, especially cycles, their averag e ESR will go up. While this is still dependant on temperature the basic fa ctor of it is worse, higher ESR.

Leiberman was up here like last year with a chart he made with a pot of wat er, thermometer and ESR meter. I would almost like to see how those same ca ps fare after being in service for a couple of years. If he still has the t ime to burn of course.

I would not buy an ESR meter, I can measure that with a scope or a few othe r means. But after being in service they deteriorate and we all know that. But if they are not leaking fluid out or anything, what is causing it ?

The only explanation is that the electrolyte is losing its electrolytic pro perties. There was no leak, the plates did not change, the application of t he electrodes to the plates did not change, or did they ? You know a bit mo re about the construction of these things so you speculate on that. However I think that a problem with the connections would cause higher ESL, moreso than ESR. The only thing left is the electrolyte. And I remember the almos t wars over their formulae.

The leaky ones were a different story, they leaked all over the boards in a lot of shit. I got to the point where I did not replace them, they already done leaked. I washed the shit out of the board and was usually done with i t. The thing is seven years old and this is fixed now, see you in seven yea rs. The only real problem is when that shit ate the copper right off the bo ard. But that is why I got the big bucks.

He's off his trolley again - its best to leave him be when he gets like this.

Yeah I forgot - you're one of the shonky dealers flogging them overpriced crap.

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** What utter lunacy.

I do not and never have dealt in any products.

You are totally nuts.

.... Phil

Your explanations are very clear I must say. Now, given that v. large value electros with v. low ESR are expensive, would there be any saving from buying lots of smaller values and placing them all in parallel (space permitting) to equal the value of the one large one? And would there be any performance hit to take from doing things this way? Thanks again.

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** Bout as clear as mud - you trolling retard.

** Doing this has been a common practice for decades when low levels of output noise are required for a SMPS. The total capacitance value is not as important to this task as the final ESR of the combined parts.

Of course, using a DC choke between two sets of electros works even better.

.... Phil

.... Phil