Electrolytic capacitor voltage headroom

Salvador · Feb 15, 2016

Hi , I'm in need for some 10+ caps for my smps secondary side smoothing.
the thing is electrolytics are not cheap , so I need about 1000uF a piece but there is a big difference between whether it's 200v rated or 100v rated.
the voltage across the each cap would be about 92v DC , I wonder would a 100v rated cap be just fine in this application or do a higher voltage rating would ensure longer life , I mean how close to its rated voltage limit can a electrolytic capacitor work for long periods of time ?

thanks.

jim hardy · Feb 15, 2016

You might enjoy going beyond a quick answer.

http://www.cde.com/resources/catalogs/AEappGUIDE.pdf

http://www.digikey.com/Web Export/Supplier Content/CDE_338/PDF/CDE_SelectingCaps.pdf?redirected=1

digest those two and you will become a resource for your friends.

myself
I won't use them above 70% rated voltage
and i buy 105degC rated ones.

berkeman · Feb 15, 2016

Salvador said:

Hi , I'm in need for some 10+ caps for my smps secondary side smoothing.
the thing is electrolytics are not cheap , so I need about 1000uF a piece but there is a big difference between whether it's 200v rated or 100v rated.
the voltage across the each cap would be about 92v DC , I wonder would a 100v rated cap be just fine in this application or do a higher voltage rating would ensure longer life , I mean how close to its rated voltage limit can a electrolytic capacitor work for long periods of time ?

thanks.

I'm with Jim on this one -- you need more than "about* 8% headroom. Big caps can fail rather loudly!

Why do you need so much capacitance? If you use low ESR caps instead of standard ones, that may help to reduce your output ripple without needing so much capacitance...

Salvador · Feb 15, 2016

yes lower esr caps and also I'm just redesigning my smps because previously it had too much ripple still going through and also some leakage parasitics going thorugh my wires.
well it was basically an experiment whether it works now that it works it's time to put it together with more attention to detail.

Ok I will go through the materials Jim provided.So if 90 volts is my operating voltage then I assume the next standard I can choose is 160v then 200.

CWatters · Feb 16, 2016

Electrolytic caps are temperature sensitive. If you run them a bit hot their life expectancy can reduce dramatically. Some have a life of just a few thousand hours if run a bit hot but still within their specified max temperature. Some well known manufacturers of set top boxes have had whole batches fail just out of warranty because their engineers got it wrong.

jim hardy · Feb 16, 2016

The electrolyte itself is something of a "trade secret" - just try to find details on it.
I suspect there was a learning curve, and the new guys stumbled.

CWatters said:

Some well known manufacturers of set top boxes have had whole batches fail just out of warranty because their engineers got it wrong.

On the bright side - my kid pulled a huge flatscreen TV out of the trash, replaced the caps with bulged tops and that fixed it. No kidding it was so big i teased him about his "Drive-in " .

Cost of parts - about six bucks .
the grin on his face - priceless .

Salvador · Feb 17, 2016

yeah that wonderful feeling when you get something expensive for free and on top of that repair it yourself and even on top of that it costs you all in all a few bucks.
I once got an old Japanese Luxman amplifier from electronics scrapyard , and all it had was blown fuses , replaced them and it runs for many years now.
I still have no clue why the fuses were blown with everything else in perfect order but well whatever.
I have actually got quite a few things from scrap and with a bit of repair and knowledge they still work so it's a kind of 21st century hobby.

On the topic I wanted to ask , how much miliOhms is considered a low ESR cap ? The frequency with which my transformers run is about 50khz.
I just wonder what to better look for when searching the caps.

jim hardy · Feb 17, 2016

Salvador said:

On the topic I wanted to ask , how much miliOhms is considered a low ESR cap ?

Now that'd depend on its capacitance, wouldn't it ? Because the ripple current flowing through ESR is what heats the capacitor .

Look at the CDE datasheets and study ripple current ratings
pick a cap rated 2X or more the ripple current you intend it to handle .

If you don't find ripple current rated per se, look for power dissipation rating probably tabulated by case size. Iripple² X ESR gives power your cap will have to handle. Leave yourself at least 100% headroom.

Salvador · Feb 19, 2016

Electrolytic capacitor voltage headroom

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