Is it okay to use 25v rated caps in 24v rail output of SMPS

[zape]

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[retiredcaps]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

Nope. Too close to the rated voltage. Get a 35V cap.

The 24V rail could be 22 to 26 depending on regulation and/or load.

[zape]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

I tested it is 24.1v
Anyway, will order 35v caps then

[Per Hansson]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

It's better to use higher rated caps.
Unless the capacitor you use can handle overvoltage.
Here is an example of a 24VDC PSU with a 25v bulk cap:
https://www.badcaps.net/forum/showthread.php?t=7413

[stj]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

the old design rule was 1.5x running voltage,
so 24x1.5 = 36
so 35 is pretty much perfect.

[zape]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

So when the cap says 25v what exactly does this mean?
I thought exactly what is says - up to 25v is OK. I would even expect that there is always some room for overvoltage. Why should I divide by 1.5?
Is this just to make its life longer (the question is then for how much longer?)
I'm only using top brand caps like Rubycon or Panasonic.
The cap specs may say 10000 hrs at 105C I guess at it's rated voltage, so why divide by 1.5 then? to make it 11000?

[Per Hansson]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

It's a general "rule of thumb"
That's why I gave you the link I did:
To show that it's possible to do it with 25v caps.
(If you know what you are doing).

But in general I would not recommend it.
I was looking for specs that say how much you can briefly overvolt that UCC KMH capacitor.
Because I'm quite sure I've seen it somewhere but now I can't find it anymore.

[stj]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

voltage has ripple, or you wouldnt need the cap.
the cap must handle the level of ripple.

how much is that?
well the common thing is mains.
it's a 50% duty cycle sine-wave.
if you rectify it to dc the peak is 1.4x the rated level.

so that's probably how the 1.5 rule started.
it's also not wise to run caps or any other part on the limit because it could reduce the lifespan.

[zape]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

voltage has ripple, or you wouldnt need the cap.
the cap must handle the level of ripple.

how much is that?
well the common thing is mains.
it's a 50% duty cycle sine-wave.
if you rectify it to dc the peak is 1.4x the rated level.

so that's probably how the 1.5 rule started.
it's also not wise to run caps or any other part on the limit because it could reduce the lifespan.

Good point, but when you have the cap in circuit the ripple level is reduced, so there is no more (significant) ripple.

[redwire]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

You can do the research and you'll find electrolytic capacitor applied voltage (to rated voltage) does not matter much, for quality manufacturers.

Electrolytic caps in (linear or SMPS primary) power supplies are subjected to mains tolerances, so voltage moves around and you'd always leave some headroom in the WVDC.


UCC Electrolytic Capacitor Technical Notes
Shows no difference Graph 5-2, unless larger 350V parts.
5-6-1 Overvoltage shows 107% WV has small effect and 114% WV much more on lifetime.

Nichicon shows a 35V part (Fig.2-4) at >40V is heading into drama. 2-9-4 Operating at rated voltage has no effect.

Cornell Dubilier Aluminum Electrolytic Capacitor Application Guide
"...Aluminum electrolytic capacitors [are] made with formation voltages at least 35% higher than rated voltage and with rated temperatures of 85ºC or higher, don't require much voltage derating. In applications operating at less than 45ºC no derating is needed, and with up to 75ºC, 10% is sufficient. For higher temperatures and with high ripple current, 15% or 20% is appropriate. Since operating life continues to increase for further derating, military and space applications use 50% voltage derating."

Panasonic alum. electro cap lifetime estimation
"...Applied voltage does not influence life time too much if an applied voltage to the aluminum electrolytic capacitor is below 0.9x the rated voltage but can be a factor of sudden failure."

[stj]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

the ripple is only reduced because the cap is doing it!

[Per Hansson]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

I was looking for specs that say how much you can briefly overvolt that UCC KMH capacitor.
Because I'm quite sure I've seen it somewhere but now I can't find it anymore.

I used my eyes when looking at the datasheet again.
It's listed right under the rated voltage.
For example a 25v KMH cap is rated for 32v surge voltage...

There is an explanation here under "Guidelines for Using Aluminum Electrolytic Capacitors"
Surge Voltage
The surge voltage rating is the maximum over-voltage including DC, peak AC and transients to which the capacitor may be subjected for short periods of time (not exceeding 30 seconds every 5 minutes). According to JIS C5141, the test shall be conducted for 1000 cycles at room temperature under test condition W of JIS C5141or at the maximum operating temperature under test condition B and C of JIS C5141. Under test, the capacitor shall have voltage applied through a current limiting resistor of 1000 ohms without discharge. After test, the electrical characteristics of the capacitor are specified in JIS C5141.

[stj]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

yesn surge is not ripple, it's not continuous.

[Techgeeky]

Re: Is it okay to use 25v rated caps in 24v rail output of SMPS

35 v or 36 v will be good to go.