Author Topic: Increase life of electrolytic capacitors?  (Read 3504 times)

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Offline bdunham7

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Re: Increase life of electrolytic capacitors?
« Reply #25 on: July 11, 2021, 02:08:11 pm »
Thanks, actually you are all correct.

The series capacitor thing is a gamble that we wont get widely disparate leakage current capacitors.....and i now dont think its a gamble worth taking.
The trouble with two 15uF, 450V in parallel is that  (generally) they will have worse overall ripple than one 33uF.

Sometimes the key is to realize when you have solved the problem.  I misstated the ripple current derating above (I used 100kHz, not 100Hz), but according to your calculated parameters, I'd still say your first selection solves the problem--although the 16x25 model from the same line would be even better if it fits.  It has very long life at its max ratings and in your application you have derated it some, so I think the only thing left is to make sure it doesn't get cooked. 

It might be more comforting so see 3X or 4X derating on ripple, but unless you have specific data showing what the life extension would be for derating various parameters, you don't really know if any changes you make will offer any actual improvement.  Also, I'm curious--how do you get 200mA ripple with a 15W PSU?
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Offline FaringdonTopic starter

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Re: Increase life of electrolytic capacitors?
« Reply #26 on: July 11, 2021, 02:26:10 pm »
Hi,
Its the thing with the capacitor only getting change current for a small time due to the poor power factor operation.
Ill append the LTspice if you wish. (its attached now)
Its 269mA AC RMS ripple at 220vac in and 15w out.
Attached is the ripple current scope, and the FFT of it....it shows the majority is at 50-800Hz (unless the Blackman-Harris FFT function got it wrong)
« Last Edit: July 11, 2021, 02:51:46 pm by Faringdon »
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Offline fcb

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Re: Increase life of electrolytic capacitors?
« Reply #27 on: July 11, 2021, 02:44:26 pm »
Heat is the primary life-shortener (as I'm sure everyone else has mentioned). Ripple current causes heating. Derate on ripple if you care about life, by how much is anyone's guess.

If I was concerned, I'd build two prototypes and measure just how hot the caps get on each, you should be able to estimate the caps life from the mfr datasheets.

And 270mA RMS ripple is mad for a small supply, you sure?
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Offline FaringdonTopic starter

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Re: Increase life of electrolytic capacitors?
« Reply #28 on: July 11, 2021, 02:55:10 pm »
Thansk, yes Ltspice confirms it as in post above but one.
Worse still, most of this ripple is at 50-800Hz, and not at  the 65Khz of the switching frequency.
As you understand, we dont want to derate ripple by having more capacitance, as it means more inrush.
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Offline bdunham7

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Re: Increase life of electrolytic capacitors?
« Reply #29 on: July 11, 2021, 03:05:54 pm »
Hi,
Its the thing with the capacitor only getting change current for a small time due to the poor power factor operation.
Ill append the LTspice if you wish. (its attached now)
Its 269mA AC RMS ripple at 220vac in and 15w out.
Attached is the ripple current scope, and the FFT of it....it shows the majority is at 50-800Hz (unless the Blackman-Harris FFT function got it wrong)

Can you post the assumed schematic?  Also, are you using the recommended circuit design for your PSU, including the NTC and EMI chokes?

I think there is actually a fair amount of energy at higher frequencies, although I'm glad to see it is down -320dB @ 100MHz.  :)   Try replacing the PSU circuitry with a resistor that draws exactly 15 watts and see what the calculated RMS is then.  I'm thinking that the various parasitics in the circuit will blunt the peaks enough that your ripple to load ratio will be less than an incomplete LTSpice simulation shows.  I would at least try to experimentally measure it in an actual circuit before you go into production.  Just based on experience and observation, 33uF is not a small primary filter cap for a 15W PSU, and even lower quality third-tier caps are usually not the failure point in these types of supplies.

In any case, I think your (or my) selected capacitor will likely outlast this economy PSU module.  If you really want to worry about something, start looking at the secondary capacitors.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline FaringdonTopic starter

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Re: Increase life of electrolytic capacitors?
« Reply #30 on: July 11, 2021, 03:11:45 pm »
Thanks, yes, the attached is it..
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Offline bdunham7

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Re: Increase life of electrolytic capacitors?
« Reply #31 on: July 11, 2021, 04:07:42 pm »
OK, there's a few things I don't see.

How does LTSpice model the PSU unit itself?

Where is the problem with inrush current if you use a larger capacitor? (not that I think you need to)

Can you try running the simulation with the capacitor changed to 15 and then 68uF?  I think the main reason you have such a large ratio of ripple to load is that the capacitor is more than large enough. 



A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline FaringdonTopic starter

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Re: Increase life of electrolytic capacitors?
« Reply #32 on: July 11, 2021, 04:20:35 pm »
Thanks, yes you're right the capacitor is a wee bit too big. Thing is,  as such, it will offer a decent bit of small_mains_transient protection without activating the MOV. And as you know, MOVs die a little bit after every transient that they quench. So the too big Elco helps the mov live a little longer, or so the story goes.

Even though the vishay AC-05 can handle the inrush, its datasheet says nothing as to how much it will be slightly damaged after every inrush event....and says nothing about whether  such damages are cumulative or not, and may  result in the inrush resistor dying at some point.

Quote
How does LTSpice model the PSU unit itself?
LTspice (in the sim a few posts back)  is modelling a DCM flyback at 65KHZ, which is whats on the Mornsun AC/DC
« Last Edit: July 11, 2021, 04:26:06 pm by Faringdon »
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