Overrated Capacitors?

[mcel]

Having just replaced the 11 1500uf Jackons on my Abit BE6-II, I noted the difference in size between these and the Panasonic FCs that I put in. It led me to thinking about why it's a problem to get 8mm caps in 1500uf, 6.3V. The third most important parameter when choosing capacitors in a design is of course the current rating. As CPUs have got faster, and the core is pulling out more big current pulses from the reservoir caps, then they will demand more current capacity from the caps. I haven't checked the current rating of the Jackons compared to the Panasonics, but maybe Panasonic are more conservative about their ratings than Jackon and others, and that's why their caps are bigger?
I had about 5 of the 11 on the board with bulging tops and bottoms, though no real leakage. Some of the remainder had bottoms bulging. The Vcore, which was noisy, is now rock solid, so what the real capacitance left in the old Jackons was I don't know! Not much I would suspect..

[arneson]

Re: Overrated Capacitors?

Size certainly does matter, but quantity may alieviate load.
I would think that a jump up in capacitance ( more filtering ) and v rating (ability to take abuse) within whatever limiting height and width spacing is allowed, would be a good thing.

[PeteS in CA]

Re: Overrated Capacitors?

ESR, ripple current rating, and rated life are an inter-related part of the picture. Ability to dissipate heat from the cap's core is important. The thermal ambient around the cap - proximity to heat sources, airflow - is important. The quality of the materials - foil and electrolyte - and process control is important. I think the electro-chemical technology of the caps is being so stressed by the applications that near perfection in materials and process are necessary.

[arneson]

Re: Overrated Capacitors?

True and well stated.

[japlytic]

Re: Overrated Capacitors?

If I designed set-top boxes (as well as other mid to high-end consumer electronics), I would use around twice the recommended reliable uF ratings (eg. using 10000uF in a rail that recommends 4700uF). For further reliability, I would also use rectifiers at 50% of their maximum ratings. Even though this will increase cost a little, this will considerably reduce the chance of failures, and increase the lifetime of the equipment - this may be true for industrial and military equipment!

Brands of capacitors I intend to use in order of preference:

1. Nippon Chemi-Con
2. Matsushita
3. Rubycon
4. Elna
5. Sanyo
6. Nichicon (Probably because of bad batches of HM and HN series)

[Chris1992]

Re: Overrated Capacitors?

Elna before Sanyo? IIRC Pete ranked cap companies by quality and Elna was around the level of Teapo. But I have a ~1985 Kenwood amp/tuner set with Elna caps and both components work great.

[linuxguru]

Re: Overrated Capacitors?

Elna is great for audio applications - audiophiles seem to swear by Elna. However, these are typically in higher voltage ratings (16 to 50V) and the requirements for good audiophile-quality cap (low DA, low hysteresis, linearity) differ substantially from the requirements for a good computer-grade cap (low ESR, high ripple-current, extended high temperature operation).

Good computer-grade caps may actually sound horrible in an audio application, and good audio-grade caps may fail quickly as motherboard bypass/filter caps. Some bad caps reported on this board (CapXon, Luxon) are fairly good in audio applications.

I've been using Panasonic FC (for high-values/bypass) and Chemicon SMG (for low-values/interstage coupling) for a vintage Hi-Fi restoration project (more details after I finish). No particular reasons for using those caps, they just happened to be lying around. I'd have used Elna if they were at hand - a 12-year old Alpine car power amplifier that I have is filled with Elna, and it's working fine and sounds good to this date.

[Chris1992]

Re: Overrated Capacitors?

Ah. Are Elna's RJJ series caps okay for motherboard applications? Never seen Elna on a board.

[linuxguru]

Re: Overrated Capacitors?

Elna RJH (successor to RJJ) has slightly better specs, but both should be OK for motherboard use. Availability of RJH seems to be better than RJJ, but YMMV.

[PeteS in CA]

Re: Overrated Capacitors?

Re Elna, they have never seemed to be an industry leader in low Z caps. Also, a company I worked for 1980-1982 had an unfortunate experience with Elna.

Cap ratings can be misleading. For example, if you look at ESR, ripple current rating, and life, these series from the three '''Cons" seem to be peers:

5K Hours:
HD, KZE, and ZL

10K Hours:
HE, KY, and YXG
KZM and ZLH

In my very abusive testing, HE and ZL are approximately peers, and KZE is close. YXG didn't fare as well. Because I have a hard time getting Rubycon parts (their sales/marketing in the US is execrable, and has been since at least the late 80s), I haven't tested as many of their parts. YXG did do better than PJ, PL, PW, and LXV, however.

[PeteS in CA]

Re: Overrated Capacitors?

Part 2:

That said, I do not at all doubt the mfr's ratings for their parts. My tests probably are showing the parts performance under a certain kind of abuse (excessive ripple current). If (there's that big two-letter word again) the MB mfr's VRM designs are good if they use good parts, and if the system cooling is good, any of the above series should do very well. To be honest, I wonder whether one or more of those "if's" might contribute to some cap failures, while the "'Cons'" parts have the robustness to tolerate the slight abuse.
As linuxguru pointed out, P/Ss (including VRMs) impose very specific stresses, and Elna, Lelon ( in my tests), etc. may do well with other types of applications.

[willawake]

Re: Overrated Capacitors?

did you test any panasonic?

is it possible that low quality psu can affect cap lifetimes on a motherboard. how does eliminating caps from the original design for cost savings affect also.

[arneson]

Re: Overrated Capacitors?

Anyone know how it is that the Mac motherboards do not use electrolytics?
I was investigating an unstable mac and subbed the power supply, which was not very different from any AT type PSU, but the board itself had problems.
Only one very small can cap on these boards, so why does mine have 23 can caps?

[linuxguru]

Re: Overrated Capacitors?

> is it possible that low quality psu can affect cap lifetimes on a motherboard.

It can happen - if there's poor quality or insufficient caps on +5v and +3.3v on the secondary of the PSU, and the ESR increases due to cap failure in the PSU, then some of the excess ripple current from the PSU secondary rectifiers will be diverted through the motherboard bypass caps on +5v and +3.3v. If these were marginally speced, they can fail quickly.

That's why it's important to diagnose a bad PSU early - it can lead to a succession of motherboard failures. It's also easier to recap a PSU compared to a motherboard.

>how does eliminating caps from the original design for cost savings affect also.

Same as above - high ripple current spread over fewer functioning capacitors.

[willawake]

Re: Overrated Capacitors?

thanks linuxguru.

Anyone know how it is that the Mac motherboards do not use electrolytics?

which boards are you talking about? the g5 has the most caps i have ever seen on a board.

here is iwill design for vcore without al-caps

[Rainbow]

Re: Overrated Capacitors?

That might be true only for old Macs. See for example a 386 or older 486 board - there aren't any electrolytics on most of them too.

[linuxguru]

Re: Overrated Capacitors?

here is iwill design for vcore without al-caps

Interesting stuff - they're using SMT ferrite inductors and SMT ceramic capacitors for an all solid-state VRM. No doubt, it's likely to be far more reliable - I'll certainly consider such an Iwill board for my next upgrade if the price-premium is not significant. However, tolerances on ceramic caps are fairly loose, and vary a lot with temperature and humidity. Will it work well for a high-current server motherboard? They claim it does work - we'll see.

[Rainbow]

Re: Overrated Capacitors?

And what about tantalum caps? My notebook has a couple of them in Vcore switching regulator. But it's for Socket 7 CPUs so the current draw is rather low (I have P166MMX there now).

[willawake]

Re: Overrated Capacitors?

iwill were also talking about a sff with the digital vrm
http://www.theinquirer.net/?article=23630

Specifications: Nvidia nForce Pro 2200 chipset, dual Opteron CPUs, dual SLI graphics cards, 400 watt PSU (450 W planned). Selling price is expected to be similar to the ZMAXdp, which costs around $700 in barebones form. µ

[linuxguru]

Re: Overrated Capacitors?

Tantalums are dry electrolytic - that's their major feature, apart from compactness and low inductance. They do fail occasionally, but usually only due to overvoltage, reverse polarity and humidity seeping through cracks in the epoxy coat. They don't age and dry out in the same manner as Al electrolytics.