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Motherboard capacitor voltage tolerances.

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plazmatron

Fri Sept 14 2012, 11:46AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

I have recently bought some new capacitors, to replace a couple of bulging caps on my motherboard.

This keeps bugging me though, Why is it that motherboard manufacturers use such a tight voltage tolerance for caps on their boards??

On the 5v rail, there are caps rated for just 6.3v, and on the 3.3v rail, they are rated for just 4v.

On PSU's I have designed and built, I use caps rated at 1.5 to 2 x the rail voltage. Granted, this means my caps are physically a little bigger than they need to be, and are perhaps fractions of a pence more expensive than their lower voltage counterparts, but is this really just a cost and space issue, or is there something more technical I am missing here??

Les

Ash Small

Fri Sept 14 2012, 12:14PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I have read in the past that 'lytics should be used near their rated voltage. Something to do with the way the dielectric is formed, or maintained, but I'm only repeating what I read years ago.

Sulaiman

Fri Sept 14 2012, 12:41PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

No, if you read manufacturers data sheets you will see that service life increases as you reduce the working voltage below the rated voltage,
motherboard manufacturers only need their products to last a few years or at least until the warranty expires
so why use capacitors (or any other components) rated for longer life?
Also the capacitor size hence board size would have to increase to fit higher voltage capacitors.

plazmatron

Fri Sept 14 2012, 03:30PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Sulaiman wrote ...

No, if you read manufacturers data sheets you will see that service life increases as you reduce the working voltage below the rated voltage.......

That is exactly what I thought!

I am considering using higher rated voltage caps on mine, but thought I would make sure that there wasn't a 'real' reason why they might be using such close tolerances. The caps I have will physically fit on the board, with plenty of clearance regarding other components, so I figure why not use caps rated at 10v on all the 5v rails.

Patrick

Fri Sept 14 2012, 06:43PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i think the bulging is most often cuased by high temp, and high ripple currents ant high freq, i think the voltage being close to theyre max rating is less contributory, cheap caps live the shorter life compared to a well made cap. So, bulging in your case is primarily an ESR determined event.

GeordieBoy

Fri Sept 14 2012, 09:15PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The voltages on the 3.3V and 5V rails inside a PC have to be tightly controlled to ensure correct operation, so there's no need to fit capacitors with much voltage headroom. Modern CPUs draw large currents that can change very abruptly, so the power supply has to be able to respond quickly to this varying demand in order to keep the supply voltage rails within spec.

Capacitor sizes for the same technology of electrolytics are roughly proportional to CV product, so any over-engineering on the voltage rating would increase the can size, price, shipping weight and board space required. Higher voltage capacitors also tend to have a higher ESR value, and this is the one parameter that is probably more critical for capacitors in this application than any other...

When replacing such capacitors the ESR figure and temperature ratings are likely to be the most critical parameters. When the load current on a SMPSU increases abruptly, the output capacitors have to hold the fort and maintain the output voltage until the power supply's control loop responds. Then it takes time to ramp up the buck inductor current, and finally only then start to replenish the rapidly depleting energy in the reservoir capacitors! This is a relatively slow process so all the burden of an instantaneous change in load current goes on those electrolytic capacitors. The problem is that during a step increase in load current any ESR present in these capacitors causes an immediate sudden voltage drop in the supply rail, and there's nothing the control loop of the power supply can do about it for quite a long time.

That's why low ESR is so important in this application. You often see large banks of electrolytics in parallel instead of one electrolytic in SMPSUs because this gives a lower overall ESR. It also spreads the heating among multiple capacitors which helps with MTBF. The actual total capacitance is largely irrelevant since above the cutoff frequency of the LC filter it is the ESR that dominates and determines the voltage ripple too.

I hope this helps,

-Richie,

Conundrum

Sat Sept 15 2012, 07:26AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Yes, ESR is vital.
I'd preform the capacitors first and discard any with high leakage, don't forget that if it goes dead short it could cause a serious mess.

In addition, manufacturers like to use lead free solder which is evil to remove.
I have had to cut capacitors open then pull the leads out of the board one at a time before

If you want I can send you some 62C alloy, solder wick and a little paste to reflow with.

-A
#include "$0.02.h"

plazmatron

Sat Sept 15 2012, 12:19PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Thanks guys, I will stick with the original voltage and ESR ratings then, just thought I would ask first, and it seems to make a lot of sense.

Conundrum wrote ...

In addition, manufacturers like to use lead free solder which is evil to remove.
I have had to cut capacitors open then pull the leads out of the board one at a time before..

Yes I have noticed that! Before hammering away at my own motherboard, I have been practicing desoldering and replacing caps, on some old motherboards I was hanging on to. It takes a lot more care than desoldering off of regular PCB's. Still, its a lot easier than dealing with SMT stuff!

Les

Pinky's Brain

Sat Sept 15 2012, 04:08PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Why would insulation thickness affect ESR? It will increase the loss factor for AC ... but is that relevant?

Ash Small

Sat Sept 15 2012, 05:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Pinky's Brain wrote ...

Why would insulation thickness affect ESR? It will increase the loss factor for AC ... but is that relevant?

I'm not certain I'm correct, but isn't ESR related to electrolyte?

The more electrolyte in the can, the greater the ESR?

I seem to remember recently reading about 'capacitor plague' being a result of certain Taiwanese manufacturers incorrectly 'copying' new 'low ESR' electrolytes developed in Japan.

"A major cause of the plague of faulty capacitors was industrial espionage in connection with the theft of an electrolyte formula. A researcher is suspected of having taken, when moving from Japan to Taiwan, the secret chemical composition of a new low-resistance, inexpensive, water-containing electrolyte. The researcher subsequently tried to imitate this electrolyte formula in Taiwan, to undersell the pricing of the Japanese manufacturers. However, the secret formula had apparently been copied incompletely, and it lacked important proprietary ingredients which were essential to the long-term stability of the capacitors"

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