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Old 12th March 2009, 07:05 PM   #1
Deofol is offline Deofol  United States
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Question PSU Reservoir Capacitors (Single large vs Multiple small)

In the context of designing the PSU. Is there an advantage of using a single large reservoir capacitor of adequate size per rail vs using a multitude of sizes in series or parallel to archive the same capacitance?

As an example if a design has the need for 10,000uf per rail, would it be beneficial to have a single 10,000uf cap, or 2x5,000uf capacitors? Or is it even better to go both ways and over compensate with a larger capacitor (say 8,000uf) in conjunction with a couple of mid and smaller (say 3,000uf & 1,000uf).

Or am I just over complicating the whole thing for no reason and just go with what I can find most economically?

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- Bill
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Old 12th March 2009, 07:20 PM   #2
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Theoretically, multiple small caps in parallel would result in a lower ESR than the single large cap, improving the capacitor bank's ability to dump large amounts of current for large/fast transients.

You'd need to look at the capacitor specs and do some math to figure if this would yield a benefit at the frequencies of interest. You also need to consider the inductance of the path between the cap and the load (the output transistors).
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Old 12th March 2009, 07:26 PM   #3
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Caps are usually paralleled to gain an advantage of lower esr Usually only needed for SMPS output filters because of their high switching frequencies and limits of loop stability. (smaller parts work better at HF).
For linear power supplies no real advantage to parallel here because it costs more for OEMs w/o much to gain. Most quality bulk caps will spec esr at 120Hz. BTW higher temp rating are indicative of further cap quality.
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Old 12th March 2009, 07:40 PM   #4
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COST is the biggie here.. when I bought my 80V PS caps.
A 15k - 20k @ 80V was 25$ , but 3- 4700u/80v are 13$
(current mouser prices) ,4700@100v is also much cheaper
3 X $4.75=$14.25
also the added benefits of lower profile and ESR are a
bonus..
OS
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Old 12th March 2009, 07:51 PM   #5
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for DIY costs are different
part close-outs deals
added PCB's, standoffs, size changes, wiring changes, reliability hits aren't usually caculated for total cost comparisons.

Believe me if the costs were lower you'd see it in OEM products. Their margins are so low as it is, they look at the cost everything 3 times with a magnifiying glass.
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Old 12th March 2009, 08:21 PM   #6
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Quote:
Believe me if the costs were lower you'd see it in OEM products.
yes , they just use 1- 4700 uf@80 v per rail..
"bottom line"
OS
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Old 12th March 2009, 08:29 PM   #7
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yea only for 8 ohm min /channel
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Old 12th March 2009, 11:35 PM   #8
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Default Re: PSU Reservoir Capacitors (Single large vs Multiple small)

Quote:
Originally posted by Deofol
As an example if a design has the need for 10,000uf per rail, would it be beneficial to have a single 10,000uf cap, or 2x5,000uf capacitors? Or is it even better to go both ways and over compensate with a larger capacitor (say 8,000uf) in conjunction with a couple of mid and smaller (say 3,000uf & 1,000uf).
- Bill
For the main power supply, I think it is best to only use identical capacitors to maintain balanced current draw. For power amps, I think it is best to use two identical 105C rated capacitors per rail voltage, since this allows a small common ground bus bar. Almost every power amp with +/- rails uses 4 large capacitors screwed to a common star-ground copper bus bar. For superior high frequency response, you can solder in smaller secondary filter capacitors right next to each output transistor.


If you check the 105C capacitor reliability data you will find that large can-type screw terminal capacitors have significantly higher MTTF than small solder down caps. Ten capacitors each with a 10 year MTTF might have a combined 8 year MTTF.
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Old 13th March 2009, 12:42 AM   #9
Deofol is offline Deofol  United States
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Great incite all. Thanks.

Quote:
yes , they just use 1- 4700 uf@80 v per rail..
Funny that you mentioned that. I'm looking at my blown Sony ES, and its rated at 5x80W RMS. And there sits 2 single 4700ufs, one per rail. nice.
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Old 13th March 2009, 01:39 AM   #10
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Default Re: Re: PSU Reservoir Capacitors (Single large vs Multiple small)

Quote:
Originally posted by LineSource



If you check the 105C capacitor reliability data you will find that large can-type screw terminal capacitors have significantly higher MTTF than small solder down caps.
spot on for "computer grade caps"
parts count is the real killer of MTBF
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