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Old 24th January 2013, 07:44 AM   #1
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Default Electrolytic rating advice needed for filter application

As i understand from a good bit of searching here and w/ google, a simple electrolytic of sufficent rating can be used as a filter for dc out on amplifier power supplies.

My question is, powered off mains 120v AC, what voltage, farad, and esr rating should i use for 35v 9.6a DC out from a smps?

Also, will this cause a voltage drop? My smps is 34.5v-40v variable, so i can account for that, but another idea is re-routing the voltage sense after the filter, so the smps is in charge of controlling the voltage.

good/bad idea? Or should i just account for voltage drop by increasing the voltage out of the smps?

This is powering a ddx320, with 2 8ohm 100w stereo speakers, and a 3 ohm 130w sub.
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Old 24th January 2013, 11:48 AM   #2
DUG is online now DUG  Canada
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What capacitance is in place now?

How much does the supply voltage drop under load now?

What is the switching frequency?

Is the PS rated for 9.6A or is that your load?
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Old 26th January 2013, 09:16 AM   #3
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Switching power supply for sub, its well. But when its used for fuFilter cap spec for smps is very much depends on switching frequency of the smps and the output current rating. Always go for very low ESR type to reduce cap heating. Value in Farad directly proportional to output current and inversely proportional to switching current.
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Old 27th January 2013, 04:05 PM   #4
DUG is online now DUG  Canada
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Quote:
Originally Posted by stevensoosai2527 View Post
... Value in Farad directly proportional to output current and inversely proportional to switching current.
So,

A 1A load with a 1A switching current give you 1 Farad?

Seems a little high, don't you think?
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Old 27th January 2013, 04:09 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by DUG View Post
............A 1A load with a 1A switching current give you 1 Farad?..............
You missed one term in the relationship:
When 1F is discharged or charged at the rate of 1A then the voltage changes at the rate of 1V/s.
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Old 27th January 2013, 04:22 PM   #6
DUG is online now DUG  Canada
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AndrewT

Please note question mark and the post that it applied to.

I know what you have posted but I am at a loss as to see that it answers my question.

There is something missing in stevensoosai2527's generalization and that is what I would like to find out.

Just looking for answers and knowledge.

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Old 27th January 2013, 05:33 PM   #7
PChi is offline PChi  United Kingdom
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Is this capacitor the output filter for a smps? Or is it an additional capacitor?
Can you post a schematic?

For an smps the ripple current rating and ESR are the most important parameters. The ripple current through the output capacitor depends on the circuit design so measurement and / or a schematic are required.
SMPS capacitors are under a lot of stress so another factor is what is the ambient temperature and how long do you want the power supply to last along with how much space is available and how much do you want to spend.

The ripple current rating isn't usually much a problem for the typical unregulated power supply used in an auduio amplifier due to the large peak to average ratio of the level so the amplifier is rarely delivering full power. But an smps output capacitor could well have a fair level of ripple current even at a low load.
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Old 27th January 2013, 05:52 PM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by DUG
There is something missing in stevensoosai2527's generalization and that is what I would like to find out.
What is missing is the time period, and the allowable voltage droop. Required capacitance is proportional to current. Inversely proportional to charging frequency, not charging current.

However, I would expect that a capacitor attached directly to the output of an SMPS would have to be accounted for in the design of the SMPS, because a big cap could change the shape of charging pulses or increase EMC problems. I know enough about electronics to know that SMPS design is difficult, so I leave that to others.
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