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LadyN

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Joined: 26/01/2019
Location: United States
Posts: 408

Posted: 09:38pm 20 Mar 2019

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Capacitors in parallel: effect on ESR and ripple rating

Assume each capacitor in a bank has following specs:

4700uf
80V
ESR 0.092
Ripple@120Hz: 3.31A

If I parallel 10 of them:

The eq. capacitance is 10 * 4700uf = 47000uf

Questions:

1. I would also assume the ESR to drop by 10x as well? So the eq. capacitance has ESR of 0.0092
2. As a result, the Ripple@120Hz should increase?

mackoffgrid

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Joined: 13/03/2017
Location: Australia
Posts: 460

Posted: 10:14pm 20 Mar 2019

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yes
yes, the total allowable ripple current increases.
Its why when you look inside a aerosharp inverter they have something like 20 capacitors in parallel.

SolarMike has employed this idea on numerous times.

The trick is effeciency, of PCB area, cost, picking the right components, ie, price, size, effort sweet spot.

Cheers
AndrewEdited by mackoffgrid 2019-03-22

LadyN

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Joined: 26/01/2019
Location: United States
Posts: 408

Posted: 01:16am 21 Mar 2019

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It can't be as simple as that, right?

There must be a reason why a 0.001 ESR cap costs more than 10x of 0.01 ESR caps.

Infact, I typically see 0.01 ESR caps to cost, say $2/pc while the 0.001 ESR can cost say, $120/pc, which is a factor of 60x, not 10x, so what's going on?

Feels like I am missing something or a few things.

For example:

1. my calculations do not take into account the additional resistance introduced by the leads that connects each cap together to form a bank (this might not be an issue with the ripple rating as the heat dissipation due to additional trace/line resistance happens in the trace/line which is external to the caps)
2. maybe that increases the amount of (unintended) inductance in the bank as well (which might not be an issue at low freq.)
3. maybe the space savings of having a smaller capacitor that has 10x less ESR but not 10x bigger is worth the added 6x premium?

Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406

Posted: 01:47am 21 Mar 2019

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Andrew is quite right, you can use lots of little ones, up to a point...

The problem is getting lots of smaller capacitors to equally share the ripple current and in such a way that the ESR is not degraded by over long interconnections.

Its more a matter of judgement and personal preference than anything else.

Cheers, �Tony.

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