Paralleling caps to meet ripple current requirements?

[John]

A DC-DC converter I'd like to use requires an input cap with a 10A
ripple current rating (at the coverter's rated output). I'm assuming
it's a 100KHz ripple rating, the data sheet doesn't say.

I can easily find 4A or so rated caps at the capacity/voltage/ESR I'm
looking for and was wondering if I can just parallel three of those
caps to achieve the 10A ripple current requirement? Assume that all
PCB traces are low impedance/inductance and that the caps are mounted
close together and close to the input of the converter.

Thanks!
John
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[Eeyore]

A DC-DC converter I'd like to use requires an input cap with a 10A
ripple current rating (at the coverter's rated output). I'm assuming
it's a 100KHz ripple rating, the data sheet doesn't say.

I can easily find 4A or so rated caps at the capacity/voltage/ESR I'm
looking for and was wondering if I can just parallel three of those
caps to achieve the 10A ripple current requirement? Assume that all
PCB traces are low impedance/inductance and that the caps are mounted
close together and close to the input of the converter.

What value cap are you looking for ?

Paralleling caps for increased ripple current is quite normal btw.

Graham

 

[John Popelish]

A DC-DC converter I'd like to use requires an input cap with a 10A
ripple current rating (at the coverter's rated output). I'm assuming
it's a 100KHz ripple rating, the data sheet doesn't say.

I can easily find 4A or so rated caps at the capacity/voltage/ESR I'm
looking for and was wondering if I can just parallel three of those
caps to achieve the 10A ripple current requirement? Assume that all
PCB traces are low impedance/inductance and that the caps are mounted
close together and close to the input of the converter.

Yes, you can. But since ripple current is what heats the
caps and determines their life, 4 might be better. I would
like to leave a fat margin (perhaps 50%) between the ratings
and the actual current.

If you want to keep almost all of that ripple current
isolated between those caps and the converter, add a little
inductance between the upstream supply and the capacitors.
This will pass almost all the ripple current through the
caps, instead of sharing it between the source and the caps,
but that is where it belongs.

 

[John]

What value cap are you looking for ?

It's a 12V-to-0.9V converter.
Need 270uF for the input and 2000uF (or so) for the output. Looking
at the Vishay OS-CON right now...nice ESR specs. They're pricey but
this is just a one-off converter I'll be using "in-house".

It seemed that idea would work just fine. Just wasn't sure if there
was a "gotcha" somewhere. Thanks!

John
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[John]

Yes, you can. But since ripple current is what heats the
Good tip, thanks.

Interesting...the test-rig schematic for the converter (an Artesyn
SIL40C) showed a 270uF cap, 1.5uH series inductor, and then another
270uF cap at the Vin to the converter. Wasn't quite sure of the
inductor's function...now I know.

Do I still need the full 10A ripple requirement for both sets of caps,
before the inductor and after the inductor?

John
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[Eeyore]

Yes, you can. But since ripple current is what heats the
caps and determines their life, 4 might be better. I would
like to leave a fat margin (perhaps 50%) between the ratings
and the actual current.

Really !

If you're worried about lifetime just use higher temperature parts.

To the OP. Electrolytic caps are usually rated as having x thousand hours life
based on operation at their max rating (ripple current) in a specified ambient
temperature, the 'wear out' mechanism basically being heat which slowly
evaporates the electrolyte).

Lifetime can be increased by reducing the ambient temp OR using a cap with a
higher temperature rating. Lifetime doubles for every 10C reduction in temp
typically.

For example a cap rated for 2000 hours at 85C would last 8000 hours at an
ambient temp of 65C and a '2000 hour 105C' cap would last 32000 hours under the
same conditions.

Graham

 

[Eeyore]

Good tip, thanks.


Interesting...the test-rig schematic for the converter (an Artesyn
SIL40C) showed a 270uF cap, 1.5uH series inductor, and then another
270uF cap at the Vin to the converter. Wasn't quite sure of the
inductor's function...now I know.

Do I still need the full 10A ripple requirement for both sets of caps,
before the inductor and after the inductor?

Ummm, it starts getting more complicated when you add inductance. The inductor
actually reduces the ripple current in the caps since it stores some energy too.

You won't ever go wrong by being generous with ripple current specs though.
There's also no reason you can't simply increase the capacitance of the input
cap (rather than fit several smaller caps - provided you still meet any required
ESR specs) and get extra ripple current capability that way.

Graham

 

[John Popelish]

Good tip, thanks.


Interesting...the test-rig schematic for the converter (an Artesyn
SIL40C) showed a 270uF cap, 1.5uH series inductor, and then another
270uF cap at the Vin to the converter. Wasn't quite sure of the
inductor's function...now I know.

Do I still need the full 10A ripple requirement for both sets of caps,
before the inductor and after the inductor?

I seriously doubt it, if the design does not involve any
resonances. The cap between inductors would normally see
very little ripple current. I also doubt that having two
equal capacitors is optimal. The one between inductors is
essentially there just to capacitively divide the RF current
that jumps through the inductor via its stray capacitance.
I good, low equivalent series inductance (i.e. stacked film
or ceramic multilayer) might well do better than the 270 uF
even if only a fraction of its capacitance. But you may
need a parallel larger capacitor to suppress any resonances
between those inductors and the small capacitor that the
converter generates.

After you get past the ripple current requirement of the
final cap, you have to deal with where that current goes.
The loop area this current surrounds needs to be minimized
to get the most use from those capacitors and minimize
magnetic fields caused by the ripple.

 

[John Popelish]

Really !

If you're worried about lifetime just use higher temperature parts.

Similar sized parts from the same maker with different
temperature ratings usually have shorted life ratings for
the higher temperature units, and lower ripple current
ratings. In other words, the main difference between the
lower and higher temperature rated units is a derating of
the ripple current rating and a shortened life prediction.
I'm sure they tweak other things but this is the man difference.

For this reason, if you derate the ripple current rating, as
long as you don't exceed the ambient temperature rating, the
lifetime can safely be assumed to improve.

 

[John]

I seriously doubt it, if the design does not involve any
The test rig schematic only had one inductor.

+-------+
1.5uH | |
12V---+------- ___ -------+-------+Vin |
| -UUU- | | |
--- --- +-------+
---270uF 270uF--- Converter
| |
GND GND

Draws about 4.4A at 12V and delivers 40A at 0.9V.


Good tip. I was going to try to minimize it as much as possible
thinking that I really didn't want to sling whatever EMI was coming
off of that 10A of ripple.

Thanks!
John
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[John]

For example a cap rated for 2000 hours at 85C would last 8000 hours at an
Very good point. I'm not sure how many options I have with the low
ESR I'm looking for but I guess it can be a tradeoff between ESR and
temp rating...especially if the price difference is large.

John
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[John Popelish]

The test rig schematic only had one inductor.

+-------+
1.5uH | |
12V---+------- ___ -------+-------+Vin |
| -UUU- | | |
--- --- +-------+
---270uF 270uF--- Converter
| |
GND GND

Draws about 4.4A at 12V and delivers 40A at 0.9V.



Good tip. I was going to try to minimize it as much as possible
thinking that I really didn't want to sling whatever EMI was coming
off of that 10A of ripple.

I don't know the physical shape of the converter and whether
it is a package or a bunch of components, but since you are
paralleling capacitors for the final storage, you may be
able to make two groups and route their ground sides to the
common point on the converter by two equal and opposite
loops (from a current standpoint) so that the bulk of the
external field is canceled.

 

[John]

Ummm, it starts getting more complicated when you add inductance. The inductor
I noticed that the ripple rating went up as the capacitance went up.
Since my load for the converter is pretty constant (other than
ON/OFF), does that ease the requirement for ultra-low ESR caps?

The data sheet and app notes for the converter emphasize ESR but I'm
thinking that if I have only a slowly varying load (if varying at all)
that low ESR caps aren't nearly as important for voltage regulation
but might only needed to satisfy any input requirements for the
converter?

I guess the lower ESR caps would make it easier to keep the caps cool
though, taking the 10A ripple current into account.

John
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[John]

I don't know the physical shape of the converter and whether
Great idea!

It's a vertically mounted SIP with 24 pins (single row) spread out
over 2.4".

There are three Vin pins in the center with the Vout and GND pins
alternating for pins 15-25 and scattered GND in pins 1-8 to go with
the POWER GOOD, TRIM, and CURRENT SHARE signal pins (pins 1-8).

I guess I have either side of that row of pins to play with.

John
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[Eeyore]

Very good point. I'm not sure how many options I have with the low
ESR I'm looking for but I guess it can be a tradeoff between ESR and
temp rating...especially if the price difference is large.

You can end up juggling these things forever.

Graham

 

[John]

You can end up juggling these things forever.

LOL, you read my mind on that one! I guess I just gotta' dive in and
start buying caps. It's only a one-off device. OK, maybe 2 or 3 if
it works well.

John
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[John Popelish]

I noticed that the ripple rating went up as the capacitance went up.

That is just the equivalent of paralleling several smaller
caps, inside the can.

Since my load for the converter is pretty constant (other than
ON/OFF), does that ease the requirement for ultra-low ESR caps?

Not at all. The ripple current is related to the value of
the output current, not how fast it changes. It is the load
current being drawn from the input capacitors, but only
during part of the converter cycle, and not at the other
part of the cycle. Remove the average from that pulse train
and you get an alternating direction current of about half
the DC output current.

The data sheet and app notes for the converter emphasize ESR but I'm
thinking that if I have only a slowly varying load (if varying at all)
that low ESR caps aren't nearly as important for voltage regulation
but might only needed to satisfy any input requirements for the
converter?

I guess the lower ESR caps would make it easier to keep the caps cool
though, taking the 10A ripple current into account.

It lowers the actual watts dumped into the caps, but also
lowers the peak spike voltage that appears across them and
that propagates upstream through the filter inductor.

 

[John Popelish]

Great idea!

It's a vertically mounted SIP with 24 pins (single row) spread out
over 2.4".

There are three Vin pins in the center with the Vout and GND pins
alternating for pins 15-25 and scattered GND in pins 1-8 to go with
the POWER GOOD, TRIM, and CURRENT SHARE signal pins (pins 1-8).

I guess I have either side of that row of pins to play with.

That is what I was picturing.

 

[John Popelish]

LOL, you read my mind on that one! I guess I just gotta' dive in and
start buying caps. It's only a one-off device. OK, maybe 2 or 3 if
it works well.

In that case, please include a few 1uF, 50 or 63 volt
stacked film caps (Panasonic type V) to experiment with as
high frequency bypass for the electrolytics, if you can get
them. These would go as close to the SIP pins as possible,
to keep the highest frequencies out of the larger loops.

 

[John]

Not at all. The ripple current is related to the value of

Ahhhh...OK.

So the ESR times the ripple current = the voltage spike?
I can see how reducing the ESR would be valuable here.

Thanks!
John
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