Organic Aluminum Polymer capacitors in series for higher voltage?

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I need a low ESR high ripple current capacitor for the output of a
switching DC-DC boost converter, and I am considering using several organic
aluminum polymer capacitors in series. What I need is at least about 50 uF
at 60 VDC or more, with ripple current at 100 kHz of about 3 amps RMS. The
closest I could find in standard type low ESR capacitors was a Nichicon UPW
series 470 uF 63 VDC with 55 mOhm ESR and 2090 mA in a 12.5 x 31.5 mm
radial package, which is about the largest I can fit. I can get a 270 uF LF
series polymer cap in a 16 VDC rating that is good for 4800 mA, with 9 mOhm
ESR, in an 8 x 12 mm package, and I think I can fit four of them in the
space I have available.

My concern is that the applied voltage may not be distributed evenly enough
among the four capacitors, and may cause an overvoltage condition on one or
more. The same charging current will be going through each, but the voltage
will depend on the actual capacitance, which can vary 20%, so I might see
12 to 18 VDC on them with 60 VDC maximum total voltage. The other concern
is during unloaded discharge, where leakage current will vary and some will
discharge faster than others. However, if leakage increases with applied
voltage, and does not vary too greatly, it may be self-correcting, as those
capacitors with more voltage will bleed off faster than others. I would
rather not add bleeder resistors across each one to equalize the voltage,
if possible.

I have looked at large multilayer ceramic capacitors and they are
ridiculously expensive for more than 2 or 3 uF at 100 VDC. I saw some when
I checked stock at Mouser that were nearly $1000 each! Obviously intended
for military, aerospace, or audiophools.

TIA for sharing any experience with series capacitors. I have ordered some
and I will do some bench testing, but that does not always provide enough
information to be sure of reliability in a production device.

Paul



Re: Organic Aluminum Polymer capacitors in series for higher voltage?

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Err, I get a few like this:
http://www.mouser.com/Search/ProductDetail.aspx?qs=hnsWAVXnr6FyB9xGVTFjpA%3d%3d
From $7 in 1-off to under $1 in QTY. 4.7uF 100V 2220 case.

Digikey have ones up to 15uF:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name56%5-3242-1-ND
But they are more pricey.

What is your space concern - height?, area?, both?
There are ways to reduce both.

Dave.



Re: Organic Aluminum Polymer capacitors in series for higher voltage?
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In theory you could use zeners in parallel with those caps.
As long as the zener voltage is lower then the Vmax of the cap, and
the sum
of the zener voltages higher then the total voltage....
Have not tried, maybe it would go poof.

Re: Organic Aluminum Polymer capacitors in series for higher voltage?
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I have done basically the same thing.  I put resistors across the
capacitors to hold the voltages in place with time.  I used  high
enough voltage capacitors so that even with the most extreme value
mismatch the voltage rating would not be exceeded.  I also placed some
1N400X diodes across them for the discharge at power off even though I
doubt it would really be a problem.



with ripple current at 100 kHz of about 3 amps RMS. The
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Re: Organic Aluminum Polymer capacitors in series for higher voltage?
On Wed, 25 Jun 2008 03:46:05 -0400, "Paul E. Schoen"

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I tested some 12-volt aluminum poly caps. Charging them at low
current, through a resistor, leakage was very low until about 25
volts, when they instantly failed shorted. This is different from a
regular aluminum or tantalum cap, where leakage increases with voltage
so there is some free series-stack equalizing effect. So I'd think
they need equalizing parts; Jan's zeners, maybe.

We stock this ceramic one...


  PART:  1814   CAPACITOR       CHIP 1206        10UF 25V    


  STOCK   UM      MIN   ALLOC    USED     OUT     BUY     PRICE    TOT VAL
    996   EA        0     404     737       0       0      0.33     328.03


  Mfr1   MURATA             GRM31CR61E106KA12L                        
  Mfr2   DIGIKEY            490-3373-1-ND              SAME AS M1      
  Mfr3   MOUSER             81-GRM31CR61E106KA12L      SAME AS M1      
  Mfr4                                                                
  Mfr5                                                                


which you could stack up in various permutations.

I really like the alum polys for switcher output filter caps, as long
as you allow for their very low esr re: loop stability.

John



Re: Organic Aluminum Polymer capacitors in series for higher voltage?

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I am using a PIC16HV616 or PIC16F684 for the PWM, and the output is
actually current regulated at about 800 mA to a string of 7 or 13 high
power LEDs. Using the A/D to sample the output current every 1 mSec or so,
I am relying on the output capacitor to slow the output response enough to
avoid a large overshoot, although this would usually not be a problem
unless the input voltage rose suddenly, which is unlikely with an SLA
battery. Ripple at 100 kHz on the LEDs is not a factor. My new design also
senses the MOSFET source current, using a comparator which can react much
more quickly to such conditions, and I can probably get by with a much
smaller capacitor. I was able to run the prototype (with an older board)
with only 2 uF output capacitance, but that was on a current limited
supply. I am afraid that with an SLA battery it could ramp up to
destructive current levels before the PIC can sense it and throttle back,
at least with the old design that did not have a drain current sense.

I am already using a 1.5 uF 100 volt MLCC in SMT1210 that is only $0.53
ea/100, and there is also a 1.0 uF 100 volt in SMT1206 for $0.31 ea/100. So
I could lay out maybe 10 of these in parallel on the bottom of the board,
and that works out to only about $3 for 10 uF or $5 for 15 uF, which is
probably enough. I will have to consider the extra cost of multiple SMT
parts on the bottom of the board, which are not easily done by pick and
place, and will add extra labor cost if hand soldered. But it won't cost
anything to add the pads as an option. Maybe I could make a small separate
board for the capacitors and wire it to the main board as a module.

Using the capacitor you suggest works out to about $5 for 10 uF at 100V,
and would require 16 pieces as well as resistors or zeners for series
connection.

Thanks for checking this out. Adding zeners or resistors will be a
labor-intensive operation if the lower voltage series capacitors are used.

Paul



Re: Organic Aluminum Polymer capacitors in series for higher voltage?
On Wed, 25 Jun 2008 15:36:33 -0400, "Paul E. Schoen"

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Oh, on your ceramic cap, check the actual capacitance *at 100 volts*.
Some of these hi-K dielectrics can lose half their C or more as
voltage goes up.

John


Re: Organic Aluminum Polymer capacitors in series for higher voltage?
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Have you looked at film capacitors?
e.g.
http://www.avx.com/docs/Catalogs/ffb.pdf

The 68 uF, 75 volt 6.2 amp 1.7 m ohm unit runs about $21
(recently raised from $15) at Digikey, quantity 1.




--
Regards,

John Popelish

Re: Organic Aluminum Polymer capacitors in series for higher voltage?

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This part is 31 x 17 x 43 mm and will not fit. The 33 uF version is 31 x 13
x 22 and is just barely too large to fit. The PCB is 0.95" wide x 2.5" long
and sits in the middle of a 25 mm ID tube, so the 12.5 mm round can just
barely fits. A rectangular footprint is more difficult.

Thanks,

Paul



Re: Organic Aluminum Polymer capacitors in series for higher voltage?
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I'm sure I have seen 10uF 100V X5R ceramic multi layer SM
caps for about 10$, so 5 of them in parallel should work and
be small enough.  Not a thousand dollars either.

--
Regards,

John Popelish

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