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