capacitors in parallel

MICRO-Farads??

Electrolytics make for lousy, inaccurate filters. ...Jim Thompson

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Man, I was about to blithely answer the question without even noticing the units.

Mr Pan:

What are you _really_ trying to do? Filtering at the frequencies that such capacitor values seem to imply is usually done actively using op- amps, much smaller capacitor values, and much higher resistances than would be 'normal' in a power signal path -- then once the signal is the way you want it, you amplify the snot out of it.

Putting aside concerns about the wisdom of using electrolytics in a pi filter, yes, you can parallel capacitors. If not with impunity, then often with success, and often (particularly when you're on the upper boarder of the range of a particular capacitor technology's commonly available values) to the benefit of cost and/or performance.

If you're using the caps at the upper end of their frequency range then odd things can happen, but generally you wouldn't be paralleling caps in that case, unless you have made some very unwise decisions about the impedances of the circuit you're designing.

e,

ttdesign.com

It's for the output of a DC/DC converter, I'm following a design similar to the one here

there's a pi filter at the 5V output

reasons I asked about paralleling caps is cause I'm trying to avoid using electrolytics, im thinking i'll put some ceramics in parallel to meet this capacitance

thanks

Software

Paralleling reduces parasitic resistance and inductance (ESR and ESL) so works better than a single cap.

That's a lot of capacitance to get from ceramics. Take a look at polymer electrolytics: they have very low ESR and don't dry out like wet caps.

It's a power supply filter. Microfarads make sense, although 1000 is maybe a little overkill for a 1.7 amp switcher.

re,

But for this use he really needs to use low ESR caps. Panasonic FMs or Nichicon HZs. Organic polymers?

G=B2

ure,

cottdesign.com

I hope it's OK if I hijack some of your thread. I wanted to ask a silly question.

I've always thought of pi filters as 'filtering both ways'. That is you've got an L (or R) and C, putting a C on the other end snuffs out signals coming back into the circuit. In the circuit panfilero posted this doesn't seem to be the case. So what's the first cap doing? (C8 in app note) If it's part of the output filter, what impedance is it working against?

George H.