Cap selection for industrial power supply

[atergis]

Hello Badcaps -

Since you folks are brainiacs on capacitor selection, I thought you might be able to give me some advice on selecting alternate caps for an industrial power supply we are constructing.

A tiny bit of background - The supply is primarily for energy conservation sensors. We are working with an incoming voltage rail of 45-60 Volts and buck converting it down to one 3.3v rail and one 24v rail. Current flow is lower than 500mA on all rails. Currently we have Nichicon PW 33uF 63V UPW1J330MED caps providing filtering for the incoming voltage. The 24V buck converter currently has a Nichicon PW 56uF 50V UPW1H560MED cap, and the 3.3V buck converter has a Panasonic SMT SD 120uF 6.3V EEFSD0J121R.

I know most of this forum deals with higher capacitance and lower voltage, but do you have any suggestions for caps that would be the same size or smaller, and would offer a better lifetime or lower ESR?

Buck converters, being that they use inductors to produce the current flow for the caps, rely heavily on ESR for voltage ripple calculations. Right now I calculate our voltage ripple to be approximately 105mV for the 24V rail and 900uV for the 3.3V rail. Both of these figures are well within our absolute necessary spec, but less voltage ripple is always better.

Thank you in advance for your time
-Andrew Tergis, EtherMetrics LLC

[gonzo0815]

Re: Cap selection for industrial power supply

Are you using any particular switcher device like Simple Switchers (LM2XXX) or is it an proprietary, discrete design?
What environmental conditions are to be expected, what lifetime should the converter should have?
Without this information it is not that easy to give recommendations.

[atergis]

Re: Cap selection for industrial power supply

Switching devices are Linear Devices LT3437 for the 24V and Maxim MAX5033AUSA for 3.3V. The device will be enclosed in some plastic snap on enclosure (still being designed) but may be placed in potentially warm and humid environments (like around air handlers).

The idea would be to maximize the lifetime within our space requirements. Is there something better than the Nichicon PW series caps we are using? Even better, are there lower ESR caps of a similar size with better lifetime? More ESR = more voltage ripple in our case, but we have very strict size requirements.

[gonzo0815]

Re: Cap selection for industrial power supply

I think your cap choice is already a good compromise for the the task. I can`t find any other series with same load life and lower ESR.
Generally, i would add more then one capcitor, so you can reduce ESR, increase ripple current capability and at least height is not the problem.
But may be you should look into MLCC`s, as the available capacitance has much increased since the last years.
I am not that sure about their reliability, but at least they can`t dry out and have excellent ESL & ESR.

[atergis]

Re: Cap selection for industrial power supply

Thanks. I'll look into some of the larger-rating MLCCs. We already use MLCCs elsewhere on the circuit, but not for primary power rail filtering. Thanks for the suggestion!