Reducing SMPS noise

hondgm · 2016-09-06T16:46:13+00:00

I'm working on a PCB that contains nearly 30 resistors in a ladder configur ation, each step switched in or out by a SMT solid state relay. It's essen tially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the S MPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. Replacing the SMPS with a linear would be ideal, but the amount of space re quired is a big problem. So I'm looking at filtering the output of the SMP S. I'm a digital and DC analog guy, so this switching noise scares me. So me research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-dow n advice that will hopefully get me a solution without days of headaches. The SMPS claims a 100kHz switching frequency, and spikes on the scope confi rm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything.

Sep 06, 2016 43 Replies

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Tim Williams 9 years ago

Maybe you just couldn't 'hear' it. :^)

On a related subject, I once built a complete type D flip-flop from CML. Just to see how fast 2N3904s can possibly go. There was too much inductance and delay in the feedback loops though (which, being fairly small signals, is analog feedback too, not hard saturated digital), and it oscillated. Oh well. ;)

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com

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John Larkin 9 years ago

Level translation: start near ground, shift up, shift back down.

I did the circuit with SOT23s, and it worked, but they got absurdly hot. Too much voltage, too much current. SOT89 is a nice little package.

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This was too easy. Something's wrong.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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John Larkin 9 years ago

It just needed some ferrite beads!

Or base resistors, which do pretty much the same thing.

John Larkin Highland Technology, Inc lunatic fringe electronics

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John Larkin 9 years ago

The data sheets have Z versus current.

You seem crabby lately.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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