cheap switching supply cleanup - Page 1
Forum section: electronics / projects

author: cncjerry
author_rank: Unknown
timestamp: December 02, 2017, 10:36:15 pm
content: OK, idiot I can be, I needed a higher current 24V supply than what my various Tek supplies can provide, around 2amps, I was thinking at the time, so I bought one of those 0-30v, 10Amp supplies and of course it is noisy as hell. I was using it to power some rubidium frequency standards for Allan deviation testing (yes, I know this multiplies my idiotcracy) and saw some funky noise +/- 15khz from the 10M carrier. These RFS units have a small spike in that area already that I have to explore. At first I thought the problem was my recently repaired 8566B as I didn't see it on the 8568B in the other room but then I was using a good Tek supply on the 8568B test setup.Anyway, enough said, can I filter this out easily? Is there a simple notch filter that people have used successfully on switching supplies or should I just get a choke or two and some outboard caps or am I wasting my time trying to clean it up?I'm thinking of now just buying another Tek PS5010 as I really only need about .7amps and the PS 5010 can do it in a high power compartment.thanks,Jerry

author: cncjerry
author_rank: Unknown
timestamp: December 02, 2017, 11:07:10 pm
content: Yes, I thought about doing that during testing and pulled out two large SLAs I had which are now dead, expanded, etc. I had planned to float charge them.So I ordered this POS. I 've tried a few chokes, etc. but no luck yet. I'll have to get serious about it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2017, 09:47:59 am
content: If it's the kind of supply I'm thinking of, I've been down that road before.My application was a 100V 10A switching supply for low voltage testing of industrial equipment.The common mode noise was so bad, it was visible on the switching output. Like, 10% of the full output voltage.I added a big choke (2.4" ferrite toroid with 2 x 15 turns 12AWG hookup wire) on the mains side (where the internal CMC clearly wasn't doing enough) and on the output side. I also added a 10uF film cap across the output, and some Y caps to ground.This reduced it to tolerable levels (under a volt), which isn't saying much.Later, the PSU destroyed itself, deliveringrated currentinto a low voltage load. Piece of shit.Anyway, common mode noise is usually the trickier offender. You can throw as much diff mode filtering at it as you want, and never fix the problem!First, you must demonstrate your problem: make a standard setup and measure how much noise the PSU is actually delivering. Ideally, you'd set up over a ground plane, using a mains LISN to power it, connecting a typical load to the output, and measuring the output +/- noise levels, at a high enough bandwidth (i.e., not the 20MHz typically quoted for PSU ripple!).Tim

author: 741
author_rank: Frequent Contributor
timestamp: December 05, 2017, 03:59:57 pm
content: Jim Williams's AN101 might help. From memory he says to use carefully chosen ferrite beads.I was going to add a link, but this forum seems to shows the whole url itself, and it's long and ugly...(For muchlowerfrequency issues, H&H 3rd ed Fig 8.121 shows a 1-transistor capacitative multiplier).