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Old 24th October 2009, 12:04 AM   #1
at77 is offline at77  Canada
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Default Quieting a Switching PSU? or easier to replace it with a linear PSU?

hey all,
i have a switching PSU in a digi 002 rack recording interface. It's pretty noisy (power wise). I'm trying to decide whether to try to clean it up by some simple method - replacing the caps with better, for example - or to replacing the whole thing with a linear psu.
it's powering only low power stuff - +/- 12v for opamps and a 48v circuit (for phantom mic power), + 5v and + 3.3v for digital and logic components.
the caps (post voltage regulators) are Hermei LE caps (mostly 2200uf and 1000uf) and i thought about swapping those with same value nichicon kz or panasonic FM perhaps.
another thought i had was just to drop a 2200uf cap on each lead (power to ground) to filter there rather than messing with the psu itself. there's already chokes on the board for the 12v, but it's still pretty noisy.
my other option is adding a psu for the analog 12v rails - which i tried unsuccessfully... has a bad ground loop noise and i'm not clever enough to source it. think it needs to be redesigned to isolate the AC side. soooo, thought about just building a whole new linear psu for everything with 3.3v, 5v and the +/-12v lines as well. would rather the simplicity of keeping the switching psu and cleaning it up though.
any advice is appreciated.
thanks,
Adam
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Old 24th October 2009, 02:29 AM   #2
star882 is offline star882  United States
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Add a few LC filters.
Linear is generally only practical for low power applications (roughly less than 5w, or less than 10w for LDO). Modify the power supply to increase the 12v and 48v outputs by a little and add LDOs.
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Old 24th October 2009, 02:33 AM   #3
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Quieting a switch mode power supply is much akin to tuning a banjo -- when you throw it in the garbage and it hits the "Ozark Harp" it's tuned.

Same could be said of the accordian.

But seriously -- an LM317/LM337 as a post regulator can rid many nasties but you will burn a couple of watts in the process.
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Old 24th October 2009, 09:15 PM   #4
at77 is offline at77  Canada
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Thank you both.
I have to do more research about the LC filters, star882. So many variables that I'm not familiar with (I'm just a hobbyist, not an engineer unforunately).
perhaps the LM3** is an ideal simple solution for me. : )
thanks for the help. may post design ideas once i have some.
cheers,
Adam
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Old 27th October 2009, 11:19 AM   #5
at77 is offline at77  Canada
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Default How bout this?

hey there,
so, after a little more research, i discerned that I didn't know:
A) whether it should be a band pass, band cut, or low pass LC filter
B) what frequency I should be aiming for/against
C) the appropriate values of the components given those things.
however, i did discover that i've made LC filters before, only i called them 'crossover networks' ; ) and used them in speakers.

I also built and use Elso's Kwak-Clock which has a little LC filter in it. It's a 2.2mh inductor with a 2200µf cap. From my research i discerned that it is a low pass PI format taking advantage of the cap at the psu.

My question is this:
without getting too deep into the math, could I just use the same simple circuit and values for my ripple/noise filter?
I know it's a bit of a trick getting an inductor with a high enough amperage rating. my positive 12v supply line (as rated on the psu) is 3.5amps, negative is 0.5amps. But from a little bit of very rough calculation, it seems that my board is only really drawing 1.5amps on the positive rail max.

Other question is this:
saw an article on boosting 12v to 19v in the 'favorites' of psu articles here. thought about boosting the 12v rails to 18v to take full advantage of the opamp headroom (they're all opa2134/4134 or 4228 - rated for +-18v). I'm not sure if i'll run into trouble though starving for amperage once i boost the voltage. any thoughts to point me in the right direction?
(i had built, as i said, a separate psu for the analog rails that was +/-18v that worked EXCEPT it had a bad 60hz hum. because i don't know enough, i haven't figured how to resolve it, so I just temporarily gave that up. point being, the board happily works with the voltage boost).

Thanks for any input.
Adam

ps edit:
thought about, with the voltage boost, doing a simple voltage doubling circuit from 12v to 24v and then regulating back to 18v with the lm317/337 - basically trying to do the boost and clean up in one fell swoop. thoughts?
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Last edited by at77; 27th October 2009 at 11:22 AM. Reason: forgot something
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Old 27th October 2009, 02:00 PM   #6
Pano is offline Pano  United States
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Quote:
Originally Posted by jackinnj View Post
Same could be said of the accordian.
Hey! I resemble that remark!

Are the 3 footed regs like the LM317 fast enough to clean up the HF hash that most switchers have? I'll have to go back and look at impedance plots.
Was thinkin' they are too slow.
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Old 27th October 2009, 05:12 PM   #7
at77 is offline at77  Canada
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hey gents, this will probably show my lack of knowledge, but...
i was reading the lm317 datasheet and thought, perhaps besides regulating, i could use it to also boost the voltage to 18v since it just runs on the principle of set resistance compared to the 1.25v signal it generates.

My first assumption was that it is like most regulators and needs supply voltage higher than its output voltage, but perhaps this isn't the case.
a word from the wise please?
Adam
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Old 27th October 2009, 05:15 PM   #8
at77 is offline at77  Canada
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ps i did see in the data sheet that it runs at about 65db ripple rejection without capacitor decoupling, but that can be increased to 80db with the addition of a 10f cap to the adjustment terminal. I assume it would also be happier with a nice big fat Low esr cap before the voltage input.
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Old 27th October 2009, 06:09 PM   #9
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Quote:
Originally Posted by at77 View Post
hey gents, this will probably show my lack of knowledge, but...
i was reading the lm317 datasheet and thought, perhaps besides regulating, i could use it to also boost the voltage to 18v since it just runs on the principle of set resistance compared to the 1.25v signal it generates.

My first assumption was that it is like most regulators and needs supply voltage higher than its output voltage, but perhaps this isn't the case.
a word from the wise please?
Adam
You can get the LM317 to act as a boost regulator -- it's in the data sheet -- but you are back to noise generation. TANSTAAFL as we used to say at the UofC.
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Old 27th October 2009, 06:28 PM   #10
at77 is offline at77  Canada
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perhaps then, 2 in series? one for boosting, the other following for smoothing? yes no?
or is the noise generation too significant to make that viable?
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