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Guidance on a switcher as a pre-reg
Guidance on a switcher as a pre-reg
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Old 18th October 2020, 02:29 PM   #1
tvrgeek is offline tvrgeek  United States
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Default Guidance on a switcher as a pre-reg

Thinking of using one of my many old laptop switchers in leu of a transformer for the input to a very good linear regulator to feed a DAC. Wondering how much capacitance switchers can handle without stability or surge failure.

Guessing they should handle a couple hundred mic, but probably not more.

Cheaper than a transformer and large filter as I have them.
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Old 19th October 2020, 01:24 AM   #2
egellings is offline egellings  United States
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Keep in mind that the HF noise on the switcher's output will ride right through the linear regulator's circuitry. The linear regulator's error amplifier is not fast enough to track this noise and null it out so it gets through. Maybe this is not an issue in your application.
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Old 19th October 2020, 10:09 AM   #3
tvrgeek is offline tvrgeek  United States
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Ah, just the information I was looking for. My regulators should get here today so I can test. Maybe a C-R-C filter feeding the linear.
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Old 19th October 2020, 10:33 AM   #4
trung224 is online now trung224
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Another problem with CRC filter after the switcher is that C (normally electrolytics in hundred microfara range) loses its effective capacitance when going to 500kHz-1MHz and higher frequencies. This make your CRC useless for filtering high frequency noise anyway.

I think if your DAC does not consume much power, you should consider TPS7A470x regulator family. Its bandwidth extends to 10 MHz, so it is useful to filter the high frequency noise from the laptop switcher.
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Old 19th October 2020, 10:39 AM   #5
synonymous is offline synonymous  United States
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C-L-C-L-C

Use an electrolytic and a ceramic or two in each C
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Old 23rd October 2020, 02:44 PM   #6
jackinnj is offline jackinnj  United States
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Guidance on a switcher as a pre-reg
Take a look at this thread:
PO89ZB , an inline DC filter for SMPS wall warts . Preamps, HPA, Korg NuTube, etc
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Old 24th October 2020, 12:38 AM   #7
tvrgeek is offline tvrgeek  United States
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TPS7A470x I'll look for boards with that. I don't do boards any more and SMT was a royal pain. Digging through junk box found some old chokes. Maybe I'll make some "pi"
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Old 24th October 2020, 01:19 AM   #8
Rick PA Stadel is offline Rick PA Stadel  United States
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Suggest you track down Mark Johnson's thread somewhere herein -- it goes by a 4-letter acronym (that of course I can't remember ) .. Wish I could give you a link, but I've struck out looking for it a couple of times, now. It's designed specifically for this situation, should remedy all of your concerns, and he's giving away the engineering, including the Gerbers! Thanks Mark.

Cheers

edit: Oops -- shouldn't have breezed over a few posts: jackinnj already had it -- "PO89ZB".

Last edited by Rick PA Stadel; 24th October 2020 at 01:25 AM.
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Old 24th October 2020, 01:29 AM   #9
6L6 is offline 6L6  United States
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Guidance on a switcher as a pre-reg
PO89ZB, link above, does an INCREDIBLE job of removing all the switcher HF junk. It's quire amazing, absolutely try it.
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Old 24th October 2020, 01:45 AM   #10
gigigirl is offline gigigirl
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HI 6L6, Interesting, Can they we bought fully populated.

Cheers
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