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Old 30th June 2004, 08:36 PM   #1
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Default LC filter design

would anyone have details or equations for designing an LC filter..the purpose is to build a lad PSU using the lm2575..i just need an extra output filter to minimise ripple..
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Old 30th June 2004, 09:19 PM   #2
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The nice thing about CLC type supply filters is that they are so
non-critical of part values. Almost any coil and reasonably big
capacitors will make a big improvement over just capacitors.

Try something like 1 to 2 mH (air core with heavy wire) and
10,000 to 30,000 uF capacitors (4 of them total for +/- supply)

Even better if you isolate the channels with separate supplies
or filters.

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Old 1st July 2004, 07:12 PM   #3
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thanks nelson..much appreciated..i will post my design..see what everyone thinks..
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Old 2nd July 2004, 12:10 AM   #4
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You should be looking into min. 1.9mm wire and 2mH.

I wind my own to get custom inductors fast and easy. A roll of 1.9mm magnet wire (approx. 20kg) will give you like 12-15 2mH inductors, depending how you wind them.

Dont forget to pot the inductors to keep them silent, most epoxies will do just fine for that purpose.

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Old 2nd July 2004, 03:58 AM   #5
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Quote:
Originally posted by Nelson Pass
The nice thing about CLC type supply filters is that they are so
non-critical of part values. Almost any coil and reasonably big
capacitors will make a big improvement over just capacitors.

Try something like 1 to 2 mH (air core with heavy wire) and
10,000 to 30,000 uF capacitors (4 of them total for +/- supply)

Even better if you isolate the channels with separate supplies
or filters.


The LM2575 is a buck-switching regulator, the noise is not ripple. It's a different set of problems, as I have said before, switcher noise just laughs at the techniques employed in linear regulators and moves mirthfully through your system. You can't blithely plug in cap values without knowing the ESR etc. and at any rate, the high switching frequency, to an extent, dominates the choice of cap.

Nat Semi provides all the equations for calculating the ripple -- subscribe to their interactive design tools. They also have webcasts on switcher design archived on their website.

Wanna quiet a switcher ? -- try using a linear post-regulator. You give up a bit in terms of efficiency, but gain a lot in terms of the quality of power supply output.
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Old 2nd July 2004, 04:06 AM   #6
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magura..how did you do your calculations?..that is what i wanted to know..btw jack..thanx for the input...didnt think of a post lin reg...maybe a lm723
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