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Old 19th July 2014, 11:21 AM   #1
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Default LC Low Pass Filter Design.

Hi folks

I am trying to come up with an LC filter for the output of a linear power supply.

The output is switchable between 6v and 12vc DC, and I need a steady 2A out of the filter with a frequency cut off as low as possible.

If someone has a solution, would it be possible to explain the method used to reach that solution, as I don't have a lot of experience with LC filters.

Thanks
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Old 19th July 2014, 07:22 PM   #2
DF96 is online now DF96  England
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"Frequency cutoff as low as possible" is not a spec for a filter so you can't start designing from that. First you need to decide what you want the filter to filter out: ripple, noise, interference or what? We can't offer a solution if you haven't posed a problem.
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Old 19th July 2014, 08:14 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by JSmith1980 View Post
and I need a steady 2A out of the filter with a frequency cut off as low as possible.
Start by winding a coil about the size of your house, just much larger, buy the whole stock of high-farad caps from RS, Mouser, Farnell, etc and you will end up with a reasonable real-world approximation of your spec: a cut-off Freq in the µHz range
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Old 19th July 2014, 09:07 PM   #4
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When I say as low as possible, I mean I want to filter out as much DC ripple, noise and RF as possible.

Seeing as it is impossible to achieve that, I'm looking for an LC filter that comes very close, yet still of a practical size.

I have found a commercially available filter that would work, yet the output current is only 1A, I would prefer a steady output of 2A.
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Old 20th July 2014, 12:21 AM   #5
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DC ripple?

What's the size/weight target you have?
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Old 20th July 2014, 01:11 AM   #6
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Why not use a LM338 for a regulated supply? It has 5A output (up to 12A for short periods of time) and can be adjusted for 6v or 12v output. If you bypass the adjustment pin with a 10uf capacitor you get 75db ripple rejection at any output. Solid tantalum is best used due to low impedance even at high frequencies, just don't forget about protection diodes.
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Old 20th July 2014, 12:45 PM   #7
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The dimensions I have to work with in the enclosure is 250x80x300. approximately.

I reckon a cut off frequency of 40hz would be enough.

The main goal is to have the supply resemble an ideal battery as much as possible, whilst taking into account the area available for parts.

The power supply will be used to power low current digital devices, but I would prefer an output of 2A to add some versatility.
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Old 20th July 2014, 12:47 PM   #8
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Quote:
Originally Posted by famousmockingbird View Post
Why not use a LM338 for a regulated supply? It has 5A output (up to 12A for short periods of time) and can be adjusted for 6v or 12v output. If you bypass the adjustment pin with a 10uf capacitor you get 75db ripple rejection at any output. Solid tantalum is best used due to low impedance even at high frequencies, just don't forget about protection diodes.
Thanks for the info, I will have a look at the datasheet. I had initially planned to have two separately regulated stages, but that sounds better and it will free up some board space.
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Old 20th July 2014, 03:55 PM   #9
DF96 is online now DF96  England
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Quote:
Originally Posted by JSmith1980
When I say as low as possible, I mean I want to filter out as much DC ripple, noise and RF as possible.
Quote:
I reckon a cut off frequency of 40hz would be enough.
40Hz cutoff will remove very little ripple.

To design a PSU you need a specification. "Noise/ripple as low as possible" is not a specification. "40Hz cutoff" is a filter specification but not a very useful one if you are concerned about ripple. "Like an ideal battery" is not a specification.
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Old 20th July 2014, 06:36 PM   #10
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Quote:
Originally Posted by DF96 View Post
40Hz cutoff will remove very little ripple.

To design a PSU you need a specification. "Noise/ripple as low as possible" is not a specification. "40Hz cutoff" is a filter specification but not a very useful one if you are concerned about ripple. "Like an ideal battery" is not a specification.
Like I had mentioned in my initial post, I don't have a lot of experience with LC filters, I am learning as I go. Surely you must have an idea of what I am looking for, from the information I have given so far.

I am looking for a power supply that than can deliver a steady 6v or 12v with a steady 2A of current without noise, RFI and minimal DC ripple.

The first part is in place, I have the DC voltage and current. All I need to do now is eradicate unwanted ripple, noise and RFI from that DC, whilst keeping the device within the dimension I have specified.
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