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dsavitsk 20th August 2007 08:49 AM

Power Supply Ringing
 
I am putting together a power supply for a DAC. The transformer I want to use is 300-0-300, but I need somewhere on the order of 220V to 280V (there is a CCS which will drop whatever voltage it needs to.) So, I thought I'd use a choke input filter to drop some volts (with a small cap before it to cut down on any noise issues that might crop up.)

I simmed it in PSUD, and I seem to have some ringing. So, the questions are, 1) any suggestions on how to eliminate this, or 2) if I am using a CCS with decent PSRR, does this even matter that much?

Taking out the second choke does get rid of it, but I would like this PS to be super duper quiet as it is for a DAC, so using a second choke would be my preference.

http://ecp.cc/images/clclc.jpg

http://ecp.cc/images/clclc_start.jpg

http://ecp.cc/images/clclc_step.jpg

MRupp 20th August 2007 08:54 AM

In a low voltage LC filter I had some ringing caused by C1 once it went above a certain value. I suggest you try lower values or leave it out altogether.

Also, your choke may still ring, in which case snubbers would be a good idea to dampen the ringing.

SY 20th August 2007 12:36 PM

I re-simmed it with the first cap still at 1u (I changed ESR to 1R), the second cap at 47u/4R7 and the third at 200u/4R7. Perfectly smooth, no ringing with a stepped load.

zigzagflux 20th August 2007 12:54 PM

General rule of thumb is it's best to increase capacitor size as you progress down the filter, just as SY shows. This keeps the bulk storage at the end of the filter where it is available "quicker", instead of the beginning.

Of course, with a properly designed CCS at the output, I doubt it will make a difference.

Stixx 20th August 2007 01:02 PM

2 Attachment(s)
What about this one?
Saves you two chunks of iron (unless you want to have them on your top plate...;) ) plus the expense and has comparable filtering.

From my (limited) experience the EZ81 likes a bit more resistance in front of "her" :o , at least that is what the old datasheets say. On my MJ headphone amp it stopped the clanging and banging noises from the rectifier upon startup...

QSerraTico_Tico 20th August 2007 01:20 PM

Quote:

Originally posted by Stixx
What about this one?
Saves you two chunks of iron (unless you want to have them on your top plate...;) ) plus the expense and has comparable filtering.

From my (limited) experience the EZ81 likes a bit more resistance in front of "her" :o , at least that is what the old datasheets say. On my MJ headphone amp it stopped the clanging and banging noises from the rectifier upon startup...

An additional benefit is that the resistors do not pick up hum stray-fields as the coils do.

dsavitsk 20th August 2007 05:38 PM

Quote:

Originally posted by SY
I re-simmed it with the first cap still at 1u (I changed ESR to 1R), the second cap at 47u/4R7 and the third at 200u/4R7. Perfectly smooth, no ringing with a stepped load.

This is a pathetic complaint, but it puts a crimp in the ability to use a multi section (150 - 150) cap. There don't seem to be any electrolytic caps that can mount via clamps that are small enough (in uF) to not stress the rectifier. Well, there are some Jensens but they cost a small fortune. Maybe some motor runs or polyprops, but they are a little large for my space. I'll keep looking.


Quote:

Originally posted by Stixx
What about this one?
Saves you two chunks of iron (unless you want to have them on your top plate...;) ) plus the expense and has comparable filtering.

Actually, they'll be inside the case. But, from my limited experience, power supplies that drop a lot of voltage via a bunch of large resistors don't sound that good. It could be the amp I tried it in, or my imagination, but even behind a CCS it didn't seem to sound as good as a choke input filter when lots of voltage drop was necessary.

Edit: It does seem that using a small multi section cap and adding an extra RC filter with a larger cap both gets rid of the ringing and improves ripple by quite a lot. Since I am going to be using two caps anyway, this seems like a good solution. So,

c(1u) L(10H) C(32u) L(10H) C(32u) R(220R) C(100) seems just about right, the last 100U being accomplished by paralleling a 50-50. Or, there is a JJ that is 20+20+ 20+40 which looks like it can be made to work, though ripple is not as good.

-d

SY 20th August 2007 05:59 PM

Then stagger the choke values or use two multisection cans. The main idea is to spread apart the resonant frequencies of each section.


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