Filtering the HF hash of a SMPS - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th July 2015, 01:19 PM   #1
diyAudio Member
 
Join Date: Mar 2015
Default Filtering the HF hash of a SMPS

Help me with designing a filter to get rid of some of the HF noise from a brick 12VDC SMPS and to learn something.

I decided to use an external SMPS based on what I have around (a 6A 12V brick or a 12V AC trafo for halogen lighting). The linear supply would require quite a bit of voltage dropping which with installing the trafo would require a bigger case.

I'm powering 4 shunt regulators drawing a total of around 3A. They will work off 10V so I have 2V spare to drop in the filter. I was thinking to use a CRC or a CRCRC, each C being a 10uF tantal paralell with 100nF ceramic. I chose this based solely by feeling so please educate me.
Also I'd like to know how to simulate the effects of different filter on the HF noise once I know what filter to use. Unfortunately I have no scope so I can't measure the HF content of my SMPS to give you some figures. I'll have design the filter blindly, expecting the worst and do the best filtering witouth breaking the bank. Please no top of the line "audio" components", i want engineering.
And a side question, what HF characteristics do PTFE/AL foil capacitors have? There is cheap russian old stock around.
Thanx,
Marko
  Reply With Quote
Old 16th July 2015, 01:41 PM   #2
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
Firstly you need to measure the HF hash if any coming out of the SMPS, then the filtering can be targeted, as you cant its all guess work... Not really recommended.
Also look at Henry Ott's sites...
home page
and this...

http://www.compliance-club.com/pdf/EMCTestingPart1.pdf
http://www.compliance-club.com/pdf/EMCTestingPart2.pdf
http://www.compliance-club.com/pdf/EMCTestingPart3.pdf
http://www.compliance-club.com/pdf/EMCTestingPart4.pdf
http://www.compliance-club.com/pdf/EMCTestingPart5.pdf
http://www.compliance-club.com/pdf/EMCTestingPart6.pdf
http://www.compliance-club.com/pdf/EMCTestingPart7.pdf
EMC Design
Design Techniques for EMC
Design Techniques for EMC: cables and connectors
http://www.compliance-club.com/curre....asp?artid=500.
Design Techniques for EMC - part 4
PCB DESIGN TECHNIQUES FOR LOWEST-COST EMC COMPLIANCE
Design for EMC Part 6: ESD, dips and dropouts, etc.

My self, I would be look at common mode and differential mode chokes as well as Pi filters with caps and ferrites...... without something to aim for though...

Leaded devices are not the best for this, you have to use RF design, that means ground planes careful layout etc.
  Reply With Quote
Old 16th July 2015, 01:44 PM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You can't design a filter by "feeling". Neither can we. You can't do "engineering" without a spec.

To filter out HF you need some vague idea of what frequencies are involved and how much filtering you need. Otherwise you don't know whether you need a filter at all, or if you need an impossible-to-practically-implement filter.

Having said that, HF can be filtered using LC - if the F is H enough. This is especially true when you need low DC impedance (you need low DC impedance to get less than 2V drop on up to 3A current).

Personally, I would not 'bypass' the caps with smaller caps - this could make things worse. I might 'bypass' the inductors with resistors, but lowish Q inductors might not need this.

You are probably dealing with frequencies from the SMPS switching rate (some 100's of kHz?) plus harmonics up to a few MHz. You can assume that almost any capacitor will act as a capacitor over this range, except highish value electrolytics - even they will maintain a lowish impedance.
  Reply With Quote
Old 16th July 2015, 01:49 PM   #4
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
also this is relevant...
Switcher EMC design
  Reply With Quote
Old 16th July 2015, 06:22 PM   #5
diyAudio Member
 
Join Date: Mar 2015
Quote:
Originally Posted by DF96 View Post
You can't design a filter by "feeling". Neither can we. You can't do "engineering" without a spec.

You are probably dealing with frequencies from the SMPS switching rate (some 100's of kHz?) plus harmonics up to a few MHz. You can assume that almost any capacitor will act as a capacitor over this range, except highish value electrolytics - even they will maintain a lowish impedance.
So I'll borrow a scope next week and see what Fs are we talking about.
Until I do let's do a practical design that would attenuate the probable switching F and harmonics. What filter would you use to get 50 kHz to 5 (10?) MHz down 60 dB @ 3A and drop two Volts.

I'd like us to come to a practical solution with this example so I can re-do the design myself when I get the measurments.

I'm powering a DCX2496 digital crossover with 4 ReflectorD shunt power supplys from this forum's member Salas. They do just fine without the HF filter for now but I'll measure if some hash does get through once I borrow the scope. And I'd like to know how to design a HF filter anyway.

And hey, thanks for your help!
  Reply With Quote
Old 16th July 2015, 07:15 PM   #6
diyAudio Member
 
Join Date: Jan 2010
Quote:
Originally Posted by m a r k o View Post
So I'll borrow a scope next week and see what Fs are we talking about.
Until I do let's do a practical design that would attenuate the probable switching F and harmonics. What filter would you use to get 50 kHz to 5 (10?) MHz down 60 dB @ 3A and drop two Volts.

I'd like us to come to a practical solution with this example so I can re-do the design myself when I get the measurments.

I'm powering a DCX2496 digital crossover with 4 ReflectorD shunt power supplys from this forum's member Salas. They do just fine without the HF filter for now but I'll measure if some hash does get through once I borrow the scope. And I'd like to know how to design a HF filter anyway.

And hey, thanks for your help!
You assume there is some noise between pos and neg output of your smps - this might be true. In that case we talk of symmetrical noise that can be reduced by a symmetrical LC filter or similar. Unfortunately, this is only the first half of the truth.
There exist asymmetrical (or common mode) disturbances as well that require a different type of filter (common mode filter). You need basic knowledge of both types to deal efficiently with filtering.
  Reply With Quote
Old 17th July 2015, 06:35 AM   #7
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
Quote:
Originally Posted by marce View Post



My self, I would be look at common mode and differential mode chokes as well as Pi filters with caps and ferrites...... without something to aim for though...

Have you looked at any of the links? The thread link is a set of papers done by Keith Armstrong on exactly what you are wanting to do, noise on switcher circuits?
  Reply With Quote
Old 17th July 2015, 07:21 AM   #8
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
a wise man once said "return the noise from whence it came".
filtering is not always the best way. filtering often will push the noise from one end to another. input to output or visa versa. sometimes you can drive the noise into the grounds to pop up again elsewhere E.g Common mode sourced.
best to optimize or add snubbers to absorb the hash energy from where it comes E.g. switches and rectifiers. once that is done look at filters and decide based on measurements and topologies. bricks for laptops is more line ripple than hash once loading near max.> brick topology is pretty bad generally, you get what you pay for, so upgrade to decent half frame SMPS can save grief, time, and money in the long term.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 17th July 2015, 08:10 AM   #9
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
On reflection we are discussing EMC, Henry Ott, Ralph Morrison etc. have written books about it, the EMC club is full of articles as well as notes and papers by many others...
On this or any other thread all we would be able to look at is a specific well defined problem.
  Reply With Quote
Old 17th July 2015, 08:47 AM   #10
diyAudio Member
 
trobbins's Avatar
 
Join Date: Feb 2009
Location: Melbourne, Oz
Perhaps if you power your audio equipment using a 12V battery for starters, and measure the noise floor of the audio signal output (somehow) - that is your reference to start with.

Then swap the battery for your very noisy, ugly, couldn't be seen dead with, smps and measure noise floor and report back :-)

Last edited by trobbins; 17th July 2015 at 08:50 AM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Filtering of HF content cwtim01 Power Supplies 2 24th March 2014 07:10 PM
Some brief questions on SMPS's filtering and grounding Moejoe Power Supplies 8 28th May 2013 07:21 PM
Filtering a cheap +/-15V dual SMPS supply for low power use (head amps, preamps) fualcr Power Supplies 7 6th January 2012 04:11 PM
12V smps EMI-filtering zilog Power Supplies 1 31st August 2008 10:13 PM
rca83 hash? garbage Tubes / Valves 40 13th February 2005 04:22 PM


New To Site? Need Help?

All times are GMT. The time now is 03:53 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2