New self oscillating post filter feedback topology... - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Class D

Class D Switching Power Amplifiers and Power D/A conversion

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 29th April 2010, 12:35 AM   #11
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
You will need op-amps for implementing active poles, the trick is to wire them in such a way that the circuit does not rely on them much above 20khz.
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote
Old 29th April 2010, 09:14 AM   #12
81bas is offline 81bas  Germany
diyAudio Member
 
Join Date: Jan 2006
Location: Kiel
Quote:
Originally Posted by Eva View Post
You will need op-amps for implementing active poles, the trick is to wire them in such a way that the circuit does not rely on them much above 20khz.
May I rephrase your sentence? I would say "the trick is to wire them in such a way that the circuit does not rely on them near Fo of the output LC filter". I am right?

Also, is the usual integrator an example of the active pole filter? I have tried to read about it somewhere in Internet/Wikipedia, but it is usually connected with too complex mathematics for me...
  Reply With Quote
Old 29th April 2010, 09:45 AM   #13
diyAudio Member
 
Join Date: Apr 2010
@EVA: Why would I need active poles?
  Reply With Quote
Old 29th April 2010, 12:05 PM   #14
81bas is offline 81bas  Germany
diyAudio Member
 
Join Date: Jan 2006
Location: Kiel
Quote:
Originally Posted by ViennaTom View Post
@EVA: Why would I need active poles?
Explanation in attached image...
Attached Images
File Type: gif pole.GIF (45.1 KB, 649 views)
  Reply With Quote
Old 29th April 2010, 01:53 PM   #15
diyAudio Member
 
Join Date: Apr 2010
Oh I see, the idea of introducing an active pole is to have loop gain rising towards lower frequencies, right? Interesting idea...But what is this good for? Output impedance at low frequencies? Isn't it better to have constant loop gain throughout the audio band?
  Reply With Quote
Old 29th April 2010, 06:39 PM   #16
81bas is offline 81bas  Germany
diyAudio Member
 
Join Date: Jan 2006
Location: Kiel
Quote:
Originally Posted by ViennaTom View Post
But what is this good for? Output impedance at low frequencies? Isn't it better to have constant loop gain throughout the audio band?
I would say, in this case each decides on its own, what is better...
But again, 25dB loop gain is too low for me...
  Reply With Quote
Old 29th April 2010, 06:58 PM   #17
diyAudio Member
 
Join Date: Apr 2010
@81bas: After some calculation and simulation, I came to the conclusion that the loop gain in my circuit is about 20 (equals 26dB) So you are quite right in assessing my circuit! You must be quite an expert! How did you calculate? I divided output pk pk amplitude by (comparator input peak peak ripple times audio frequency voltage gain). e.g. 360 / (0.32 x 56) = 20
  Reply With Quote
Old 29th April 2010, 09:37 PM   #18
81bas is offline 81bas  Germany
diyAudio Member
 
Join Date: Jan 2006
Location: Kiel
Quote:
Originally Posted by ViennaTom View Post
@81bas: After some calculation and simulation, I came to the conclusion that the loop gain in my circuit is about 20 (equals 26dB) So you are quite right in assessing my circuit! You must be quite an expert! How did you calculate? I divided output pk pk amplitude by (comparator input peak peak ripple times audio frequency voltage gain). e.g. 360 / (0.32 x 56) = 20
oh, I am not an expert at all I have seen this value in previously attached image, and supposed that most of UCD designs will have the same loop gain too...
However, I completely agree with your way to calculate the loop gain in UCD. The key point here is the amount of switching ripple at the comparator input, as you mentioned... I think also, that increasing the switching frequency will increase the loop gain of UCD, since amount of the residual ripple will be lower!
  Reply With Quote
Old 30th April 2010, 02:17 PM   #19
diyAudio Member
 
Join Date: Apr 2010
Default SOCD - implementing the pole...

So I implemented a pole @ ~20kHz (C4, R6). Of course w/o any opamp!!
Now the calculated loop gain is ~ 52dB from DC to ~1kHz, then falling linearly to ~ 26dB @20k. I think that is quite ok for ultra-high bass and lower-mid-area damping factor.
Step response: You can see from the small signal response: it is - of course - nearly perfect. Big signal step response looks like slight instability for the first glance, but it is not: If higher slew rate is demanded than output filter can deliver, the regulator loop is "overloaded" and the high loop gain "pole loaded" regulator will "remember" missed volt-second product during ascent or descent of output voltage and then answer with the overshoot. This is NOT instability. It will occur at high slew rates and can be more or less avoided by placing a bandwith limiting input filter in front of the amplifier (like 5Hz - 25kHz 2nd order, which I recommend for PA amplifier....)
Attached Images
File Type: jpg SOCD pole.jpg (159.0 KB, 592 views)
File Type: jpg SOCD pole step response 4 ohm 1.jpg (80.1 KB, 544 views)
File Type: jpg SOCD pole step response 4 ohm 2.jpg (164.9 KB, 494 views)
File Type: jpg SOCD pole step response 4 ohm 3.jpg (110.8 KB, 463 views)
File Type: jpg SOCD pole step response 4 ohm 4.jpg (114.2 KB, 103 views)
  Reply With Quote
Old 30th April 2010, 03:44 PM   #20
81bas is offline 81bas  Germany
diyAudio Member
 
Join Date: Jan 2006
Location: Kiel
Quote:
Originally Posted by ViennaTom View Post
So I implemented a pole @ ~20kHz (C4, R6). Of course w/o any opamp!!
Now the calculated loop gain is ~ 52dB from DC to ~1kHz, then falling linearly to ~ 26dB @20k. I think that is quite ok for ultra-high bass and lower-mid-area damping factor.
Cool! I am not sure, whether it will work for real world comparator, but anyway...

Could you please say, how big is the difference in switching frequency between "zero" and "near clipped" output state in your simulations? Usually there is a 2:1 frequency drop, as Eva said, but it is for "normal" UCD...

Last edited by 81bas; 30th April 2010 at 03:46 PM.
  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
Post filter AND pre filter feedback..! Lars Clausen Class D 30 4th April 2013 04:03 PM
question :post filter feedback titanchen68 Class D 38 2nd August 2006 02:00 AM
Question- post filter feedback error correction? darkfenriz Class D 11 13th June 2006 10:31 PM
Voltage Mode Pre and Post Filter Feedback Genomerics Class D 30 6th April 2006 07:27 PM


New To Site? Need Help?

All times are GMT. The time now is 07:39 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2