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 19th July 2010, 10:51 PM   #1
thadman is offline thadman  United States
diyAudio Member
 
Join Date: Dec 2005
Location: west lafayette
Default Class D output filter design

For Class D amplifiers, the linearity of the output filters is paramount with regards to sound quality.

What are the best type of output inductors? Aircore or ferromagnetic? Aircore will not suffer from saturation or core losses, but will have higher resistance for a given inductance and its magnetic field will not be contained. Will the necessary proximity of the inductor to the switching transistors negatively effect the performance of the circuit due to the generated magnetic field?

What are the best ferromagnetic materials and winding topologies?

What are the best type of output capacitors? It is obviously important that its properties do not vary dramatically with temperature or power. I have also heard of capacitors suffering from the piezoelectric effect, could anybody offer further insight on this? Multilayer ceramic capacitors (ex. those produced by AVX) look very interesting.

Thanks,
Thadman
__________________
"It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."
  Reply With Quote
Old 20th July 2010, 12:12 PM   #2
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
INDUCTOR:
air core is a bad idea because of high resistance and high capacitance, not to mention the size. Both gapped ferrites and distributed airgap powders have their pros and cons. Best powders are molybnedium permaloy (MPP) and slightly worse but way cheaper sendust materials (aka. koolmu, super mss, ...).

CAPACITOR:
basically SMD is a must, good quality film caps are good but rare, ceramic NP0/COG are the best choice, but are hard to find with higher voltage ratings. A bank of smaller value ceramic X7R is often the cheapest and still acceptable solution.
  Reply With Quote
Old 20th July 2010, 03:21 PM   #3
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
The problem should be divided into two cases (at least): feedback after output filter and feedback before filter (or no feedback). Post-filter feedback makes filter linearity less important, but it needs a good high-freq behaviour.
  Reply With Quote
Old 20th July 2010, 11:49 PM   #4
thadman is offline thadman  United States
diyAudio Member
 
Join Date: Dec 2005
Location: west lafayette
Are any of the SMD inductors suitable? Vishay makes some smps inductors which can handle over 80 amps with very little resistance. They don't appear to be the pinnacle of linearity, but I'm not sure how much of a problem that would be if post filter feedback is used.
__________________
"It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."
  Reply With Quote
Old 21st July 2010, 09:11 AM   #5
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
They are probably drum inductors, which radiate a lot of EMI unless very well shielded.
E cores and toroids behave better.
  Reply With Quote
Old 21st July 2010, 11:44 AM   #6
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
Quote:
Vishay makes some smps inductors which can handle over 80 amps with very little resistance.
DC resistance is not really an important thing. Any value under 5 % of load impedance is just fine. Parallel losses, like hysteresis and eddy current are more important, since they act all the time even at idle (wich is the typical operation of an audio amp). Basically low DC resistance and low parallel loss are antagonists, so I wouldn't expect much from such a coil.

Quote:
Are any of the SMD inductors suitable?
Mounting method is irrelevant.

Key parameters are:
- saturation current and characteristics (distortion),
- HF loss,
- capacitance,
- stray magnetic field,
- DC resistance,
- price,
- size,
- repeatability,
- mountability,
- etc...
  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
class-d output filter indispensable? motomoto Class D 10 10th September 2010 07:24 PM
Class D output low pass filter EMI suppressor rickco Class D 4 28th October 2009 05:15 PM
Class D - LC output filter - C ripple current Skeebopstop Class D 8 26th February 2009 05:08 AM
Shielded or Toroidal Inductors for Class D output filter speakerguy79 Class D 24 10th February 2009 10:33 PM
smps output filter inductor design zilog Power Supplies 7 30th August 2008 11:47 PM


New To Site? Need Help?

All times are GMT. The time now is 10:27 PM.


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