Giving class D a try - 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 26th August 2010, 05:16 AM   #1
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Default Giving class D a try

Circuit:

http://myweb.msoe.edu/williamstm/Cla..._Schematic.pdf

Note: comparator is actually LM393. Compensation network changed since the simulation (which naturally didn't work IRL).

Solder side:

Click the image to open in full size.

The filter choke and bypass caps are not shown in the schematic.

Boring component side:

Click the image to open in full size.

Choke is quadfilar wound 29AWG, 25 turns on T51-52C, wired for differential mode (bifilar 47uH per winding). At 120kHz it gets fairly warm (datasheet says ~0.75W core loss). The output transistors also burn about a watt each, due to switching loss (the driver needs an emitter follower).

Typical output waveform:

Click the image to open in full size.

Lots of ringing due to the filter. Ripple cleans up fairly well with additional filtering. Response doesn't seem to be very linear, no thanks to RFI and ground loop problems I didn't allow for in the layout. Looks like switching noise causes the oscillator to trigger sooner, which modulates frequency and PWM in a lumpy way. The error amp helps, but its gain is necessarily low, so it's not very effective at most audio frequencies.

Despite its shortcomings, it makes audio. It has a certain charm at low frequencies, and it's very gratifying to watch the power supply on the oscilloscope, watching it brown out on the bass drums.

Has much worse PSRR than I would like, and complex signals (like guitar harmonics with cymbal crashes) result in awful screeching sounds (IMD and whatnot).

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 26th August 2010, 08:07 AM   #2
Banned
 
Join Date: Jan 2005
Weird circuit And that layout is just horrible, no wonder it doesent work properly.

Layout is the number one priority in class d. What switching frequency are you using ?

Im not too sure about the use of P and N fets rather than all N channel output stage, only seen this in very cheap and older stuff.

Im sure Eva will have a few words to say about this thing.
  Reply With Quote
Old 26th August 2010, 02:03 PM   #3
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
FDS8333 is pretty handy, 0.1 ohm Rds(on) "complementary" pair (as complementary as MOS can be). It's like an ordinary CMOS inverter (cf. CD4069UBE), with twice the voltage handling and 4000 times more silicon! Complementary FETs are wonderful when you're working under 20V. No need for bootstrap.

Switching at 120kHz, 150ns rise/fall time. (Next revision gets emitter followers, as noted, which should push it to 50ns.)

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 26th August 2010, 02:21 PM   #4
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
This circuit is not a good starting point, and the layout is terrible, it looks like done without being aware of the HF current paths, and PCB track and component lead inductances at all

SMD is a step in the right direction, though, except for output capacitors due to the lack of linearity of all dielectrics except NP0/C0G.
__________________
I use to feel like the small child in The Emperor's New Clothes tale

Last edited by Eva; 26th August 2010 at 02:24 PM.
  Reply With Quote
Old 26th August 2010, 02:39 PM   #5
wwenze is offline wwenze  Singapore
diyAudio Member
 
Join Date: Mar 2008
Question (@ Eva but anyone can answer though): doesn't the length of the traces kinda negate the advantages of using SMD over through-hole?
  Reply With Quote
Old 26th August 2010, 02:44 PM   #6
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
Yes, the long traces and the absence of ground plane results in high trace inductances, so using parts with low inductances does not make much of a difference, but it's not bad as a first try.

The output MOSFET are not bad at all. Output inductor may require a better core material like -2. A reasonably fast comparator like the inexpensive LM319 or TL3016 is also required, and a better level shifting and gate drive scheme.

Buffering/amplifying the totem pole outputs of a TL3016 and coupling them capacitively to the gates may work (with some limitations, but minimum delay).

The coil for supply rail EMI filtering is ok, but the configuration is not good. The diode in parallel should be removed, and one pi filter (CLC) should be placed on each rail. These MOSFET are more than adequate for hard switching, their body diodes don't store any substantial charge.
__________________
I use to feel like the small child in The Emperor's New Clothes tale

Last edited by Eva; 26th August 2010 at 02:52 PM.
  Reply With Quote
Old 26th August 2010, 05:33 PM   #7
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Erm, the L+D is for shorting-mode commutation, not RFI. RLD snubber, without an extra resistor, since commutation duty cycle is low. You can't get away with bypassed supply rails when you drive the gates together and supply is over ~5V. I've tried it before, and measured the 20A peak pulses myself! This limits commutation current to an amp or two, on par with signal current.

Too bad I don't have any #2 cores. I'll probably go with gapped ferrite next time. The high Q could be troublesome for compensation though, since it removes feedforward action. The capacitors may not be C0G, but X7R isn't bad, and it's got plenty low ESR. I use 50V ceramics so linearity is of no concern at 12V.

If you're interested, revision 2 is here:
http://myweb.msoe.edu/williamstm/ClassD2_Schematic.pdf
Supply filtering and output CM choke also included.

Board:
http://myweb.msoe.edu/williamstm/ClassD2.pdf
(with ground plane this time!)

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 7th September 2010, 03:42 PM   #8
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Improved model:

Click the image to open in full size.

Oscillator/error amp filter added as through-hole this time. Other toroid (top) is a common mode choke, which cleans up the ugly waveforms.

Click the image to open in full size.

The four small copper areas are the emitter follower drivers, which push the gates around in about 50ns, reducing switching loss.

Compensation is a pole-zero at 50kHz, which brings loop gain to unity as the output filter hits resonance (90 degrees), dodging the phase shift a bit in the process. Overall gain droops slightly at 20kHz, peaks again at 50k, then drops past there (aside from hetrodyning effects around 125kHz, 250kHz, etc.). Square wave response is as reasonable as can be expected. No squirrely behavior was observed, unlike the previous version (which had a sinuous transfer curve due to supply ringing). Clipping behavior is quite reasonable. Large signal HF response (e.g. 10kHz triangle) doesn't look very good, because the error amp has low gain at that level. In fact, the triangle response resembles the oscillator waveform, which should come as no surprise since it's an RC oscillator. The next improvement should use current sources in the oscillator. Heck, open loop performance may then be sufficient without the error amp.

I'm particularly proud of this PCB, as it aligned *perfectly*. The 20 mil via holes are right in the center of their pads, top and bottom. Last two-sided board I made ended up with a bit of offset.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 7th September 2010, 10:48 PM   #9
AP2 is offline AP2  Italy
Banned
 
Join Date: Dec 2009
Location: Italy
Hi,
this is better than the first pic, but it is not good idea to total tin tracks.
  Reply With Quote
Old 8th September 2010, 01:17 AM   #10
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Why?

It makes placing SMTs much easier.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  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
Is my Powersupply giving enough... Power? icecoolwas Class D 11 2nd February 2009 03:09 PM
Turntable giving out too much treble. thelostpatrol Analogue Source 2 5th April 2007 01:49 AM
Giving them the horn? JRKO Multi-Way 7 3rd July 2004 05:49 PM
not giving up on the SOZ MEGA_amp Pass Labs 3 19th May 2004 08:33 PM


New To Site? Need Help?

All times are GMT. The time now is 05:54 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