My first Class D from scratch...

thefish

Member
2006-10-28 7:38 pm
As some of you may know I have been working on a building a Class D amp for a while, well I finally finished it but it is having some very strange problems.

When the Mosfets are not powered and the input is grounded everything looks great. As soon as the Mosfets are powered I get a ton of ringing in the system. The output also looks terrible, the surprising thing is that it does not sound to terrible (about like a cheap boom box). Everything is being run from 13.8V power supply. Notice the ringing in the high side gate is almost 100V.

[IMGDEAD]http://www.siliconlite.com/AMP/01_AMP1.JPG[/IMGDEAD]

Triangle Generator

High Side Gate

Low Side Gate

Output (only half the bridge signal).

Schematic

What is the best way to measure the output with the oscilloscope? I was just going to attach the probe to one side and the ground clip to the other but the manual says to never attach the ground clip to anything except true ground.
 
-Step1: Put SMD ceramic caps at your 50V rails.
Place 1 piece of these caps (everything between 0.1uF...1uF is OK) directly at each half bridge.
Ceramic material should be X7R or X5R.

-Step2: Put a RCL (series) snubber at each MosFet from drain to source. L is formed by the unavoidable inductance of your snubber geometry. A good starting point for C would be 220pF and for the R 10 Ohms.

Good luck !
Markus
 
Furtheron:
The very strong magnitude of ringing is indicating that
your circuit is dealing with quite some current peaks.
And this seems to happen already without load.
High Current Peaks without load do only happen if your dead time is to short, or let's better say: negative dead time ==> shoot through current peak.

From your circuit it also looks like almost no dead time at the input of the level shifter. ...resulting in some overlap at the gate drive voltages.
To increase your dead time you might add some 100pF...470pF from J1 to GND and J2 to GND. Place these caps as close as possible to the IR 2011. X7R SMD types should be fine.
 
The 1uF at the rails must be very close to the transistors and it really has to be a lossy ceramic like
X7R or X5R.
A MKT or MKP will not introduce enough damping.

Your 470pf should bring you several tens of ns more deadtime. But with 2us/grid you cannot see this.
It seems like you have quite a long shoot through.
With this adjustment you will probably have a lot of
heat in your MosFets, even if you manage to quite down the ringing, which is triggered by your shoot through peak.
470pF is large enough. To increase the dead time further you might increase the 100 Ohms. Probably you need several hundrets of Ohms.

Your J1 & J2 signals are clearly showing quite some overlap.
It does not look like you would have any real dead time
The idea of the dead time is to have a short time during which both MOSFETS are off. This means both Ugs must be below the treshhold voltage.

Zoom into your gate drives (i.e. 100ns/grid) and you will probably see a certain time were both gates are higher than the MosFet treshhold.
 

thefish

Member
2006-10-28 7:38 pm
I swapped out the 100ohm resistors with 330ohm resistors, there is still the ringing though the magnitude is a bit less. More imporantly the mosfets are now running cool to the touch even after letting it run for 1 hour. Its still not perfect but it is getting there. I will upload a scope image a bit later, but I think it could use a bit more dead time still. Thanks for the help!
 
This is sounding reasonable.
You should get adjusted both: Ringing and shoot through.
The ringing is triggered by the high di/dt of the current peak. So there will be probably a point of dead time adjustment where the ringing becomes suddenly much less.

But don't trust this calm no load scenario.
As soon as you go to some higher output current you will get the ringing back, because then you will have to handle the hard switching condition of the body diode.

To handle this you probably have to settle a proper snubber.
Also it might be helpful to use a 10...47 Ohm gate resistor for turn ON, with a parallel diode for turn OFF.
The gate resistor will help to reduce the turn ON di/dt and by this help to reduce the ringing.
 

thefish

Member
2006-10-28 7:38 pm
ChocoHolic said:
But don't trust this calm no load scenario.
As soon as you go to some higher output current you will get the ringing back, because then you will have to handle the hard switching condition of the body diode.

To handle this you probably have to settle a proper snubber.
Also it might be helpful to use a 10...47 Ohm gate resistor for turn ON, with a parallel diode for turn OFF.
The gate resistor will help to reduce the turn ON di/dt and by this help to reduce the ringing.

I let it play music for 2 hours with a 8 ohm speaker and still everything stayed cool. I am still holding the rails at 13.8V so when I up it to the designed 50V things will probably change... oops I forgot the power was on and shorted out one of the mosfets, it blew a couple of parts...
 

thefish

Member
2006-10-28 7:38 pm
speakerguy79 said:
The above advice is good. Snubbers are a must, I would use both RC and a Schottky if possible. Your layout is very large, have you tried using an AM or FM radio in its vicinity?


I have not yet tried a radio near by to test for interfereance, but it does not seem to have any effect on my TV antenna (about 4 feet from the amp). I wish I could do more work on it now but I have to wait for a new IR2011. When you say my layout is very large, are you talking about the PCB? The total size of the PCB is less than 2x3 inches, how would I make it smaller?
 

thefish

Member
2006-10-28 7:38 pm
I made a lot of layout changes and added some parts, a new PCB in on order...

Revision 2:
Moved MOSFETs all to one side of the PCB
Swapped the IR2011 PDIP chips with IR2011 SOIC chips
Added 470pf X7R cap from J1 and J2 to ground
Added 1uF X7R cap from mosfet power rail to ground
Added fast diodes from MOSFET source to drain
Increased size of ground plane
Increased inductor current specs (physically larger inductors)

PCB View Top:
[IMGDEAD]http://www.siliconlite.com/AMP/Top.gif[/IMGDEAD]

PCB View Bottom:
[IMGDEAD]http://www.siliconlite.com/AMP/Bottom.gif[/IMGDEAD]

The only thing I am now a bit worried about is the output filter, I was planning to use X7R 200V surface mounts for the caps, but I have since read that X7R caps are not good for an ouput filter, is this true?

P.S.
I will post some more when I get it built but it may be a while since the PCB company I used is slow 18+ days.
 

Tekko

Banned
2005-01-01 3:33 pm
Terrible layout can also cause ringing like this.

Also you need to increase the switching freq to atleast 400KHz to be able to play fullrange satisfactory, at just 170KHz, the power bandwidth makes the amp drop off in audio level as you hit just a few KHz.

Another improvement would be making it self oscillating, like UcD, SODA or SODFA, clocked designs are usually only used in car bass boosters since EMI tend to be much higvher than self oscillating.