Help with feedback

[classd4sure]

Hi,

Sorry I brought up avalanche again... posting before coffee. My best advice is to ignore my advice

Would a larger gate resistor or perhaps snubbing help control this, possibly a series resistor as subwo1 had recommended.

Which of those would be the best way, in terms of efficiency. I think a larger gate resistor would put more dissipation in the mosfet,and also lower THD, and a snubber might be easier on the mosfet in this regard?

What of changing inductors, since one amp works with one particular coil, make them the same, you'd probably want a larger one right, at this point in time, (half way through first coffee) I think it might be less demanding on the body diode ? It's also the only real difference between the one that works and the one that doesn't, different coils.

The other thing is, the body diode of the lower FET is paralled with a diode, a schottky may be better, and it seems that the top mosfet could use one as well? I've seen app notes that only do it for the synchronous switch, but that was always for power supplies.

Thanks

 

[phase_accurate]

What analogspiceman said sounds reasonable.
It could well be that most of the FETs of the type you are using are just above the necessary limits for dV/dt for this application but the odd one isn't (which it not necessarily has to be since this is an unspecified parameter) causing some amps to fail and others not.

Another thing to carefully look out for: Ringing on drivers' inputs or gate voltage of the FETs can cause failure of either FETs, drivers or both due to multiple switching for each transition. AFAIK these are clean in your case though.

Regards

Charles

 

[analogspiceman]

That's a good test to do, thanks.
My rails are +/-45V (at idle, they drop to about +/-40V at full load). The failures were produced at output currents no more than 7-10A rms.

What about these: FQP46N15 (from Fairchild)?

These parts actually are rated for reapplied dv/dt (last time I checked Fairchild would be in a minority of about a third of the mosfet houses who did so). And at 6 V/ns their parts are sort of average in that regard (and certainly good enough for typical class d use).

Your circuit may well have other fatal problems, though, such as layout or grounding issues (can't tell from just a schematic). For example, your 10 ohm gate resistor may be a little on the low side if your layout adds a lot of extra inductance to the gate circuit. Poor grounding could mess up your driver IC and possibly cause mosfet cross conduction. This condition would tend to be more sensitive to running at high load currents rather than at full (plus) supply voltages.

Keep in mind that the reapplied dv/dt limit is never exceeded by too strong of turn off drive (one need only limit the turn on drive). This is best done with a simple dv/dt feedback circuit from drain to the appropriate spot in the gate drive. Simply increasing the gate resistor will serve to limit maximum dv/dt, but it also slows down the already slow enough parts of the transition, thereby unnecessarily increasing switching losses.

 

[Pierre]

As this thread was dedicated to feedback issues, and with the only intention of keeping it useful for people who reachs it, I propose to continue this discussion about Mosfet failures in a new thread I will create "Mosfet reliability issues in Class D".

Thanks!

 

[hypnopete]

Hello Pierre,


I am using a very similar drive circuit to you, I found that it was quite picky, some mosfets ran really well, some others ran really badly. The combination i have now has run perfectly for ages. I have been giving them a hard time too. I can confirm that i had a lot of sucess with the MTW32N20E. However something like the SPN20N60 was terrible!!!!.


I am running +/-90V rails as well, as you may already know.


Regards

Peter

 

[Pierre]

Hello all again.
I would like you to advise on an issue where I can find a unique opinion, as people advices in opposite directions: ground layout.

Until now, all my prototypes have a quite solid groud plane in the power side, with special care that the decoupling capacitors and filter caps have a low resistance path to the GND connection, as well as the speaker return (that I almost always connect directly at the power supply, btw)
Then I use another ground plane for the input, triangle and comparator circuits.
Both planes are then joined at a point near the GND connection with a 1.5mm track.

Lately, I have used separate ground zones for the input opamp, triangle generator and comparator, joined at only one point. This combines with the power plane with the said 1.5mm track.

Others advice to use a single plane for all.
For example, ZAP pulse had separate planes joined with a small choke and they finally recommended to remove it and put a wire due to some "ground shifting" or so.

What is the best layout in terms of noise inmunity, etc.?

Best regards,
Pierre

 

[Bent]

hypnopete's PCB/circuit

Hi,

Hypnopete, is your circuit/pcb available, I have tried teh various links along the way of this thread and none work.

Pierre, is your available?

I am in Oz and want to build my firts class D amp for my sub.

Thanks is advance, Graeme

 

[Pierre]

I have to finish drawing it. I am basing on the design ssanmor posted in this forum about 1 year ago or so. (basically IR2110 driver, synchronous design with a triangle generator). I will re-draw it with the last updates and send it as soon as possible.

However, there is another member that has built it succesfully, his nick is hypnopete. He designed it to be synchronous but finally found that tying the triangle input of the comparator to GND made it work self-oscillating as well I think he posted the schematics. The name of the thread is "Help with Class D amplfier design (feedback)"

Best regards

 

[kASD]

Charles.......................helpful as always