Class D troubleshooting

bknauss · 16th July 2004, 06:56 PM

Hi all,
I've got a class D sub amp that has been having some troubles. The FETS on it seem to die somewhat spontaneously. I had it on originally, sounded great for a while, then just sorta died after a couple hours of use... not really hot or anything. I put in some FETs with a higher vdss, through on some pink noise, and within a couple hours the FETs gave out again. I've measured the rails, and they're pretty far under the vdss of the FETs, so I'm going to assume the power stage and the FETS are not the problem. I am going to assume there's an instability in the circuit... where should I start looking?

Thanks in advance.

dmfraser · 16th July 2004, 11:06 PM

Standard operating procedure when troubleshooting an amp. Run it with a 100W light bulb in seri4es with the AC power supply so you can troubleshoot it without blowing up the output devices.

Are the devices getting hot? If the switching time is too slow, they will get jot and die. Rise time should be a fraction of a microsecond. You do have a 25MHz or better scope? If not, a problem like this is really hard to track down.

bob123 · 17th July 2004, 12:30 AM

what you need is a hammer - big one though 20lb sledge hammer ought to do the job.

anatech · 17th July 2004, 01:49 AM

Actually, if the fets switch fast enough, the gates may get a coupled spike and short. Do you have zener diodes to protect the gates against spikes, especially reverse bias (off direction) ?
-Chris

16th July 2004, 06:56 PM	#1
bknauss diyAudio Member Join Date: Jul 2002 Location: Gaithersburg, MD	Class D troubleshooting Hi all, I've got a class D sub amp that has been having some troubles. The FETS on it seem to die somewhat spontaneously. I had it on originally, sounded great for a while, then just sorta died after a couple hours of use... not really hot or anything. I put in some FETs with a higher vdss, through on some pink noise, and within a couple hours the FETs gave out again. I've measured the rails, and they're pretty far under the vdss of the FETs, so I'm going to assume the power stage and the FETS are not the problem. I am going to assume there's an instability in the circuit... where should I start looking? Thanks in advance.

16th July 2004, 11:06 PM	#2
dmfraser diyAudio Member Join Date: Dec 2003 Location: Orange County, CA	SOP Standard operating procedure when troubleshooting an amp. Run it with a 100W light bulb in seri4es with the AC power supply so you can troubleshoot it without blowing up the output devices. Are the devices getting hot? If the switching time is too slow, they will get jot and die. Rise time should be a fraction of a microsecond. You do have a 25MHz or better scope? If not, a problem like this is really hard to track down. __________________ Dan Fraser

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17th July 2004, 12:30 AM	#3
bob123 diyAudio Member Join Date: Jun 2004 Location: Southampton UK	what you need is a hammer - big one though 20lb sledge hammer ought to do the job.

17th July 2004, 01:49 AM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Actually, if the fets switch fast enough, the gates may get a coupled spike and short. Do you have zener diodes to protect the gates against spikes, especially reverse bias (off direction) ? -Chris