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Class D Switching Power Amplifiers and Power D/A conversion

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Old 2nd August 2014, 06:49 PM   #1
cqtsdss is offline cqtsdss  Spain
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Default Help for Class D amp issue

Dear friends:

I have been working into the audio service market for more than twenty years. I have always been updating my skills and competences by trying to study all new technologies and according to this principle, Class D amplification has been one of my most recurrent subjects on the last years. But even after all the books, docs, tutorials, magazines and webs I have visited, I experience problems I can't identify with all the info collected trough the years.So I open this thread to ask for help because I am a bit lost right now.

I have a class D power amplifier of a brand called Wharfedale, it is an Active Speaker with a 12" woofer and 1" horn. I have the schematics, which I enclose, and the problem goes as follows:

When I first received the module the power supply was turned off because the protection circuitry was acting. One of the two mos fets of the LF amp was shorted. I did some test and found no other damaged components so to avoid surprises I replaced both mosfets instead of just the damaged one.

I put the amp to test on a resistive load for several hours with no trouble, then I put the module back on the cabinet and made another succesful test so the speaker was sent back to the customer. Two days after, the speaker came back with again the same problem.
I have read in some documents that it is important that Mosfets for Class D amps must be matched pairs. I had another module of the same model with a different problem so, instead of replacing the whole amp, I only removed and replaced the mosfets to see if this was the reason. Again I put the amp into several tests and after them I sent the speaker to the customer, and guess what, it is back in the shop.

At this point I am a bit lost, so if any of you with the information provided have any ideas, please give me some help. Hopefully I'll give it in return sometime.

Thanks ins advance
Attached Files
File Type: pdf TITAN CLASS D.pdf (52.4 KB, 45 views)
File Type: pdf TITAN CLASS D POWER.pdf (81.9 KB, 38 views)

Last edited by cqtsdss; 4th August 2014 at 04:08 PM.
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Old 2nd August 2014, 09:15 PM   #2
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I once blew an amp.
I fixed it then had the same fault back again straight away.
Turned out I had blown the speaker and the speaker was almost a dead short.
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Old 3rd August 2014, 09:00 AM   #3
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Quote:
Originally Posted by nigelwright7557 View Post
I once blew an amp.
I fixed it then had the same fault back again straight away.
Turned out I had blown the speaker and the speaker was almost a dead short.
I don't have any faults like u have but it heats...what gate resistor should be used for FDP2532??
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Old 3rd August 2014, 09:20 AM   #4
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Lack of snubber at the output (prior to the inductor) can't be helping matters one iota.
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Old 3rd August 2014, 10:34 AM   #5
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Snubber?
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Old 3rd August 2014, 10:45 AM   #6
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A snubber network is a series RC used for absorbing ringing/overshoots.
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Old 3rd August 2014, 10:52 AM   #7
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Yes i have 10ohm 5w and 470nf capacitor for that
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Old 3rd August 2014, 10:56 AM   #8
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That doesn't sound like the right values for classD.
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Old 3rd August 2014, 11:03 AM   #9
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Then what should be used???
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Old 3rd August 2014, 11:05 AM   #10
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Something like 220pF and 10R, rather depending on the voltages.

Also notably absent from the schematic were zeners protecting the FETs from gate overvoltage. Mandatory in my estimation.
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Last edited by abraxalito; 3rd August 2014 at 11:07 AM.
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