Help for Class D amp issue

[cqtsdss]

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

 

[nigelwright7557]

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.

 

[sweetperfume]

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??

 

[abraxalito]

Lack of snubber at the output (prior to the inductor) can't be helping matters one iota.

 

[sweetperfume]

Snubber?

 

[abraxalito]

A snubber network is a series RC used for absorbing ringing/overshoots.

 

[sweetperfume]

Yes i have 10ohm 5w and 470nf capacitor for that

 

[abraxalito]

That doesn't sound like the right values for classD.

 

[sweetperfume]

Then what should be used???

 

[abraxalito]

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.

 

[sweetperfume]

You mean one 10ohm and 220pf on positve supply and output 2nd 10ohm and 220pf on negative supply and output????

 

[abraxalito]

It would be a start yeah - where 'output' is R66/L501 junction.

 

[cqtsdss]

Thanks to all of you for the interest, I am not going to get into design's mistakes. There are thousand of units of this model, working for years, and they are OK although it's also true that it is not the first time I find real mistakes on equipment that are on the market. I am not quite sure about problems on the speaker but it is something I'll check deeply. I was more worried about the fact of the components and the need of them to be matched pairs but after I exchanged them for a pair installed on another unit, and after the failure happened again, I simply discarded this possibility. The funny thing is that the amp works for hours on the load and then with the speaker too. It is so weird. I always though about the components mentioned before the coil. I'll check them as well.
I'll keep informing

 

[manojtm]

Hi cqtsdss,

MOSFET gate resistor value increased to 47R and check your current sense work well (22A Peak level)

Regards
MANOJ

 

[cqtsdss]

Dear Friends: Thanks for all your support. I'll have to leave this subjects for a few weeks as I don't have any spare Mosfets and my local seller has run out too, as in Spain in august most companies are on holiday I'll have to order them to a foreign company and this will take some weeks for customs reasons, It's good to live where I live (ironically). As soon as I get back into it, I'll do all your tests and I'll let you know.

 

[phase_accurate]

I would also run a long-term test when the amp is built into the speaker.

And then there is something important that often gets forgotten: Nothing can be made idiot-proof because idiots are much too inventive !

Maybe it simply dies due to abuse.

Regards

Charles

 

[cqtsdss]

I would also run a long-term test when the amp is built into the speaker.

And then there is something important that often gets forgotten: Nothing can be made idiot-proof because idiots are much too inventive !

Maybe it simply dies due to abuse.

Regards

Charles

I fully agree with you the problem is when the idiot has another speaker of the same model and it is not failing. Only those, like you Mr "phase-accurate" and I, who are behind the workbench will understand the suffering. I am not here to do reverse engineering, wish I could, and is even harder when you get no support from manufacturer and even more when you are the official service for the brand. I don't like the usual technique of replacing the whole module, I love electronics, but in cases like this perhaps it is the best option. Anyway, I'll keep investigating the problem and if I finally succeed I'll let you know.

Again thank you and