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Old 19th January 2005, 06:48 AM   #1
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Default Help with Class D Amplfier Design (feedback)

Hello All,

This is my first post so sorry if i am covering something that has been covered before.

However I have designed and built a Class D , Half bridge amplifier. I am having a bit of trouble getting the feedback working correctly. I have attached my schematics for those that are interested in having a look.

In particular i am having trouble working out the best values for R20, R34 and C38.

Also does anyone know of any good techniques to stop overmodulation without clipping the input signal too much.. I am trying to get the amplifier to run stable at 4, or hopefully even 2 ohms. The input triangle wave it at 150kHz, and is 2v peak-to-peak.

Any suggestions, pointers etc would be really great.

Thanks in advance.

Peter
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Old 19th January 2005, 07:29 AM   #2
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Sorry,

It appears the pdf file didn't upload properly.

I'll post it again.
Attached Files
File Type: pdf schematic.pdf (75.4 KB, 1225 views)
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Old 19th January 2005, 07:56 AM   #3
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With the given circuit you'd have to take feedback from the switching output because you wouldn't have enough phase-marging otherwise. If you want to take it from the output filter you'd have to add some parts, though not many. so which variant do you prefer ?

Why do you want to prevent overmodualtion ? It is basically the same as clipping with a "normal" amplifier. You just have to watch out that your amp does this cleanly.

Regards

Charles
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Old 19th January 2005, 08:01 AM   #4
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Hi,

I would prefer taking the output from after the output inductor as this has given me the best response in the past, I would have thought that taking the feedback after the inductor would also reduce the THD, by automatically accounting for small changes in output inpedance for different loads.

Which method do you think would be better?.. I can try it either way..

With regards to the overmodulation, it just sounds pretty nasty at the moment, but that could quite likely be related to my current feedback.

Regards

Peter
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Old 19th January 2005, 09:43 AM   #5
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Did you produce a PCB directly from this ? I wonder because your integrator looks quite strange (inverting- and non-inverting input exchanged).
Regarding clipping: I think it doesn't help to limit the voltage in the place where you do. You should try to avoid that the integrator goes into saturation.

You have a very large gain the way your circuit is dimensioned. How much do you actually want ? What is the carrier- (i.e. triangle- ) frequency ? What is the supply voltage of the output-stage ?

Regards

Charles
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Old 19th January 2005, 10:22 AM   #6
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Yes I have produced a PCB from that Schematic. I just looked at that Intergrator and it is actually misdrawn in the schematic, However i can verify that it is infact wired the correct way on the PCB.

Okay I can easily remove that voltage clipping with the diodes. What can i do to stop the intergrator going into saturation as you were saying?

The output Stage is a split rail +/- 80V. The Driver IR2110 is fed of a floating 12V supply that is -80+12.

The Carrier is a 2 v, Peak to Peak Triangle Wave at 150kHz. I want the correct gain to give me clipping at say 1v peak to peak audio in.

Regards

Peter.
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Old 19th January 2005, 10:22 AM   #7
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Also I forgot to mention, the reason the schematic was showing the opamp inputs around the wrong way is. After building the circuit I had to swap the two inputs to the comparator to ensure the feedback would always allow the system to oscillate. I accidently swapped the inputs to the opamp, not the comparator. Hence the inputs to the LM319 are actually reversed.

Thanks for pointing that out.
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Old 20th January 2005, 08:25 AM   #8
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Having re-read your last statement I just stumbled over a discrepancy: Do you want a self-oscillating topology ? If so, why do you feed it a triangle signal ?

Regards

Charles
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Old 20th January 2005, 09:43 AM   #9
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Hello Charles.

I would like to try a self ossillating topology. However up until today I have just been using a triangle wave. But i did notice when i was playing around earlier today that if i disconnected the triangle wave source while the amp was running at low-ish power it kept modulating, and sounded really good considering. However the switching freq dropped to about 75kHz.

So If its not too much harder I would like to give the self-oscillating topology ago, well as long as i can manage it with the PCB i have made already.


Regards

Peter Reid
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Old 20th January 2005, 09:45 AM   #10
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I suppose you have rebuilt the schematics from a self-oscillating topology to a clock driven one (from positive feedback to negative feedback, hence). Is that what you meant?

About clipping, I agree with Charles in that the diodes don't help there, as feedback will try to compensate for the compression produced by the diodes.
Have a look at IRF reference design: they use more or less the same scheme but limiting the input to the integrator. This way can work better.

The gain of that circuit is about 330k/4.7k=70, a bit high.
I suppose that C38 and R20 try to produce a pole in the feedback network. Don't we want a zero in order to cancel the LC filter pole?

Please also have a look at the thread we started (help with feedback), you will find very useful discussions there, powered mainly by Charles (this forum owes a lot to him).

Best regards,
Pierre
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