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
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
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
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
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
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
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
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
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.
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.
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.
Thanks for pointing that out.
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
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
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
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
Thanks for that Pierre,
I will have a good read of the other thread you just mentioned.
I just ran RMAA with my circuit just to test things. The results don't look too bad considering i havn't got the feedback sorted properly yet..
Freq = 200kHz
Output Rails +/-20v
Here is the link if anyone is intersted.
http://tesla.reidconsulting.com.au/results/Comparison.htm
Regards
Peter
I will have a good read of the other thread you just mentioned.
I just ran RMAA with my circuit just to test things. The results don't look too bad considering i havn't got the feedback sorted properly yet..
Freq = 200kHz
Output Rails +/-20v
Here is the link if anyone is intersted.
http://tesla.reidconsulting.com.au/results/Comparison.htm
Regards
Peter
Charles,
I took your advise with regards to the Clipping diodes after the feedback, and hence removed them. I am running a little bit of feedback. ( It is still not set correctly). I reduced the resistance of the gate resistors to about 8 ohms. And used lower voltage, higher current HexFets. 200V, 50A. The clipping is now very clean it you can hardly hear it.
The switching output is very clean even while running from +/-80V. I have increased the Carrier frequency to 250kHz. I was able to obtain a continous power output into a 4 ohm load of just over 1400Watts peak ~ 950Watts RMS. It still sounded really good at this power level.
I even tested it running a 4 ohms || 8 ohms ie 2.8ohms Total. And obtained a power output 1230Watts RMS.. I am very pleased with these results.
I just need to improve the feedback now!
btw thankyou for your help up till now.
Regards
Peter
I took your advise with regards to the Clipping diodes after the feedback, and hence removed them. I am running a little bit of feedback. ( It is still not set correctly). I reduced the resistance of the gate resistors to about 8 ohms. And used lower voltage, higher current HexFets. 200V, 50A. The clipping is now very clean it you can hardly hear it.
The switching output is very clean even while running from +/-80V. I have increased the Carrier frequency to 250kHz. I was able to obtain a continous power output into a 4 ohm load of just over 1400Watts peak ~ 950Watts RMS. It still sounded really good at this power level.
I even tested it running a 4 ohms || 8 ohms ie 2.8ohms Total. And obtained a power output 1230Watts RMS.. I am very pleased with these results.
I just need to improve the feedback now!
btw thankyou for your help up till now.
Regards
Peter
I am very pleased someone gets good results with IR2xxx chips!
What changes have you done in the feedback network? Did you read the "Help with feedback" thread and used the ideas exposed there?
Have you performed any RMAA test now? Your THD was way too high (10%!) before in the "loopback test" is it better now? The IMD was also too high. Could you clarify what the conditions were in that measurements?
Good job!
Best regards.
What changes have you done in the feedback network? Did you read the "Help with feedback" thread and used the ideas exposed there?
Have you performed any RMAA test now? Your THD was way too high (10%!) before in the "loopback test" is it better now? The IMD was also too high. Could you clarify what the conditions were in that measurements?
Good job!
Best regards.
Sorry, but I have a curiosity, perhaps out of context in this thread:
What is the exact function of the resistors between gate and source in the mosfets?
I have seen that in a lot of designs...
I understand that a zener is good to avoid gate destruction due to excessive Vgs, but what is the resistor for? To increase noise and false trigger inmunity?
Thanks! and let's return to this thread's subject. ;-)
What is the exact function of the resistors between gate and source in the mosfets?
I have seen that in a lot of designs...
I understand that a zener is good to avoid gate destruction due to excessive Vgs, but what is the resistor for? To increase noise and false trigger inmunity?
Thanks! and let's return to this thread's subject. ;-)
Hi Pierre,
I have posted updated results at the same web address.
It appears that my sound card has a pretty high THD when i have got the levels really high ( i need to do this for the loop back test) however my most recent tests show the THD is down to about 1.1% at 2.8 ohms and the IMD is down to 3.8% at 8 ohms. I still need to make lots of improvements. btw these results were with the amplifier running from +/- 80V rails.
At the moment the feedback is really hardly doing anything, I have read though some of the "Help with feedback" thread but I am yet to finish reading it.
By the way those tests are all loop back.. The one i labeled loop back was the output of the soundcard connected back to the input of the soundcard (line in). The others have the amplifier inbetween. The output of the amplifier is divided down using a resistive network....
Regards
Peter
I have posted updated results at the same web address.
It appears that my sound card has a pretty high THD when i have got the levels really high ( i need to do this for the loop back test) however my most recent tests show the THD is down to about 1.1% at 2.8 ohms and the IMD is down to 3.8% at 8 ohms. I still need to make lots of improvements. btw these results were with the amplifier running from +/- 80V rails.
At the moment the feedback is really hardly doing anything, I have read though some of the "Help with feedback" thread but I am yet to finish reading it.
By the way those tests are all loop back.. The one i labeled loop back was the output of the soundcard connected back to the input of the soundcard (line in). The others have the amplifier inbetween. The output of the amplifier is divided down using a resistive network....
Regards
Peter
One important thing to watch out for: Some soundcards will have problems with the switching residual off class-da amps and will therefore show more THD than you actually have.
Have you considered a passive lowpass between your amp and the soundcard's input ?
Some info on measuring class-d amps:
http://www.aes.org/sections/la/MeetingPresentations/BHTestingClassDAmplifiers.ppt
Regards
Charles
Yes, I also read that tip.
It is curious, however, because there is a guy in "Ucd180 q&a" thread that has measured a UcD-180 without lowpass filter (and a crappy power resistor) and gets good results (even when UcD leaves lots of carrier residue!) But who knows what the inputs of his soundcard are like, anyway...
Charles, do you know what the resistors between gate and source are for? Thanks!
Best regards
It is curious, however, because there is a guy in "Ucd180 q&a" thread that has measured a UcD-180 without lowpass filter (and a crappy power resistor) and gets good results (even when UcD leaves lots of carrier residue!) But who knows what the inputs of his soundcard are like, anyway...
Charles, do you know what the resistors between gate and source are for? Thanks!
Best regards
They are there to ensure that no gate charge can stay or be built up if control by the IR2110 is lost. This could happen if the supply voltage for the output stage is still there but the supply of the IR2110 failed. But you don't see them often. In the given dimensioning the pull almost as much current as the driver's static current consumption is.
Regards
Charles
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