On this thread, I want to explore something most interesting that I do not really understand, which was mentioned on another thread.
This is the relationship between dead time in Class D switching, at the zero crossing, and the final audio output distortion.
This is something I had never considered before.
<<
Dead time between upper and lower switch is directly pushing up unpleasant distorsions, that have a similar harmonic spectrum as the well known cross over distorsion in class B amps. So everybody tries to keep it low. Reasonable dead times are below 1% of the period, better 0.1%. At 25kHz this means 0.1% ... 1% of 40us. So you should handle deadtimes of some hundret nano seconds (say: 400ns dead time is already a quite poor design), which is not easy with heavy BJTs. Please be aware that the storage time of BJTs is dramatically increasing by temperature. Means turn OFF delay is increasing and may eat up the dead time and system runs into cross conduction.
>>
from post
http://www.diyaudio.com/forums/showthread.php?postid=1001245#post1001245
<<
I would agree in most SMPS applications, where halfbridges are allowed to have long dead times. But for class D with low distorsions it is more difficult. I am mostly concerned about the storage time.
The storage time of heavy BJTs is long, means several us. You can reduce it by giving high negative base drive currents for turn OFF. Unfortunately the storage time is also depending on the overdrive factor. If you have a constant base drive then the resulting storage time will also depend on the load current. And temperature will increase the storage time by 30%...50% if you compare the behaviour at 100C vs. 25C.
Simply pick any power BJT and use it to switch ON and OFF a resistor load. Drive it with a signal generator and observe the delay between Ib and Ic. Or if you are in trouble with proper current measurement you can measure the time difference between the falling edge of the drive signal and the rising edge of Uce. Of course also there is a similar turn ON delay, usually this is much shorter, than turn OFF delay. Play around with this and gain some feeling about the values. Then go ahead with some drive buffer to allow high base drives or have a look to some of already existing standard base drive circuits. Play with it and see if you can keep the variance of delay times low no matter if you heat up and/or change the load.
I would guess that you can be lucky if you are able to drive a 250V/40A BJT in a way that the resulting turn OFF delay is about 1.5us at 25C and 2us at 100C. Which is already a variance of 500ns and does not take load into account. From my perspective short dead times for class D half bridges are really difficult to realize in a proper manner with BJTs.
>>
from post
http://www.diyaudio.com/forums/showthread.php?postid=1002100#post1002100
Now, my own interest is in a subwoofer Class D amp, running at 23.5khz, and possible using BJTs.
Neither 23.5khz, nor BJTs, are on the table for discussion on this thread.
Rather, I want to invite ChocoHolic, and others, to look at this dead time or zero crossing issue in Class D, and what its relation is to the audio distortion.
To me, this is not at all obvious.
This is the relationship between dead time in Class D switching, at the zero crossing, and the final audio output distortion.
This is something I had never considered before.
<<
Dead time between upper and lower switch is directly pushing up unpleasant distorsions, that have a similar harmonic spectrum as the well known cross over distorsion in class B amps. So everybody tries to keep it low. Reasonable dead times are below 1% of the period, better 0.1%. At 25kHz this means 0.1% ... 1% of 40us. So you should handle deadtimes of some hundret nano seconds (say: 400ns dead time is already a quite poor design), which is not easy with heavy BJTs. Please be aware that the storage time of BJTs is dramatically increasing by temperature. Means turn OFF delay is increasing and may eat up the dead time and system runs into cross conduction.
>>
from post
http://www.diyaudio.com/forums/showthread.php?postid=1001245#post1001245
<<
I would agree in most SMPS applications, where halfbridges are allowed to have long dead times. But for class D with low distorsions it is more difficult. I am mostly concerned about the storage time.
The storage time of heavy BJTs is long, means several us. You can reduce it by giving high negative base drive currents for turn OFF. Unfortunately the storage time is also depending on the overdrive factor. If you have a constant base drive then the resulting storage time will also depend on the load current. And temperature will increase the storage time by 30%...50% if you compare the behaviour at 100C vs. 25C.
Simply pick any power BJT and use it to switch ON and OFF a resistor load. Drive it with a signal generator and observe the delay between Ib and Ic. Or if you are in trouble with proper current measurement you can measure the time difference between the falling edge of the drive signal and the rising edge of Uce. Of course also there is a similar turn ON delay, usually this is much shorter, than turn OFF delay. Play around with this and gain some feeling about the values. Then go ahead with some drive buffer to allow high base drives or have a look to some of already existing standard base drive circuits. Play with it and see if you can keep the variance of delay times low no matter if you heat up and/or change the load.
I would guess that you can be lucky if you are able to drive a 250V/40A BJT in a way that the resulting turn OFF delay is about 1.5us at 25C and 2us at 100C. Which is already a variance of 500ns and does not take load into account. From my perspective short dead times for class D half bridges are really difficult to realize in a proper manner with BJTs.
>>
from post
http://www.diyaudio.com/forums/showthread.php?postid=1002100#post1002100
Now, my own interest is in a subwoofer Class D amp, running at 23.5khz, and possible using BJTs.
Neither 23.5khz, nor BJTs, are on the table for discussion on this thread.
Rather, I want to invite ChocoHolic, and others, to look at this dead time or zero crossing issue in Class D, and what its relation is to the audio distortion.
To me, this is not at all obvious.
Another fishing expedition.
See the last time you got away with alot by claiming "I could not find", because the questions you asked were so absurd there obviously was no precedence.
This time, you obviously expect to be spoonfed, because there's plenty out there on that topic, and again you're attempting to control the topical flow, which isn't going to happen.
The link provided above is just one example of you not bothering to search, instead you chose to waste our time. If you made half the effort you'd find many more such links on this very forum, and I'm sure google would also serve to find several others.
There's plenty of precedence in this case, stop your trolling expeditions and start digging, we're not here to spoonfeed you basics.
When you've done your research feel free to pose a question of worth.
Though I think what you really ought to start looking for, is a basic class d tutorial. Google can find you one of those too.
Cheers,
Chris
See the last time you got away with alot by claiming "I could not find", because the questions you asked were so absurd there obviously was no precedence.
This time, you obviously expect to be spoonfed, because there's plenty out there on that topic, and again you're attempting to control the topical flow, which isn't going to happen.
The link provided above is just one example of you not bothering to search, instead you chose to waste our time. If you made half the effort you'd find many more such links on this very forum, and I'm sure google would also serve to find several others.
There's plenty of precedence in this case, stop your trolling expeditions and start digging, we're not here to spoonfeed you basics.
When you've done your research feel free to pose a question of worth.
Though I think what you really ought to start looking for, is a basic class d tutorial. Google can find you one of those too.
Cheers,
Chris
One of the beauties of Class D is that a great deal can be learned from studying the no input signal condition. Many of Class Ds foibles are still present with no input.
A model using perfect switches but real diodes with induced dead time will show the relationships.
🙂
A model using perfect switches but real diodes with induced dead time will show the relationships.
🙂
classD4sure, there is no reason for you to be posting here.
Tim_X, I will begin reading that thread.
poobah, I will begin that thought experiment / analysis now. Thanks for the clue.
Brian
Tim_X, I will begin reading that thread.
poobah, I will begin that thought experiment / analysis now. Thanks for the clue.
Brian
When I read this thread title I thought "excellent, as a newcomer, maybe I can discover a little bit about the D class 'dead time' question, without having to wade through the fluff that accompanies most of the longer threads in this forum" ---- sadly that seems unlikely now! 🙁
I recently needed to try and understand something about the UcD 180 - so I read the 'UcD180 questions' thread - yes, all of it, nearly 2000 posts---took me about a week.
There is some excellent information, especially from Bruno of course, but I nearly gave up having to wade through 2 years of 'that's a nice colour you've got on your wiring' etc....
I do appreciate that the more prolific posters must find it very frustrating to have to cover the same ground over and over, but there are always newcomers who do not know the 'details' of a two or more year history of 'regulars,' and hope for a friendly welcome, when old ground needs to be revisited.
Maybe it's just that some forums are more tolerant than others!
I recently needed to try and understand something about the UcD 180 - so I read the 'UcD180 questions' thread - yes, all of it, nearly 2000 posts---took me about a week.
There is some excellent information, especially from Bruno of course, but I nearly gave up having to wade through 2 years of 'that's a nice colour you've got on your wiring' etc....
I do appreciate that the more prolific posters must find it very frustrating to have to cover the same ground over and over, but there are always newcomers who do not know the 'details' of a two or more year history of 'regulars,' and hope for a friendly welcome, when old ground needs to be revisited.
Maybe it's just that some forums are more tolerant than others!
I appreciate your concerns but I don't feel it's the case.
You've read the entire 180 thread and that's admirable. You've seen the kind of information given freely that'll probably never be posted again, however long it took you, I'm sure it was worth it. There's a few others of equal or greater caliber as well.
Maybe the more the same question is asked the less gets answered. If you do a little research first and gain the ability to elaborate on an old question, or bring forth some aspect that wasn't previously covered 10 times over, we all stand to gain. If you're having a problem understanding some particular aspect of anything already discussed at length and need further help on the matter, you're not going to get chased away.
However, I find a huge difference between any of that, and a pure fishing expedition by someone who'd completely ignore what you might consider basic information from the last 20 years, and purely expects to be spoonfed exactly what he wants to hear.
Welcome to a great forum, but please don't act abused when you haven't been. I'm confident you'll find your experience here to be as fulfilling as mine has been, if you just make a little effort to help yourself. From your non topical meta post it seems that won't be an issue.
Cheers.
PS: Did your get your UCD180 question answered OK?
PPS: Bruno doesn't post here much anymore, perhaps because of all the times he got asked how to wire the inputs over and over again. Quite the loss wouldn't you say?
PPPS: Use the search thing and you won't have to read the entire thread, even though I highly recommend it🙂
You've read the entire 180 thread and that's admirable. You've seen the kind of information given freely that'll probably never be posted again, however long it took you, I'm sure it was worth it. There's a few others of equal or greater caliber as well.
Maybe the more the same question is asked the less gets answered. If you do a little research first and gain the ability to elaborate on an old question, or bring forth some aspect that wasn't previously covered 10 times over, we all stand to gain. If you're having a problem understanding some particular aspect of anything already discussed at length and need further help on the matter, you're not going to get chased away.
However, I find a huge difference between any of that, and a pure fishing expedition by someone who'd completely ignore what you might consider basic information from the last 20 years, and purely expects to be spoonfed exactly what he wants to hear.
Welcome to a great forum, but please don't act abused when you haven't been. I'm confident you'll find your experience here to be as fulfilling as mine has been, if you just make a little effort to help yourself. From your non topical meta post it seems that won't be an issue.
Cheers.
PS: Did your get your UCD180 question answered OK?
PPS: Bruno doesn't post here much anymore, perhaps because of all the times he got asked how to wire the inputs over and over again. Quite the loss wouldn't you say?
PPPS: Use the search thing and you won't have to read the entire thread, even though I highly recommend it🙂
classd4sure said:
Yes -- and no!
I have a special application for a class D amplfier that would horrify most of you! --- and trying to find the answer has been difficult. Certainly, overall, the time spent reading through the UcD 180 thread was worthwhile -- there is clearly a lot of expert knowledge here, some of the more academic way above my head!
And I have to remember that it is 'DIY' audio, which is likely to be inhabited by enthusiastic amateurs ( and I do mean that in a positive way, using the word 'amateur' from it's latin root meaning 'lover of', rather than 'less worthy than professional').
By definition, you are trying to improve your listening experiences, by aspiring towards the better realisation of available technology.
But when you have a requirement that can, in theory at least, cope with 100% THD (I'm not joking!), and a further requirement of the lowest possible quiescent current, then a discussion on 'dead time' becomes potentially very interesting.
I do appreciate that this requirement may appear to be nonsense, but this might give an insight into the application I am researching- http://scitation.aip.org/getabs/ser...00020000001000042000001&idtype=cvips&gifs=yes
rogs said:
Class B amps suffer from crossover distortion, which is something you may also want to look into.
rogs, thanks for posting here.
This thread will be a constructive place.
I'm just now working to get someone off of it. For now, just ignore him. Bozo filter him if you want.
When people want to participate they post links to real things, or their own ideas. If they think there are relevant things to read, they post them.
Otherwise they keep quite.
I have an unusual project on the table too.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=84403
While there are many with extensive knowledge here, there are also some who think in very narrow ways.
But what is most imporant is that people stick to the topic, and only post if they want to add something constructive.
If junk posters are ignored, they will eventually go away.
Give me some time to read Tim_X's link.
Brian
This thread will be a constructive place.
I'm just now working to get someone off of it. For now, just ignore him. Bozo filter him if you want.
When people want to participate they post links to real things, or their own ideas. If they think there are relevant things to read, they post them.
Otherwise they keep quite.
I have an unusual project on the table too.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=84403
While there are many with extensive knowledge here, there are also some who think in very narrow ways.
But what is most imporant is that people stick to the topic, and only post if they want to add something constructive.
If junk posters are ignored, they will eventually go away.
Give me some time to read Tim_X's link.
Brian
This web site seems down, or gone. Maybe its been moved?
http://www.genomerics.org/outfilt.html
http://www.genomerics.org/index.html
There is also an Electronics Letters paper. I can't get the full text, because I am not a dues paying member.
But with enough time, I could get to a local university and read it online, and print it on paper.
I believe there is still more info online.
For now, back to that diyaudio thread.
http://www.genomerics.org/outfilt.html
http://www.genomerics.org/index.html
There is also an Electronics Letters paper. I can't get the full text, because I am not a dues paying member.
But with enough time, I could get to a local university and read it online, and print it on paper.
I believe there is still more info online.
For now, back to that diyaudio thread.
Alright, I'm certainly convinced that this dead time and distortion relation is real. But I still don't understand it.
I would need that Elecronics Letters paper to follow all that is said.
As I see it, dead time is just something that would be filtered out in the audio output.
It only becomes a problem when you near full signal swing, or if it somehow interferes with the ramp generator and comparator and feedback part of the circuit being able to track an input signal. Meaning, it is not just dead time, it is how that interacts with the rest of the circuit.
So, for me to understand this more fully, I will have to see more info.
I am not aware of any book that talks about Class D audio design. I have looked.
I know there are books that deal with SMPS. But the issues are not entirely the same.
I will keep looking and look at Tim_Xs link some more. I'm also anxious to see if others can bring some additional light to bear on this.
I would need that Elecronics Letters paper to follow all that is said.
As I see it, dead time is just something that would be filtered out in the audio output.
It only becomes a problem when you near full signal swing, or if it somehow interferes with the ramp generator and comparator and feedback part of the circuit being able to track an input signal. Meaning, it is not just dead time, it is how that interacts with the rest of the circuit.
So, for me to understand this more fully, I will have to see more info.
I am not aware of any book that talks about Class D audio design. I have looked.
I know there are books that deal with SMPS. But the issues are not entirely the same.
I will keep looking and look at Tim_Xs link some more. I'm also anxious to see if others can bring some additional light to bear on this.
Have you tried web pages, basic class d tutorials, white papers, patents, the other 3 to 6 threads already discussing the issue.... or must it all be handed to you?
What in the world makes you think it's our job to convince you? Let's leave that topic sit for awhile.
What in the world makes you think it's our job to convince you? Let's leave that topic sit for awhile.
zenmasterbrian said:
Chris, for what it's worth, I think that Brian is the real Bozo here. The guy is a complete waste of your time. He is a control freak wannabe and much, much worse, unlike you, contributes almost nothing of technical interest or merit. My advice to you -- ignore him completely.
Regards -- analogspiceman
classd4sure, either contribute something topical, or you shouldn't be posting here.
classd4sure, I am asking you not to post on this thread.
analogspiceman, greetings.
I've just spent some time reading you posts from the link Tim_X provided.
I was encouraged to see your name as the most recent poster.
I hope that this thread will prove to be of worth.
In any event, I ask that you not post negative meta. It just takes up space, and further discourages topical posts.
classd4sure, I am asking you not to post on this thread.
analogspiceman, greetings.
I've just spent some time reading you posts from the link Tim_X provided.
I was encouraged to see your name as the most recent poster.
I hope that this thread will prove to be of worth.
In any event, I ask that you not post negative meta. It just takes up space, and further discourages topical posts.
zenmasterbrian said:
Sort of, it's been switched off but it had been moved. There, I've hit the on/off button.
http://www.genomerics.org/classd/hbfl/outfilt.html
I'm not sure it will help you much though..
DNA
I'm heading there right now
(edited) It is helpful. I'm going to have to study it in more detail tomorrow. I wonder, in the case of a switching amp with a forth order filter instead of a second order filter, would this be so much of a problem.
Again, I'm just starting to try and understand this.
(edited) It is helpful. I'm going to have to study it in more detail tomorrow. I wonder, in the case of a switching amp with a forth order filter instead of a second order filter, would this be so much of a problem.
Again, I'm just starting to try and understand this.
zenmasterbrian said:
No need to wonder.
I realise the circuits given are not exceptionally good but if you get yourself a copy of LTspice, google for it, then you can download the circuits from the site by clicking on the Linear Technology logo. Then you can modify them and see what is happening yourself.
I think someone else mentioned that the problem is reduced if the ripple current in the output inductor is increased..... Think about why that might happen........
On the basis of that a fourth order filter with smaller inductive components may give you the increased ripple you require in the first inductor that will reduce the effects whilst still allowing you to achieve your desired attenuation.
I'm sure you will sit down and look at what is happening, reach your own conclusions and then proceed to investigate and make changes according to your thoughts about things and verify wether or not your ideas are right or wrong......
Something close to the truth is out there.
DNA
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