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Old 10th September 2006, 07:19 PM   #1
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Default dead time, and class D distortion

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/showt...45#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/showt...00#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.
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Old 10th September 2006, 08:40 PM   #2
Tim__x is offline Tim__x  Canada
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Dead Time and Crossover Distortion
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Old 10th September 2006, 08:58 PM   #3
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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
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Old 10th September 2006, 09:28 PM   #4
poobah is offline poobah  United States
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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.

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Old 10th September 2006, 10:51 PM   #5
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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
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Old 10th September 2006, 11:04 PM   #6
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Here's another link for you Brian, I know you haven't seen it yet either:

Basics For Brian
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Old 10th September 2006, 11:22 PM   #7
rogs is offline rogs  United Kingdom
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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!
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Old 10th September 2006, 11:33 PM   #8
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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
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Old 11th September 2006, 01:32 AM   #9
rogs is offline rogs  United Kingdom
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Quote:
Originally posted by classd4sure

PS: Did your get your UCD180 question answered OK?
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/serv...cvips&gifs=yes
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Old 11th September 2006, 01:41 AM   #10
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Hmm, so you aim to reproduce this theory or something?
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