Come on Ivan!
..don't let me die dump..
I can easily imagine that dead is influencing the distorsions as soon as the inductive energy of the choke does not properly force the halfbridge output to one of the rails during dead time.
But I have no idea, which effect causes or reduces 3rd harmonic or any other. But your plot is quite interesting, because it looks like the harmonic spectrum would not decrease to higher orders..... I will see what my measurements may show. (Still some months to go.)
Normally HiFi community would rate such a spectrum as unmusical. Or not?
..don't let me die dump..
I can easily imagine that dead is influencing the distorsions as soon as the inductive energy of the choke does not properly force the halfbridge output to one of the rails during dead time.
But I have no idea, which effect causes or reduces 3rd harmonic or any other. But your plot is quite interesting, because it looks like the harmonic spectrum would not decrease to higher orders..... I will see what my measurements may show. (Still some months to go.)
Normally HiFi community would rate such a spectrum as unmusical. Or not?
Well, i've tend to see thing simple.. When our sine just came to the "threshold" of linearity, we can see some "soft clipping" , like y=sin(x)+sin(3x)/10, so in the THD, 3th harmonic is dominate. If our sine will cross the "threshold", we'll see something similar to y=sin(x)+sin(5x)/10. See attachment, of course THD=1/10=10% is too much, but more visual too.
...hm.... so, if you look at the scope then area of the HF-ripple peak would appear as something like a 'barrier' ?
Ok, this is still fitting to my intuitive imagination, but at this point I am far from any proper model. I could just say: In this area, the HB-output will not properly slope to the opposite rail before the opposite switch is turned ON. Means HB-output is less than desired... Well, this could act like a barrier... and if we go to even higher levels, then the signal will be strong enough to pass this barrier...
Ahem, well... I must confess my brain model is starting to sound unscientific here....
Do you have screen shots or simulation results (transient analysis), which show such wave shapes? May be less strong, but visible?
Nightmare ahead:
Some math crazy class D nerd throwing 2 pages of math terms into this thread, which properly describe this behaviour... and comment: 'As we can easily see...'
Ok, this is still fitting to my intuitive imagination, but at this point I am far from any proper model. I could just say: In this area, the HB-output will not properly slope to the opposite rail before the opposite switch is turned ON. Means HB-output is less than desired... Well, this could act like a barrier... and if we go to even higher levels, then the signal will be strong enough to pass this barrier...
Ahem, well... I must confess my brain model is starting to sound unscientific here....
Do you have screen shots or simulation results (transient analysis), which show such wave shapes? May be less strong, but visible?
Nightmare ahead:
Some math crazy class D nerd throwing 2 pages of math terms into this thread, which properly describe this behaviour... and comment: 'As we can easily see...'
...looking good.
The nonlinear line coming from low left hand side going up to the right upwards.... Is this output vs input?
If yes, then the flat areas around 1m and 2m are exactly showing what I call a barrier. Or we could call it area of low gain.... or as you described it in your first post with the words 'soft clipping'...
The nonlinear line coming from low left hand side going up to the right upwards.... Is this output vs input?
If yes, then the flat areas around 1m and 2m are exactly showing what I call a barrier. Or we could call it area of low gain.... or as you described it in your first post with the words 'soft clipping'...
beyond class D?
Hello diyaudio. I have been away for a while, preoccupied with a number of RL matters.
Dead Time Induced Distortion
Clearly the analysis of class D noise and distortion, and its relation to dead time, is going to be complex. It will be all the more so because some is harmonically related to the drive, but some is harmonically related to the switching signal. Some is an intermodulation.
Then there are also the effects of the filtering, and the feedback loop.
Interest in Class D
Some of the current commercial interest in class D is driven by the desire for the ultra compact, low cost, low heat dissipation, and high efficiency. This is your portable and shelf systems, and PC sound systems.
This is not my present concern. I am looking at all this from a diy audiophile extremist perspective. The size, cost, heat, and power involved in having a few hundred watts of solid state linear power is of no consequence. It is insignificant compared to what is required for the drivers and the enclosure.
My interest is in ultra high power for subsonics. I'm speaking of power levels that really would be otherwise impractical, and sometimes may not even be obtainable out of a power mains wall outlet.
Opening the Envelope
I've expressed an interest in using BJTs and much lower switching frequencies than are currently the norm. I am also interested in the possibilities of using things like SCRs, IGBTs, GTOs, and some of the other new power handling devices. Most of these require induced commutation.
I am also interested in the possibility of going unisolated as far as the power mains, so long as it is done in a manner which in no way reduces safety, and is done with the oversight of persons qualified to make such assessments.
People build and fly their own airplanes. They do it in conjunction with a community of experts, and the appropriate regulatory bodies.
First Alternative, Class H
An obvious alternative to class D, and the dead time distortion issue, and the filtering of a lower switching frequency, does exist. That would be class H, where the power supplies feeding it are tracking SMPS.
You might think of the actual output devices as a kind of filter, via their PSRR, on the switching frequency. But this filter is nonlinear. Greater dB reduction of the lower switching frequency might be more easily obtainable. The power levels obtainable with just one pair of output devices on a large heat sink could be very large, at least 1kW.
Usual discussions of Class H assume complementary tracking power supplies that are independent. But this probably is not really necessary. You could have just one tracking SMPS that provides both the positive and negative voltages, and responds to either polarity of input signal.
Of course, this SMPS does not have any dead time issue to contend with. I had always said that a Class D audio amp is not really the same as an SMPS.
Second Alternative, Beyond Class D?
The class D dead time issue does seem unsolvable. You cannot design something that will always have zero dead time and no shoot through. Thermal variance and device parameter variance are significant.
Dead time means ambiguity. It is not the problem of there being no output device turned on. There are free wheeling diodes. The ambiguity is when there is and when there is not dead time, and how this corresponds to the input signal.
Its kind of like the ambiguity built into something that is exactly class B.
So why not just design for dead time? Once you do that, you lose the ambiguity. So, any distortion that results will be independent of the drive signal. Instead, it will be harmonically related to the switching signal. It will be inaudible, and it will be filtered out. So it really is not audio distortion. It is only ultrasonic noise of the type that is already present.
Consider how this might work. You have a switching cycle.
For a class D amp, when the input signal is zero, it goes to a state of 50% duty cycle for the positive switching alternation, and 50% duty cycle for the negative alternation.
Consider a different way? Consider an amp where a zero input causes 5% duty cycle for the positive, and 5% for the negative. Then as the input signal goes positive, it increases the output duty cycle of the positive and reduces the negative. The maximum positive duty cycle might be 45%. As it goes to this, the negative switch duty cycle might be reduced to zero. Now, this is another ambiguity being introduced, but it is when the amp is already at large signal extension.
This corresponds to the behavior of a class AB amp. When one of the devices is driven hard, the other device does finally turn fully off. This makes an insignificant contribution to distortion, while preserving your full power capability.
So, such an amp might have a zero drive state of 5% duty cycle pulses for each of the output switches.
It might have a maximum duty cycle for either of the two output switches of say 45% duty cycle. This means that you will need more current in this pulse, but thermally it still works out the same as class D.
When one output switch starts to be driven far beyond the 5% level, the other device is reduced to less than 5%, and finally to zero. The ambiguity, and hence the harmonic distortion, introduced by this should not be significant.
Maybe someone has already explored this? Maybe there is even a name for it? Maybe there is some literature?
***************
come join Audio Explorations
http://groups-beta.google.com/group/audioex_amps_dght
Hello diyaudio. I have been away for a while, preoccupied with a number of RL matters.
Dead Time Induced Distortion
Clearly the analysis of class D noise and distortion, and its relation to dead time, is going to be complex. It will be all the more so because some is harmonically related to the drive, but some is harmonically related to the switching signal. Some is an intermodulation.
Then there are also the effects of the filtering, and the feedback loop.
Interest in Class D
Some of the current commercial interest in class D is driven by the desire for the ultra compact, low cost, low heat dissipation, and high efficiency. This is your portable and shelf systems, and PC sound systems.
This is not my present concern. I am looking at all this from a diy audiophile extremist perspective. The size, cost, heat, and power involved in having a few hundred watts of solid state linear power is of no consequence. It is insignificant compared to what is required for the drivers and the enclosure.
My interest is in ultra high power for subsonics. I'm speaking of power levels that really would be otherwise impractical, and sometimes may not even be obtainable out of a power mains wall outlet.
Opening the Envelope
I've expressed an interest in using BJTs and much lower switching frequencies than are currently the norm. I am also interested in the possibilities of using things like SCRs, IGBTs, GTOs, and some of the other new power handling devices. Most of these require induced commutation.
I am also interested in the possibility of going unisolated as far as the power mains, so long as it is done in a manner which in no way reduces safety, and is done with the oversight of persons qualified to make such assessments.
People build and fly their own airplanes. They do it in conjunction with a community of experts, and the appropriate regulatory bodies.
First Alternative, Class H
An obvious alternative to class D, and the dead time distortion issue, and the filtering of a lower switching frequency, does exist. That would be class H, where the power supplies feeding it are tracking SMPS.
You might think of the actual output devices as a kind of filter, via their PSRR, on the switching frequency. But this filter is nonlinear. Greater dB reduction of the lower switching frequency might be more easily obtainable. The power levels obtainable with just one pair of output devices on a large heat sink could be very large, at least 1kW.
Usual discussions of Class H assume complementary tracking power supplies that are independent. But this probably is not really necessary. You could have just one tracking SMPS that provides both the positive and negative voltages, and responds to either polarity of input signal.
Of course, this SMPS does not have any dead time issue to contend with. I had always said that a Class D audio amp is not really the same as an SMPS.
Second Alternative, Beyond Class D?
The class D dead time issue does seem unsolvable. You cannot design something that will always have zero dead time and no shoot through. Thermal variance and device parameter variance are significant.
Dead time means ambiguity. It is not the problem of there being no output device turned on. There are free wheeling diodes. The ambiguity is when there is and when there is not dead time, and how this corresponds to the input signal.
Its kind of like the ambiguity built into something that is exactly class B.
So why not just design for dead time? Once you do that, you lose the ambiguity. So, any distortion that results will be independent of the drive signal. Instead, it will be harmonically related to the switching signal. It will be inaudible, and it will be filtered out. So it really is not audio distortion. It is only ultrasonic noise of the type that is already present.
Consider how this might work. You have a switching cycle.
For a class D amp, when the input signal is zero, it goes to a state of 50% duty cycle for the positive switching alternation, and 50% duty cycle for the negative alternation.
Consider a different way? Consider an amp where a zero input causes 5% duty cycle for the positive, and 5% for the negative. Then as the input signal goes positive, it increases the output duty cycle of the positive and reduces the negative. The maximum positive duty cycle might be 45%. As it goes to this, the negative switch duty cycle might be reduced to zero. Now, this is another ambiguity being introduced, but it is when the amp is already at large signal extension.
This corresponds to the behavior of a class AB amp. When one of the devices is driven hard, the other device does finally turn fully off. This makes an insignificant contribution to distortion, while preserving your full power capability.
So, such an amp might have a zero drive state of 5% duty cycle pulses for each of the output switches.
It might have a maximum duty cycle for either of the two output switches of say 45% duty cycle. This means that you will need more current in this pulse, but thermally it still works out the same as class D.
When one output switch starts to be driven far beyond the 5% level, the other device is reduced to less than 5%, and finally to zero. The ambiguity, and hence the harmonic distortion, introduced by this should not be significant.
Maybe someone has already explored this? Maybe there is even a name for it? Maybe there is some literature?
***************
come join Audio Explorations
http://groups-beta.google.com/group/audioex_amps_dght
If you are only looking for a subwoofer amp, you need not worry so much about dead time distortion. Even with like 100nS dead time, you will still get decent THD up to 500-800 Hz.
There is another effect, when you reduce dead time, your idle power consumtion goes up (up to x2). This is because the coil is no longer pulling the charge out of the off-switching mosfet. So if you can, you should keep some dead time distortion, to preserve power, if your application allows it.
If you are looking for full range audio bandwidth, there is however no way around getting rid of dead time distortion all together. (< 10nS is ok, <1nS is better).
There is another effect, when you reduce dead time, your idle power consumtion goes up (up to x2). This is because the coil is no longer pulling the charge out of the off-switching mosfet. So if you can, you should keep some dead time distortion, to preserve power, if your application allows it.
If you are looking for full range audio bandwidth, there is however no way around getting rid of dead time distortion all together. (< 10nS is ok, <1nS is better).
Lars Clausen said:
A) Mentioning a dead time vs amplification frequency haven't a sense, without the switching frequency mentioning, because dead time is a switching error. Final THD will very depends of the loop gain, e.g. 500ns at 50kHz easyly can give .01% THD@200Hz-3db (please note it's 1/2 max power, but not 1W).
B) Even if the dead time is completely zero, recuperation will go through mosfets Rds_on, but not through body diode (if I*Rds_on still lower then .7V), actually it's will just improve efficiency. Shot through is reason of higher losses at the lower dead time.
C) I don't know if <10ns (>100V swing) amp does exist, not saying about <1ns, but if you'll show THD/Power sweep 1kHz/4Ohm from 1W to clipping, it will stop my grin smiling.
IVX: Thank You for your comments, which are as usual very theoretically based. However your friends from Holland have shown previously, that even an amplifier with huge amounts of dead time does in practical distortion measurement show low THD, but only below 5-700 Hz. You can find these graphs (posted by Jan-Peter) here on the forum.
This is most probably - as you mention - because of higher loop gain at lower frequencies. But still a valid cause for not worrying about dead time for a subwoofer amplifier - as was my point
B) Again based on your theoretical thoughts. Those of us who work with the stuff in real life, know that efficiency does not go down with lower dead time. Sorry
C) It is possible, and it does exist. This is how we keep our THD to one of lowest levels in the world. But happy to make you smile
This is most probably - as you mention - because of higher loop gain at lower frequencies. But still a valid cause for not worrying about dead time for a subwoofer amplifier - as was my point
B) Again based on your theoretical thoughts. Those of us who work with the stuff in real life, know that efficiency does not go down with lower dead time. Sorry
C) It is possible, and it does exist. This is how we keep our THD to one of lowest levels in the world. But happy to make you smile
Lars, it would be more then a street talking, if you'll show us your power sweep plot once. I'm would be glad to be "very theoretically", but i'm just a DIYer, however skilled enough to see marketing BS through mile or two ones.
This quote exactly about 1W THD tested for 700W rated amp, author is quite famous:
This quote exactly about 1W THD tested for 700W rated amp, author is quite famous:
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