Beyond Class D?
zenmasterbrian
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?
fum...@gmail.com
my group
http://groups.google.com/group/class-d-amp
wellcome
On 1月21日, 上午10时47分, "zenmasterbrian" <brian.zenmas...@gmail.com>
wrote: