Class D amps with ext clock over 1MHz?

[Joerg]

Hello All,

Is there a fairly comprehensive list on the web about class D audio amps
that run above 1MHz, synchronized or externally clocked?

Ideally I am looking for something from the mainstream suppliers such as
National, TI, ST, Philips. Their web sites aren't all that helpful here.
Their product selectors are sometimes incomplete and don't list clock
speeds so you have to trundle through every single datasheet to pry out
that info. Done that for several dozen and the only one I found was the
MAX9712 but nothing for the big suppliers.

Regards, Joerg

http://www.analogconsultants.com

 

[Pooh Bear]

Joerg wrote:

Hello All,

Is there a fairly comprehensive list on the web about class D audio amps
that run above 1MHz, synchronized or externally clocked?

Ideally I am looking for something from the mainstream suppliers such as
National, TI, ST, Philips. Their web sites aren't all that helpful here.
Their product selectors are sometimes incomplete and don't list clock
speeds so you have to trundle through every single datasheet to pry out
that info. Done that for several dozen and the only one I found was the
MAX9712 but nothing for the big suppliers.

None that I know off offhand. Tripath's 'spread spectrum' design maxes out
at about 600kHz IIRC.

Haven't seen any device featuring sync either.

Any special reason to go that high ?


Graham

 

[Joerg]

Hi Graham,

Any special reason to go that high ?


I wanted to see if they can be used as precision pulse width modulators

up to 1-2MHz. With the THD levels they claim, these amps should be quite
precise. Most of all they would cost a whole lot less than rolling your
own from CMOS chips and discretes.

Regards, Joerg

http://www.analogconsultants.com

 

[Winfield Hill]

Pooh Bear wrote...

Joerg wrote:

Is there a fairly comprehensive list on the web about class D audio
amps that run above 1MHz, synchronized or externally clocked?

None that I know off offhand. Tripath's 'spread spectrum' design
maxes out at about 600kHz IIRC.

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

 

[Ken Smith]

In article <cls6l802jum@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:

Pooh Bear wrote...

Joerg wrote:

Is there a fairly comprehensive list on the web about class D audio
amps that run above 1MHz, synchronized or externally clocked?

None that I know off offhand. Tripath's 'spread spectrum' design
maxes out at about 600kHz IIRC.

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.

This sounds like a job for a feedback loop. You could measure the
difference between the turn on delay and the turn off delay and apply that
correction on the next cycle.

--
--
kensmith@rahul.net forging knowledge

 

[Joerg]

Hi Winfield,

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.


With a monolithic solution the chip designer would probably compensate

at least for some of these transition differences. One of the Maxim
class D chips (MAX9712) is claimed to run at 0.01% THD when clocking at
1.1MHz. It doesn't go much above 0.02% for most of the output power
range. It might get worse when running it up to its 2MHz clock maximum
but probably not by a lot.

Regards, Joerg

http://www.analogconsultants.com

 

[Winfield Hill]

Ken Smith wrote...

Winfield Hill wrote:
Pooh Bear wrote...
Joerg wrote:

Is there a fairly comprehensive list on the web about class D audio
amps that run above 1MHz, synchronized or externally clocked?

None that I know off offhand. Tripath's 'spread spectrum' design
maxes out at about 600kHz IIRC.

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.

This sounds like a job for a feedback loop. You could measure the
difference between the turn on delay and the turn off delay and
apply that correction on the next cycle.

Right, but then you're analog at a critical spot, and no longer a
cool completely-digital-to-the-speakers system.


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

 

[Ken Smith]

In article <m5l5o0hfbb8rhpirnpgopi093ivl8v6jmh@4ax.com>,
Tony <tony_roe@tpg.com.au> wrote:

[... Class D and MOSFET switching ..]

Yes, dead time remains the critical missing block in good class D
design. Even a few ns can cause serious (to an audiophile) crossover
distortion, and a new ns of overlap can start to heat things up quite
a bit.

If you want real big fun, try it with bipolars.

Many years ago, I made a class D with bipolars as the switches. Even with
very complex Baker clamping circuits, the switching times were way
different. I ended up wrapping the switches with Schottkys and enclosing
the whole thing in a servo loop. The result sound quite good. AM radios
didn't like working near the circuit however.


--
--
kensmith@rahul.net forging knowledge

 

[Pooh Bear]

"N. Thornton" wrote:

Winfield Hill <Winfield_member@newsguy.com> wrote in message news:<clufib0631@drn.newsguy.com>...
Ken Smith wrote...

Winfield Hill wrote:
Pooh Bear wrote...
Joerg wrote:

Is there a fairly comprehensive list on the web about class D audio
amps that run above 1MHz, synchronized or externally clocked?

None that I know off offhand. Tripath's 'spread spectrum' design
maxes out at about 600kHz IIRC.

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.

This sounds like a job for a feedback loop. You could measure the
difference between the turn on delay and the turn off delay and
apply that correction on the next cycle.

Right, but then you're analog at a critical spot, and no longer a
cool completely-digital-to-the-speakers system.

I assumed that rather than implementing audio feedback, he meant just
measure the device turn on and turn off times, and feedforwardly tweak
the on and off times by that to cancel the problem. Thus all digital.

This is a 'yeah but' argument.

Switching times alter with load current and temperature. You can never fix the problem reliably that
way.

Graham

 

[Ken Smith]

In article <41837938.55898093@hotmail.com>,
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[....]

I assumed that rather than implementing audio feedback, he meant just
measure the device turn on and turn off times, and feedforwardly tweak
the on and off times by that to cancel the problem. Thus all digital.

Yes this is what I was arguing and I believe Win understood the point.
Win is quite right that the system would not be truely completely digital.
The measuement would contain at least a small analog section.

Assuming that, I will now disagree with Win (or maybe this is just a
misunderstanding):

Switching times alter with load current and temperature. You can never
fix the problem reliably that
way.

If we assume that the switching frequency is well above the highest signal
frequency and that the amplifier is never driven to clipping, I think we
can make the following simplifications:

(1)
The die temperature does not change much from on cycle to the next.

(2)
The current in the MOS-FET, the next time its on, will be predictable
based on the previous input data and the newest point.

(3)
The delay characteristics of a MOSFET at a certain temperature and current
changes only very slowly as it ages.


Based on this, I suggest that a clever enough circuit that contains the
following would work:

You will need some circuit to convert the large amplitude signal that
indicates whether the MOS-FET is conducting or not into a digital logic
level.

This logic signal would be connected to some sort of counter clocked at,
lets say, 5GHz. This counter would measure the time between the ideal
turn on time and the true turn on time and the ideal turn off to true turn
off.

either:

(A)
The numbers from this counter are fed back to the PWM circuit which is
also clocked at 5GHz. In the PWM circuit, the delay times are subtracted
from the ideal times and these corrected times are then used to drive the
outputs.

or:

(B)
The numbers from the counter are used to keep a table discribing the
MOS-FETS characteristics up to date. Values from this table are then used
to adjust the numbers before they are presented to the PWM output section.


The weakness in this idea is that it assumes that the MOSFET delays and
then switches perfectly after that delay. This circuit can improve
matters but without correcting for the actual turn on and turn off shapes,
the results will still be less than ideal.
--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <3SPgd.399$9t2.354@newsfe5-win.ntli.net>,
Genome <genome@nothere.net> wrote:
[...]

IGBTs are a different class of beasty.

They are not really that different. They look a lot like a N-MOSFET
driving a PNP. I think we should be developing class D audio amplifiers
using mercury ignitrons. Using the capacitively coupled quenching
circuits, you'd have to have some very tricky DSP code to get the PWM
right. You would have lots of power and that nice "warm tube sound" so
there would be a market.

--
--
kensmith@rahul.net forging knowledge

 

[Pooh Bear]

Genome wrote:

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:41829C7A.8A63E733@hotmail.com...
|
|
| Winfield Hill wrote:
|
| > Pooh Bear wrote...
|
| > > Joerg wrote:
|
| > >> Is there a fairly comprehensive list on the web about class D audio
| > >> amps that run above 1MHz, synchronized or externally clocked?
|
| > > None that I know off offhand. Tripath's 'spread spectrum' design
| > > maxes out at about 600kHz IIRC.
|
| > One serious issue that's not often talked about is an asymmetric
| > power MOSFET turn-on and turn-off time. Moreover, FET turnoff
| > time has a slow recovery tail, and is memory dependent for short
| > time intervals. The distortions from this issue are exacerbated
| > at high PWM frequencies, and lead to a degraded performance.
|
| Absolutely. Just noticed the same today looking at IGBTs.
|
| Tripath get 'better than most' results by taking feedback from the output
| to counter this. They have a programmable dead-time too. Needless to say -
| the shortest dead-time gives the highest performance with the greatest
| risk of cross-conduction.
|
| Driving gates at high frequencies takes some current too !
|
|
| Graham
|

IGBTs are a different class of beasty.

Oh indeed. The effect ( switching time ) is more pronounced. Just commenting,
since I was looking at some 'Fairchild' parts yesterday ( for an SMPS ).

Graham

 

[Ken Smith]

In article <6wSgd.604$9t2.349@newsfe5-win.ntli.net>,
Genome <genome@nothere.net> wrote:

"Ken Smith" <kensmith@green.rahul.net> wrote in message
news:cm0itc$8pd$11@blue.rahul.net...
| In article <3SPgd.399$9t2.354@newsfe5-win.ntli.net>,
| Genome <genome@nothere.net> wrote:
| [...]
| >IGBTs are a different class of beasty.
|
| They are not really that different. They look a lot like a N-MOSFET
| driving a PNP.
| --
| --
| kensmith@rahul.net forging knowledge

Ken Smith compares the basic mosfet to an IGBT and realises that they are
'not really that different'.

A mosfet looks a lot like a mosfet.

An IGBT looks a lot like a mosfet driving a bipolar transistor.

What are you on about?

I think what I said is perfectly clear.

--
--
kensmith@rahul.net forging knowledge

 

[Pooh Bear]

classd101 wrote:

Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote in message news:<41837A3B.D445C487@hotmail.com>...
classd101 wrote:

Winfield Hill <Winfield_member@newsguy.com> wrote in message news:<clufib0631@drn.newsguy.com>...
Ken Smith wrote...

Winfield Hill wrote:
Pooh Bear wrote...
Joerg wrote:

Is there a fairly comprehensive list on the web about class D audio
amps that run above 1MHz, synchronized or externally clocked?

None that I know off offhand. Tripath's 'spread spectrum' design
maxes out at about 600kHz IIRC.

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.

This sounds like a job for a feedback loop. You could measure the
difference between the turn on delay and the turn off delay and
apply that correction on the next cycle.

Right, but then you're analog at a critical spot, and no longer a
cool completely-digital-to-the-speakers system.

Hi,

Tripath's hysteretic modulators, or "spread spectrum frequency
modulator" can go up to over 1Mhz during idle/low signal conditions.

Maybe on newer versions. It topped out at about 600-800 kHz a few yrs back.

It's an analog modulator, with adaptive filtering to learn and
digitally control the timing of the output stage, I'm not sure it
qualifies as taking feedback from after the filter, don't think it
does though.

It takes feedback pre the output filter IIRC.


What is it that we don't like about fully analog, self oscillating
designs? There's a number of them out there, some of which are of the
highest quality out of any class of amp, considering the power levels
that is. Non switch near 1Mhz either.

Which were you thinking of ?

Agreed that none switch at 1 MHz or near that.


Graham

Hi Graham,

I think I agree that Triphath's feedback is pre filter, seems to be
some confusion online about that.

Even 800 KHz is high, but yeah I can't recall what version it may have
been I was reading about when I saw 1 MHz so.. its really not a
"feature" anyway.

Which was I thinking of?

Well there's a good handful of them out there that I've seen, even TI
I believe has a dual channel 500W reference design which is self
oscillating.

There are a few which are certainly noteworthy, but I won't mention
because they aren't available for anything other than mass production,
and have little to no literature available for them.

So with that I'll mention:
Bang & Olufsen's "IcePower" modules, takes feedback pre & post filter.

LC Audio's ZapPulse modules: feedback pre filter

UcD modules: feedback post filter, fully discrete, very simple
circuit. Widely regarded as the top dog, ultimate available today, for
too many reasons to bother mentioning, basically if you haven't yet
heard of it, you're behind the times.

There's also another which is self oscillating and uses a single pulse
error correction scheme but since there is no litterature on it, and
only available in large quantities, I won't bother to dig the name up,
but it is out there

I'll finish up my little wish list with the Mueta, some interesting
reading material is available on it, though it remains mythical at
this point, the technology certainly sounds promising. Personally I
don't think it will dethrone the UcD.

I don't mention these to discourage anyone from trying something new,
but if you want to see what is cutting the edge these day's, there you
have it, might save ya re-inventing the wheel.

Also these modules (UcD specifically) are said to have better sound
than pure digital systems in direct comparison, sorry.

Thanks for the input. I haven't heard of some of these - maybe being in the Uk may be the reason ?

You, in turn, might care to look at the Audio amplifier products being offered by Powersoft. www.powersoft.it

They are highly regarded in the pro-audio sound reinforcement market.

I met their guys at PLASA ( UK pro-audio trade show ) last month. Very impressive figures generally and a nice
new OEM module. 1 kg provides 1kW of audio - that's everything including the PSU ! Connectors at one end take
AC line - at the other end, audio line in and spkr out. Switching is - surprisingly - < 200kHz but doesn't seem
to impair performance. DSP based modulator btw.


Graham

 

[Ken Smith]

In article <clufib0631@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:

Ken Smith wrote...

Winfield Hill wrote:
Pooh Bear wrote...
Joerg wrote:

Is there a fairly comprehensive list on the web about class D audio
amps that run above 1MHz, synchronized or externally clocked?

None that I know off offhand. Tripath's 'spread spectrum' design
maxes out at about 600kHz IIRC.

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.

This sounds like a job for a feedback loop. You could measure the
difference between the turn on delay and the turn off delay and
apply that correction on the next cycle.

Right, but then you're analog at a critical spot, and no longer a
cool completely-digital-to-the-speakers system.

I won't tell if you don't.

You could run the switching stuff into the timing circuit running at many
GHZ and then feed that number back into a DSP thus closing the loop
digitally.


Or, you could just enclose the whole thing in an analog servo loop and get
better linearity at the cost of some marketing boost.

--
--
kensmith@rahul.net forging knowledge

 

[Joerg]

Hi Chris,

What is it that we don't like about fully analog, self oscillating
designs? There's a number of them out there, some of which are of the
highest quality out of any class of amp, considering the power levels
that is. Non switch near 1Mhz either.


Actually the MAX9712 does switch there. Even higher since it is spec'd

up to 2MHz. It can be synchronized so it clocks exactly where you want
it to. So far I haven't found anything similar from the large companies
though.

Regards, Joerg

http://www.analogconsultants.com

 

[Genome]

"Ken Smith" <kensmith@green.rahul.net> wrote in message
news:cm0itc$8pd$11@blue.rahul.net...
| In article <3SPgd.399$9t2.354@newsfe5-win.ntli.net>,
| Genome <genome@nothere.net> wrote:
| [...]
| >IGBTs are a different class of beasty.
|
| They are not really that different. They look a lot like a N-MOSFET
| driving a PNP.
| --
| --
| kensmith@rahul.net forging knowledge

Ken Smith compares the basic mosfet to an IGBT and realises that they are
'not really that different'.

A mosfet looks a lot like a mosfet.

An IGBT looks a lot like a mosfet driving a bipolar transistor.


Ken Smiths Wife

'Why did you shag me up the arse last night?'

Ken Smith

'That's why I had shit on my knob this morning!'

Don't worry, it's subtle.

DNA

 

[Pooh Bear]

Terry Given wrote:

N. Thornton wrote:
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote in message news:<41837938.55898093@hotmail.com>...

"N. Thornton" wrote:


Winfield Hill <Winfield_member@newsguy.com> wrote in message news:<clufib0631@drn.newsguy.com>...

Ken Smith wrote...

Winfield Hill wrote:

Pooh Bear wrote...

Joerg wrote:


Is there a fairly comprehensive list on the web about class D audio
amps that run above 1MHz, synchronized or externally clocked?

None that I know off offhand. Tripath's 'spread spectrum' design
maxes out at about 600kHz IIRC.

One serious issue that's not often talked about is an asymmetric
power MOSFET turn-on and turn-off time. Moreover, FET turnoff
time has a slow recovery tail, and is memory dependent for short
time intervals. The distortions from this issue are exacerbated
at high PWM frequencies, and lead to a degraded performance.

This sounds like a job for a feedback loop. You could measure the
difference between the turn on delay and the turn off delay and
apply that correction on the next cycle.

Right, but then you're analog at a critical spot, and no longer a
cool completely-digital-to-the-speakers system.

I assumed that rather than implementing audio feedback, he meant just
measure the device turn on and turn off times, and feedforwardly tweak
the on and off times by that to cancel the problem. Thus all digital.


This is a 'yeah but' argument.

Switching times alter with load current and temperature. You can never fix the problem reliably that
way.

Graham


yeah but...
temp is slow changing compared to the 600kHz etc switching speed, so
thats not a problem
load current ditto, though its faster moving than theta.

NT

its trivial to build fairly accurate thermal models in software, and in
general you ought to know what the system thermal model is anyway
(temperature is important for reliability). Thermal sensors can also be
used, along with sufficient experimentation to ascertain the correlation
between the measured temperature and the temperature(s) of interest.

It's rather less than trivial to model the die temp accurately on a dynamic basis..

You can also design the control loop to improve its rejection of
dead-time related effects - google internal model control. Better yet,
do all of the above. Non-linear modulator gain can take care of
distortion due to storage time etc.

If you monitored the switching speed in 'real time' then it would be possible to make a good adjustment for it.
Maybe this is what Tripath do ?

"all-digital" is kind of a meaningless thing - virtually every digital
controller has some form of analogue input. It might be more meaningful
to talk about the requirement for extra circuitry. The feed-forward
approaches probably dont need any extra hardware to implement, whereas
feedback methods perhaps do. Most of this stuff is a lot easier to do in
software, if you can live with the few tens of microseconds computation
time (modern DSPs kick ass)

Don't forget we're talking about nanoseconds of switching speed. From memory I think that Tripath offer a
selection of 25 / 50 / 100 ns dead time.

Oh - and the lower the load impedance - the longer the required dead time too. Note that audio amps don't drive
resistive loads - so that impedance isn't fixed, nor purely real ( just to make things more interesting ).

Don't forget to factor in the power supply droop and ripple to the PWM equation while you're at it.


Graham

 

[Pooh Bear]

Terry Given wrote:

Pooh Bear wrote:
Terry Given wrote:

its trivial to build fairly accurate thermal models in software, and in
general you ought to know what the system thermal model is anyway
(temperature is important for reliability). Thermal sensors can also be
used, along with sufficient experimentation to ascertain the correlation
between the measured temperature and the temperature(s) of interest.


It's rather less than trivial to model the die temp accurately on a dynamic basis..

not really, all you need are the relevant dimensions and materials. And
there are sneaky ways of measuring your models too - I have seen some
scarily accurate dynamic thermal models of IGBT and mosfet assemblies
(Semikrons mathcad worksheets are an edifying read, but they wont give
them to you

LOL - I bet !

I guess we simply disagree on the meaning of 'trivial' in this instance.

You can also design the control loop to improve its rejection of
dead-time related effects - google internal model control. Better yet,
do all of the above. Non-linear modulator gain can take care of
distortion due to storage time etc.


If you monitored the switching speed in 'real time' then it would be possible to make a good adjustment for it.
Maybe this is what Tripath do ?


pass. I have seen some gate drive circuits that force dead-time to a
(measured) minimum, but they cost extra, and gate-drive circuits
typically are built 4(6) at a time, so are cost sensitive.

Gate drive is - of course yet another factor in the overall performance. You can't charge ( or discharge ) the gate
capacitance instantly - and in the meantime the mosfet is running in 'linear operation'.

"all-digital" is kind of a meaningless thing - virtually every digital
controller has some form of analogue input. It might be more meaningful
to talk about the requirement for extra circuitry. The feed-forward
approaches probably dont need any extra hardware to implement, whereas
feedback methods perhaps do. Most of this stuff is a lot easier to do in
software, if you can live with the few tens of microseconds computation
time (modern DSPs kick ass)


Don't forget we're talking about nanoseconds of switching speed. From memory I think that Tripath offer a
selection of 25 / 50 / 100 ns dead time.

Oh - and the lower the load impedance - the longer the required dead time too. Note that audio amps don't drive
resistive loads - so that impedance isn't fixed, nor purely real ( just to make things more interesting ).

Don't forget to factor in the power supply droop and ripple to the PWM equation while you're at it.


why not measure actual power supply voltage, and use that to calculate
the required duty cycle. but dont forget switch forward voltage drop
(which is line, load and sock-colour dependant)

On a cycle by cycle basis - that would be most accurate. But it'll be a wicked DSP that can do all these calcs in
say 2-5us.


Graham

 

[Terry Given]

Pooh Bear wrote:

Terry Given wrote:


Pooh Bear wrote:

Terry Given wrote:

its trivial to build fairly accurate thermal models in software, and in
general you ought to know what the system thermal model is anyway
(temperature is important for reliability). Thermal sensors can also be
used, along with sufficient experimentation to ascertain the correlation
between the measured temperature and the temperature(s) of interest.


It's rather less than trivial to model the die temp accurately on a dynamic basis..

not really, all you need are the relevant dimensions and materials. And
there are sneaky ways of measuring your models too - I have seen some
scarily accurate dynamic thermal models of IGBT and mosfet assemblies
(Semikrons mathcad worksheets are an edifying read, but they wont give
them to you


LOL - I bet !

I guess we simply disagree on the meaning of 'trivial' in this instance.

lol! Once you sort out how to do it, transient thermal impedance-type
calculations arent really that complex, and of course using spice makes
the modelling very simple - once you have meaningful (ah, thats the
tricky bit) values of the relevant thermal resistances and capacitances.

many FET models have a temperature input, so you can measure the power
thru the FET (yay 4 spice function blocks), run it through an
electro-thermal model (ie r's and c's) of the heatsink and feed back the
actual junction temperature. It is fun to show positive thermal feedback
(ie thermal runaway) in mosfets - something I have seen in practice, and
had some great arguments over (amazing how many people dont read the
RTdson-vs-Tj graph

You can also design the control loop to improve its rejection of
dead-time related effects - google internal model control. Better yet,
do all of the above. Non-linear modulator gain can take care of
distortion due to storage time etc.


If you monitored the switching speed in 'real time' then it would be possible to make a good adjustment for it.
Maybe this is what Tripath do ?


pass. I have seen some gate drive circuits that force dead-time to a
(measured) minimum, but they cost extra, and gate-drive circuits
typically are built 4(6) at a time, so are cost sensitive.


Gate drive is - of course yet another factor in the overall performance. You can't charge ( or discharge ) the gate
capacitance instantly - and in the meantime the mosfet is running in 'linear operation'.

yep. I'm analysing a self-oscillating flyback design at the moment, and
I suspect their rotten efficiency (78% for 25W?!) is due to piss-poor
gatedrive - ie exactly what you are talking about.

"all-digital" is kind of a meaningless thing - virtually every digital
controller has some form of analogue input. It might be more meaningful
to talk about the requirement for extra circuitry. The feed-forward
approaches probably dont need any extra hardware to implement, whereas
feedback methods perhaps do. Most of this stuff is a lot easier to do in
software, if you can live with the few tens of microseconds computation
time (modern DSPs kick ass)


Don't forget we're talking about nanoseconds of switching speed. From memory I think that Tripath offer a
selection of 25 / 50 / 100 ns dead time.

Oh - and the lower the load impedance - the longer the required dead time too. Note that audio amps don't drive
resistive loads - so that impedance isn't fixed, nor purely real ( just to make things more interesting ).

Don't forget to factor in the power supply droop and ripple to the PWM equation while you're at it.


why not measure actual power supply voltage, and use that to calculate
the required duty cycle. but dont forget switch forward voltage drop
(which is line, load and sock-colour dependant)


On a cycle by cycle basis - that would be most accurate. But it'll be a wicked DSP that can do all these calcs in
say 2-5us.

technically you only need to go as fast as the supply ripple - and if
you know something of the supply ripple characteristics, you dont even
have to go that fast.

I am looking at a 150MHz 30MIPS dsp for a job at the moment. 30MIPS =
150 instructions in 5us - thats more than enough for quite a complex
controller, if you're sneaky about it. My PC programmer mates laugh at
the puny throughput I get from my DSPs compared to their pentiums -
several instructions per ns!

Graham

Cheers
Terry