[[email protected]]

Back in March I posted a short comment on using pulse-width modulation
to improve the Baxandall Class-D oscillator


http://groups.google.com/group/sci.electronics.design/msg/a47c2f7e1fc...


I got some useful reactions.

Since then I've spent some time modelling the circuit in LTSpice.

Anybody interested in the results can see them on my web-site

www.sophia-elektronica.com

You've got to go to the bottom of the page and click on "examples" to
get to the good stuff, and that only takes you to a page showing the
resutls of simulatiing the classic Baxandall Class-D oscillator. You've
got to get to the bottom of that page before you find the links to the
interesting stuff.

I could do a bit more modelling in LTSpice, but first I'd need to work
up an eight- or nine-bit synchronous counter - the four bit counter I
put together eventually worked fine, but it took me long enough to get
the bugs out of that I hesitant about modelling anything bigger.

I'd be grateful to a pointer to counter sub-circuit for LTSpice.

 

[Genome]

Back in March I posted a short comment on using pulse-width modulation
to improve the Baxandall Class-D oscillator


http://groups.google.com/group/sci.electronics.design/msg/a47c2f7e1fc...


I got some useful reactions.

Since then I've spent some time modelling the circuit in LTSpice.

Anybody interested in the results can see them on my web-site

www.sophia-elektronica.com

I thought I'd have a play.....because I am a bit of a pedant when it comes
to 'pretty' circuit diagrams and pictures of them.

http://genomerics.org/bills/spice/baxa.asc
http://genomerics.org/bills/spice/pics/baxa.gif

Not pedantic enough though....

Errrr, it sort of oscillates but. I don't know.

Anyway, have fun

DNA

 

[Winfield Hill]

[email protected] wrote...

Back in March I posted a short comment on using pulse-width
modulation to improve the Baxandall Class-D oscillator
http://groups.google.com/group/sci.electronics.design/msg/a47c2f7e1fc...

I got some useful reactions. Since then I've spent some time
modelling the circuit in LTSpice. www.sophia-elektronica.com http://home.planet.nl/~sloma000/Baxandall parallel-resonant Class-D oscillator1.htm

You've got to go to the bottom of the page and click on "examples"
to get to the good stuff, and that only takes you to a page showing
the resutls of simulatiing the classic Baxandall Class-D oscillator.
You've got to get to the bottom of that page before you find the
links to the interesting stuff.

I could do a bit more modelling in LTSpice, but first I'd need to
work up an eight- or nine-bit synchronous counter - the four bit
counter I put together eventually worked fine, but it took me long
enough to get the bugs out of that I hesitant about modelling
anything bigger.

Why the counter? It's not for a delayed-sweep scope equivalent.
If it's for a PWM time delay, couldn't you use a time delay, and
manually adjust the delay to meet your needs? Crude, but...

 

[Winfield Hill]

Winfield Hill wrote...

[email protected] wrote...

Bill, I enjoyed a 2003 12-post thread titled,
"Peter Baxandall's class-D oscillator revisited."
Message-ID: <[email protected]>

But I wasn't able to find the March 2006 thread you mentioned.

Why the counter? It's not for a delayed-sweep scope equivalent.
If it's for a PWM time delay, couldn't you use a time delay, and
manually adjust the delay to meet your needs? Crude, but...

It clear to me a PWM version of the Baxandall oscillator would be
useful, at least for the small crowd who needs a wide output-power
range. For example, I spent a few months designing a 300kHz 500W
phase-shift PWM driver for my 10kV transformer, featuring PWM and
power-supply-voltage variation together, so it could precisely
operate over a 1000:1 voltage range, from 10V TO 1kV. Note, the
feedback loop included a square-root converter to account for the
double parallel feedback terms. I doubt I could have achieved the
wide voltage range without the PWM modulation. A Baxandall power
oscillator might have greatly simplified my circuit, eliminating
the ucc3895 phase-shift controller and hip4081A H-bridge driver,
but it would have needed PWM as well as power-voltage modulation.

I would have needed to drive the Baxandall resonant circuit with
an external frequency reference and tune the load, as opposed to
letting the switching follow the resonance, but that's OK.

 

[[email protected]]

[email protected] wrote...

Why the counter? It's not for a delayed-sweep scope equivalent.
If it's for a PWM time delay, couldn't you use a time delay, and
manually adjust the delay to meet your needs? Crude, but...

It would let me realise a "magic sinewave" approximation to the perfect
drive waveform (at least for high-Q tank circuits) plus non-overlapping
drives for M3 and M4. The nearest thing I can find to a time delay in
LTSpice is a lossless transmission line, which could be made to work,
but is worryingly unrealistic.

 

[[email protected]]

I thought I'd have a play.....because I am a bit of a pedant when it comes
to 'pretty' circuit diagrams and pictures of them.

http://genomerics.org/bills/spice/baxa.asc
http://genomerics.org/bills/spice/pics/baxa.gif

Not pedantic enough though....

Errrr, it sort of oscillates but. I don't know.

Anyway, have fun

You seem to have managed to set up non-overlapping switching between S1
and S2 - I'm not enough of a Spice guru to understand how - which means
that you need the catching diode DSTOMP to make the circuit work. The
third harmonic spike is worse (37dB down as opposed to 40dB down) than
it is on my "pretty" educational model of the classic Baxanadall
circuit. I've not yet managed to build a real Baxandall oscillator that
needed that catching diode.

 

[Genome]

You seem to have managed to set up non-overlapping switching between S1
and S2 - I'm not enough of a Spice guru to understand how - which means
that you need the catching diode DSTOMP to make the circuit work. The
third harmonic spike is worse (37dB down as opposed to 40dB down) than
it is on my "pretty" educational model of the classic Baxanadall
circuit. I've not yet managed to build a real Baxandall oscillator that
needed that catching diode.

Anything that did something was more by luck than design.... I threw DSTOMP
in there because node VCT was swinging off to -millions of volts. Of course
the thing that prevents that in 'real' life is the body-source diodes of the
mosfets, being silly I left them out.

I'm not enough of anything to initially know why it behaves the way it does
but I like to keep things simple so I can selectively prod parts and scratch
my head over what things might be doing.....

http://genomerics.org/bills/spice/baxb.asc
http://genomerics.org/bills/spice/pics/baxb.gif

DNA

 

[[email protected]]

Anything that did something was more by luck than design.... I threw DSTOMP
in there because node VCT was swinging off to -millions of volts. Of course
the thing that prevents that in 'real' life is the body-source diodes of the
mosfets, being silly I left them out.

I'm not enough of anything to initially know why it behaves the way it does
but I like to keep things simple so I can selectively prod parts and scratch
my head over what things might be doing.....

http://genomerics.org/bills/spice/baxb.asc
http://genomerics.org/bills/spice/pics/baxb.gif

That pretty much duplicates my classic Baxandall circuit, including the
initial burst of "squegging". I had to run the simulation for 10msec
and ignore the first 5msec to get a clean waveform for the DFT routine,
and it gives pretty much the same thrid harmonic content that I got.

 

[Genome]

That pretty much duplicates my classic Baxandall circuit, including the
initial burst of "squegging". I had to run the simulation for 10msec
and ignore the first 5msec to get a clean waveform for the DFT routine,
and it gives pretty much the same thrid harmonic content that I got.

I personally dislike these self oscillating things......

http://genomerics.org/bills/spice/baxc.asc
http://genomerics.org/bills/spice/pics/baxc.gif

DNA

 

[Genome]

That pretty much duplicates my classic Baxandall circuit, including the
initial burst of "squegging". I had to run the simulation for 10msec
and ignore the first 5msec to get a clean waveform for the DFT routine,
and it gives pretty much the same thrid harmonic content that I got.

It looks like your "squegging" is the presence of two resonances in the
circuit, maybe.

The fundamental operating frequency might want to be set by the transformer
primary magnetising inductance and the resonant capacitance. However the
input inductor also resonates with it and the only source of damping is the
output load... sort of...

Try....

http://genomerics.org/bills/spice/baxd.asc
http://genomerics.org/bills/spice/pics/baxd.gif
http://genomerics.org/bills/spice/pics/baxdout.gif


I made LIN 10mH and set the two load resistors for critical damping assuming
that the capacitors appear in parallel referred to the input inductor.

It's still a bit woofty, like it's not going to weld heads together at fifty
paces.

DNA

 

[Genome]

It looks like your "squegging" is the presence of two resonances in the
circuit, maybe.

The fundamental operating frequency might want to be set by the
transformer primary magnetising inductance and the resonant capacitance.
However the input inductor also resonates with it and the only source of
damping is the output load... sort of...

Try....

http://genomerics.org/bills/spice/baxd.asc
http://genomerics.org/bills/spice/pics/baxd.gif
http://genomerics.org/bills/spice/pics/baxdout.gif


I made LIN 10mH and set the two load resistors for critical damping
assuming that the capacitors appear in parallel referred to the input
inductor.

It's still a bit woofty, like it's not going to weld heads together at
fifty paces.

DNA

OK.... I changed that a bit because it looked overdamped so maybe I was
thinking about something wrong.

DNA

 

[[email protected]]

OK.... I changed that a bit because it looked overdamped so maybe I was
thinking about something wrong.

The squegging doesn't seem to be a problem in practice - it just limits
LIN. Make LIN too big, and the squegging never dies away.

I'm with you on the problems of self-oscillating circuits - my
pulse-width-modulated circuit started out self-oscillating, but proved
to oscillate in a whole lots of modes that I didn't like and couldn't
get rid of.

Unfortunately, the Baxandall circuit does require zero-voltage
switching, and if you don't let the thing self-oscillate, you need some
kind of control loop to move the oscillation frequency to match the
resonant frequency of the tank circuit (or adjust the resonant
frequency of the tank circuit to match the desired resonant frequency -
as in Win's application).

 

[Genome]

The squegging doesn't seem to be a problem in practice - it just limits
LIN. Make LIN too big, and the squegging never dies away.

I'm with you on the problems of self-oscillating circuits - my
pulse-width-modulated circuit started out self-oscillating, but proved
to oscillate in a whole lots of modes that I didn't like and couldn't
get rid of.

Unfortunately, the Baxandall circuit does require zero-voltage
switching, and if you don't let the thing self-oscillate, you need some
kind of control loop to move the oscillation frequency to match the
resonant frequency of the tank circuit (or adjust the resonant
frequency of the tank circuit to match the desired resonant frequency -
as in Win's application).

I'll still say the "squegging" is the input inductor resonating with the
resonating capacitor and if you properly damp it with the load it gets
sorted. Of course that's not a very good solution.

I was chasing some sort of deadtime inclusion type thing but I finally
figured that this is a current fed coverter so having the switches on at the
same time isn't so much of a problem and that leads me on to the next
one....

http://genomerics.org/bills/spice/baxe.asc
http://genomerics.org/bills/spice/pics/baxe.gif

Now we might be cooking. I've tagged a current mode control buck converter
on the front end of things. Watch out, as I've been diddling I changed the
transformer turns ratio.

It seems that there are two limiting conditions, maybe. The first is you
shouldn't operate with a load close to the resonant impedance of the tank
(above it but not too close to it). The second is you shouldn't try to
develop a reflected voltage across the load that is twice the input voltage.

Anyway..... there's something to have a play with. It looks like the input
control stomps on the "squegging". You can regulate the output using PWM
and... I dunno.... generally poke about and see what happens.

Cheers

DNA

 

[Tony Williams]

I'll still say the "squegging" is the input inductor resonating
with the resonating capacitor and if you properly damp it with
the load it gets sorted. Of course that's not a very good
solution.

Baxandall's value for the feed inductor was:

Lfeed = Rload(referred back across coll-coll)/2w.

Notice that with Rload referred back across the
collector-collector then there is a relationship
between Lpri (collector-collector), Q, and Lfeed.


Yarrow is supposed to have based his value for Lfeed
on O.H Schade's L-C power supply calculations.

Lmin = Vcc/3.w.I and then he used 5*Lmin.

I've used Baxandall's, but not Yarrow's.

 

[Genome]

I'll still say the "squegging" is the input inductor resonating with the
resonating capacitor and if you properly damp it with the load it gets
sorted. Of course that's not a very good solution.

I was chasing some sort of deadtime inclusion type thing but I finally
figured that this is a current fed coverter so having the switches on at
the same time isn't so much of a problem and that leads me on to the next
one....

http://genomerics.org/bills/spice/baxe.asc
http://genomerics.org/bills/spice/pics/baxe.gif

Now we might be cooking. I've tagged a current mode control buck converter
on the front end of things. Watch out, as I've been diddling I changed the
transformer turns ratio.

It seems that there are two limiting conditions, maybe. The first is you
shouldn't operate with a load close to the resonant impedance of the tank
(above it but not too close to it). The second is you shouldn't try to
develop a reflected voltage across the load that is twice the input
voltage.

Anyway..... there's something to have a play with. It looks like the input
control stomps on the "squegging". You can regulate the output using PWM
and... I dunno.... generally poke about and see what happens.

Cheers

DNA

Here you go..... LET'S SHUNT SOME POWER......

How do you fancy 1KW into 50K........?

http://genomerics.org/bills/spice/baxplay.asc
http://genomerics.org/bills/spice/pics/baxplayout.gif

Took a bit of fiddling because I haven't figured it out yet but I'm close.
Mind you it's probably a bit tra...laa...laa.

DNA

 

[[email protected]]

Took a bit of fiddling because I haven't figured it out yet but I'm close.
Mind you it's probably a bit tra...laa...laa.

Just when things start to get interesting, my wife drags me off to
Paris - we won't be back until next Monday. Sod's Law working at full
strength.

 

[[email protected]]

Baxandall's value for the feed inductor was:

Lfeed = Rload(referred back across coll-coll)/2w.

Notice that with Rload referred back across the
collector-collector then there is a relationship
between Lpri (collector-collector), Q, and Lfeed.


Yarrow is supposed to have based his value for Lfeed
on O.H Schade's L-C power supply calculations.

Lmin = Vcc/3.w.I and then he used 5*Lmin.

I've used Baxandall's, but not Yarrow's.

Thanks Tony. That is interesting and important - I completely missed
that when I read Baxandall's paper, back in 1968 (when I knew very
little about electronics).

 

[Terry Given]

It would let me realise a "magic sinewave" approximation to the perfect
drive waveform (at least for high-Q tank circuits) plus non-overlapping
drives for M3 and M4. The nearest thing I can find to a time delay in
LTSpice is a lossless transmission line, which could be made to work,
but is worryingly unrealistic.

think harder.

the ideal switch makes a great comparator, R-Cs can easily add the
delays you want. lots of different ways to do that. Not necessarily a
good idea tou build real circuits like that, but SPICE dont care.

I have a spice model of a 3-phase regenerative rectifier using parke and
clarke transforms, incorporating a behavioural model of the TMS320F2811
3-phase PWM + interlock output stage, all done with blocks (laplace etc).

And I built a behavioural thermal model of my IGBTs too, which includes
switching loss. took perhaps 30 minutes.

Cheers
Terry

 

[Terry Given]

Baxandall's value for the feed inductor was:

Lfeed = Rload(referred back across coll-coll)/2w.

Notice that with Rload referred back across the
collector-collector then there is a relationship
between Lpri (collector-collector), Q, and Lfeed.


Yarrow is supposed to have based his value for Lfeed
on O.H Schade's L-C power supply calculations.

Lmin = Vcc/3.w.I and then he used 5*Lmin.

I've used Baxandall's, but not Yarrow's.

I just got my copy of Roddam yesterday. good, isnt it.

Cheers
Terry

 

[Tony Williams]

I just got my copy of Roddam yesterday. good, isnt it.

That's him. As mentioned in previous threads I was
unable to obtain a copy of Baxandall's original paper
so had to do with Thomas Roddam's description.

Now then, move on to page 191, where TR developes
the voltage-driven series-resonant form into a
half-section filter. Can you follow the final step,
to figure 10.25, and add values to it?

Figure 10.25 is given below.

L1 C2
Squarewave voltage----((((((---+---||---+---+
| | |
| ) \
C1=== L2) /Rload
| ) \
| | |
-----------------+--------+---+

L2 is the primary of the output transformer, with
Rload referred to the primary.


On page 192, do you know what a "Boucherot Impedance
Transforming Network" is?