[markp]

Hi All,

If I had a Baxandall class D resonant oscillator, would it be possible by
modulating the current input to the drive circuit to produce a rounded off
triangle like waveform?

Bill Sloman's excellent work
http://home.planet.nl/~sloma000/Baxandall parallel-resonant Class-D oscillator1.htm
found that the odd harmonic distortion was caused mainly by the AC ripple
current flowing through the source inductor (and hence the driving
windings). This presumably causes a perturbation in the dB/dt of the flux
which modifies the output waveform.

So would it be possible (in theory) by controlling this drive current more
accurately to produce a rounded off triangle waveform without losing all the
efficiencies and advantages of a resonant class D oscillator?

Mark.

 

[markp]

It would be messy. In theory, to create a triangular waveform, you
want to add the odd harmonics of the fundamental, with each harmonic
added at an amplitude that is related to the amplitude of the
fundamental in proportion to the inverse of the square of the harmonic
number - the third harmonic at one nineth of the fundamental, and the
fifth harmonic at one 25th (4%) would seem to be as much as you'd
need.

So, three centre-tapped tank circuits, tuned to be resonant at the
fundamental, the third harmonic and the fifth harmonic. Then three
separate feed inductors, each going from the same voltage rail to a
different centre-tap, and three pairs of MOS-FET switching transistors
to drive the three separate tank circuits.

Then a 4046 running at - say - thirty times the fundamental frequency,
divided by six to drive the fifth harmonic tank, by ten to drive third
harmonic tank and by thirty to drive the fundamental tank, with a
second divide by thirty output in quadrature with the first that you
can phase lock to the output from the fundamental tank.

This would give you three sychronised sine waves; put a 225 turn
floating coil on the fundamental tank circuit, a 25 turn floating coil
on the third harmonic tank circuit and a 9 turn floating coil on the
5th harmonic tank circuit, and connect the three coils in series ands
you should be able to end up with a not-too-round triangular wave.

The feed inductors could probably have quite a lot more inductance
than the inductance of the tank circuits - the original Baxandall
class-D oscillator built with bipolar transistor switches "squegs"
when the feed inductor is too big, but oscillators driven by MOS-FETs
don't seem to have this problem.

Blimey, I didn't consider that solution! Thanks for this. The tanks I assume
would be close to resonance but may be a little out due to tolerances and
drift, which I guess will add a bit of cross-over distortion, but that might
get filtered out eventually.

Anyway, I was thinking more on the lines of modulating the current through
the single source inductor. Maybe by reducing the source inductor to a much
lower value, and PWMing it such that the resulting dB/dt in the core
generates a triangle wave. The reason I say that is because your work seems
to suggest the ripple through this source inductor (due to the centre tap
voltage rising and falling) actually adds harmonic distortion to the output
waveform, and one of the solutions was to try to remove it by PWMing.

Also, this source inductor is quite a large component and suffers from I2R
losses, so couldn't you make it look much bigger to the circuit by tracking
a proportion of the centre tap voltage and PWMing a much smaller source
inductor with it? The result from that I think would be to effectively put a
much smaller ripple voltage across the inductor, and you could get away with
a smaller one as a result.

BTW I have made a large-ish class D oscillator which worked fine even with a
large source inductor relative to the drive inductance, and yes I did use
MOSFETs

One thing of note, your experiments with PWMing produced some ringing which
you suggest non-overlapping drive might help. I came to the conclusion that
you *need* a small amount of overlapping, because when both transitors are
off the current stops flowing instantly, which would mean the source
inductor voltage would rise to try to keep its current flowing and there's
nowhere for the current to go (both transitors are off). The one I built has
a PLD that guarantees a few 100ns of overlap and had no issues with noise.

Mark.

 

[John Devereux]

Doesn't this work?

ftp://jjlarkin.lmi.net/Triangle_Cap.JPG

What's interesting is that, once it's all going, the power supply can
be cranked down to zero and you can make the triangle forever, for
free, since the ideal circuit is lossless. The slopes are technically
segments of sine waves, not linear bits, so there will be some small
curvature, less as L gets bigger. Given a real inductor, simple tweaks
could make the slopes straight.

Nice!

Though why you would want to put a triangle across a cap in the first
place is a bit of a mystery

 

[Fred Bartoli]

John Fields a écrit :
But it does.

Even with real switches and inductor...


Version 4
SHEET 1 1072 680
WIRE 96 -16 -48 -16
WIRE 336 -16 176 -16
WIRE 448 -16 336 -16
WIRE 576 -16 448 -16
WIRE 336 48 336 -16
WIRE 576 48 576 -16
WIRE 992 48 912 48
WIRE 288 64 272 64
WIRE 640 64 624 64
WIRE 288 112 240 112
WIRE 672 112 624 112
WIRE -48 160 -48 -16
WIRE 912 176 912 48
WIRE 336 192 336 128
WIRE 416 192 336 192
WIRE 576 192 576 128
WIRE 576 192 480 192
WIRE 864 192 576 192
WIRE 336 240 336 192
WIRE 864 240 336 240
WIRE 336 272 336 240
WIRE 576 272 576 192
WIRE 288 288 272 288
WIRE 640 288 624 288
WIRE 64 336 64 304
WIRE 176 336 176 304
WIRE -48 432 -48 240
WIRE 64 432 64 416
WIRE 64 432 -48 432
WIRE 176 432 176 416
WIRE 176 432 64 432
WIRE 240 432 240 112
WIRE 240 432 176 432
WIRE 288 432 288 336
WIRE 288 432 240 432
WIRE 336 432 336 352
WIRE 336 432 288 432
WIRE 576 432 576 352
WIRE 576 432 336 432
WIRE 624 432 624 336
WIRE 624 432 576 432
WIRE 672 432 672 112
WIRE 672 432 624 432
WIRE 912 432 912 256
WIRE 912 432 672 432
WIRE -48 512 -48 432
FLAG -48 512 0
FLAG 64 304 A
FLAG 272 64 A
FLAG 640 288 A
FLAG 176 304 B
FLAG 272 288 B
FLAG 640 64 B
FLAG 992 48 Vcap
FLAG 448 -16 Sup
SYMBOL voltage -48 144 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 0.022
SYMBOL sw 336 368 M180
WINDOW 0 32 15 Left 0
WINDOW 3 32 44 Left 0
SYMATTR InstName S1
SYMBOL sw 336 144 M180
WINDOW 0 32 15 Left 0
WINDOW 3 32 44 Left 0
SYMATTR InstName S2
SYMBOL voltage 64 320 R0
WINDOW 0 -53 5 Left 0
WINDOW 3 -242 110 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(1 0 0 1E-6 1E-6 .004999 .01)
SYMBOL sw 576 144 R180
WINDOW 0 32 15 Left 0
WINDOW 3 32 44 Left 0
SYMATTR InstName S3
SYMBOL sw 576 368 R180
WINDOW 0 32 15 Left 0
WINDOW 3 32 44 Left 0
SYMATTR InstName S4
SYMBOL cap 416 208 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName C2
SYMATTR Value 3e-6
SYMBOL ind 80 0 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L2
SYMATTR Value 10
SYMATTR SpiceLine Ipk=12m Rser=1
SYMBOL voltage 176 320 R0
WINDOW 0 -53 5 Left 0
WINDOW 3 -242 110 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value PULSE(0 1 0 1E-6 1E-6 .004999 .01)
SYMBOL e 912 160 R0
SYMATTR InstName E1
SYMATTR Value 1
TEXT -40 480 Left 0 !.model SW SW(Ron=0.1 Roff=1E6 Vt=0.5 Vh=0)
TEXT -32 456 Left 0 !.tran 0 25 24.9 1m

 

[Tim Williams]

..
..
..

Where did your post go?

Snip your fucking bottom posts. Evidently you don't want people to find what you've written.

Tim

 

[ehsjr]

Nice!

Though why you would want to put a triangle across a cap in the first
place is a bit of a mystery

It's the schematic symbol for a new chip they're developing for
Robocop, called the COPAmp

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[Grant]

He may have trouble with visual tracking, target acquisition
essentially. People differ a lot here. I'm OK, not great but adequate,
but what really bugs me is when the time sequences of a series are out
of order, as with top posting in a newsgroup. That is, to me, like
reading a book in which the pages have been randomly reorganized.

Agent indents various posts with > and >> and >>>, so I don't find it
difficuly to scroll down and find the new stuff, interleaved or at the
end. Maybe he can't afford scroll bars.

I have Agent set to show replies in grey, so the new stuff stands
out more. Trying to change others' posting style seems a pointless
exercise. Does one try to stop another waving their hands in face
to face conversation?

As far as the topic goes, I'd still like to know what the triangle
wave is driving, 'cos I like to see some of the bigger picture shed
some light on the smaller details.

Grant.

 

[Grant]

Yeah, but you know how sometimes people ask for the wrong thing, it
often helps to have a bigger slice of the picture.

Like square voltage makes triangle current in inductor, square
current a triangle voltage on capacitor.

So one might generate a lovely triangle with the current resonating
circuit with selected harmonics, only to have it all fall over when
loaded.

Grant.

 

[Tim Williams]

We have here what is arguably a fascinating and beautiful circuit, and
you are whining about posting styles?

Some of us don't read SPICE. All I see is a complete disregard for readers. Therefore you haven't shown a reason for me to behave otherwise.

Tim

 

[Tim Williams]

I didn't do Spice

Then why repeat it? There is no shortage of posts, I can go anywhere in the thread and find the same data if I want it. It's not going anywhere, it's archived on Google in as many posts, for all the world to see.

As far as disregard, I post about like most people post.

Ah, so rudeness isn't rudeness when everyone else is a cretin?

Except me, apparently (except for the obvious rudeness of calling you out).

This is casual conversation, like shooting the bull at a bar
not like some city council meeting under Robert's Rules.

So when "shooting the bull", you repeat all words that have been said up to that point? You must be dreary for conversation...

But you don't use LT Spice? It can be handy.

Better, I use PaperSpice ;-)

Tim