Baxandall class D oscillator, can it produce a triangle like waveform?
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From: markp on 22 Jul 2010 10:04 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%20parallel-resonant%20Class-D%20oscillator1.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.
From: Bill Sloman on 23 Jul 2010 10:32 On Jul 23, 12:04 am, "markp" <map.nos...(a)f2s.com> wrote: > 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 workhttp://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class... > 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? 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. -- Bill Sloman, Nijmegen
From: John Larkin on 23 Jul 2010 11:53 On Thu, 22 Jul 2010 15:04:46 +0100, "markp" <map.nospam(a)f2s.com> wrote: >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%20parallel-resonant%20Class-D%20oscillator1.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. > 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. John
From: markp on 23 Jul 2010 12:12 >"Bill Sloman" <bill.sloman(a)ieee.org> wrote in message >news:5b2f3288-a527-4198-8648-66137c925b25(a)s24g2000pri.googlegroups.com... >On Jul 23, 12:04 am, "markp" <map.nos...(a)f2s.com> wrote: >> 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 >> workhttp://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class... >> 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? > >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. > >-- >Bill Sloman, Nijmegen 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.
From: John Fields on 23 Jul 2010 13:31
On Fri, 23 Jul 2010 08:53:46 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 22 Jul 2010 15:04:46 +0100, "markp" <map.nospam(a)f2s.com> >wrote: > >>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%20parallel-resonant%20Class-D%20oscillator1.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. >> > >Doesn't this work? > >ftp://jjlarkin.lmi.net/Triangle_Cap.JPG --- Sorry, not even close. Not at Mark's frequency and cap spec, at any rate. Version 4 SHEET 1 880 680 WIRE 96 -16 -48 -16 WIRE 336 -16 176 -16 WIRE 576 -16 336 -16 WIRE 336 48 336 -16 WIRE 576 48 576 -16 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 336 192 336 128 WIRE 416 192 336 192 WIRE 576 192 576 128 WIRE 576 192 480 192 WIRE 336 272 336 192 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 -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 SYMBOL voltage -48 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 12 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 .005 .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 .845 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 .005 .01) TEXT -40 480 Left 0 !.model SW SW(Ron=1 Roff=1E8 Vt=0.5 Vh=0) TEXT -32 456 Left 0 !.tran .1 --- >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. --- Sorry, but no. Version 4 SHEET 1 880 680 WIRE 96 -16 -48 -16 WIRE 336 -16 176 -16 WIRE 576 -16 336 -16 WIRE 336 48 336 -16 WIRE 576 48 576 -16 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 336 192 336 128 WIRE 416 192 336 192 WIRE 576 192 576 128 WIRE 576 192 480 192 WIRE 336 272 336 192 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 -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 SYMBOL voltage -48 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 12 0 1e-6 1e-6 5 0 1) 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 .005 .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 .845 SYMATTR SpiceLine Rser=0 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 .005 .01) TEXT -40 480 Left 0 !.model SW SW(Ron=1 Roff=1E8 Vt=0.5 Vh=0) TEXT -32 456 Left 0 !.tran 10 --- >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. --- Blah, blah, blah, coulda, shoulda, woulda. "It's all just words..." |