Baxandall class-D oscillator squegging and the VBIC model

(LTSpice _schematic_ (Phil) snipped)

Since it's balanced, why do you need a monster inductor there? Why not use a resistor, or put a resistor in series with the L to dampen the squegging?

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

It seems to wobble a bit on startup, but doesn't actually squegg even with a full henry as the feed inductor.

Oscillators sure simulate slowly.

John

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That's what I'm complaining about. A real circuit would almost certainly squeg.

-- Bill Sloman, Nijmegen

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Using a big inductor reduces the level of odd harmonics (3rd, 5th etc) in the sine wave generated. Using an inductor at all is the kern of the Class-D oscillator

Adding more resistance in series with the inductor makes the circuit less efficient. Jim William's versions get over 90% efficiency - Linear Technology application notes AN45, AN49, AN51, AN55, AN61, AN65.

If it were big enough, it might damp the overshoot on start-up enough to prevent the initial reversal of current in the inductor, which might - in turn - stop the squegging from starting.

-- Bill Sloman, Nijmegen

Well there's one way to find out isn't there? :)

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John Devereux

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The last one I built with an over-sized oscillator (which was the first one I ever built, more than forty years ago) certainly squegged. Building another just to prove the point would be something of a waste of time. An example with a diode to prevent the current through the inductor going the wrong way might make more sense.

-- Bill Sloman, Nijmegen

Well of course the next step is to fix the squegging. But if it does not show up in the simulation, then the simulation is not going to be much use for fixing it!

--

John Devereux

On Jan 14, 3:41=A0pm, John Devereux wrote:

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If the problem is the current going the wrong way through the inductor

- which is what the simulations seem to suggest - then one extra diode might well fix it. If this were to work, I'd have found something - thanks to the simulation - which Peter Baxandall (amongst many others, including me and Tony Williams) had missed. If it were that simple, someone should have found it out earlier, so it probably won't work.

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+)=3D0.0 V(Vout-)=3D-0.0 I(L3)=3D0.00 I(L1)=3D0 I(L2)=3D0 I(L4)=3D-0.0 I(L5)=3D-0.0 I(L7)=3D-0.0033 TEXT -480 800 Left 2 !K1 L1 L2 L3 L4 L5 L6 0.99 TEXT -488 840 Left 2 !.model 2N3906A PNP(ibei=3D2.22716E-18 xii=3D3 Ea=3D1.11 Vef=3D16.8 \n+ Iben=3D7.558f Nen=3D1.536 Ikf=3D.293583 Xin=3D1.= 5 Ver=3D50 ibcn=3D529.3E-18 \n+ Ncn=3D15.51 Ikr=3D10. Rcx=3D.8508 Cjc=3D10.1= 3p Mc=3D. 6993 pc=3D1.006 Fc=3D.5 \n+ Cje=3D10.39p Me=3D.6931 pe=3D.9937 Tr=3D10n Tf= =3D181.2p Itf=3D4.881m rth=3D0. cth=3D0.\n+ Xtf=3D.7939 Vtf=3D10 Rbx=3D10)\n+ Level= =3D9

You might also consider putting diodes in series with the transistor collectors, to force the transistors to only work one way.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

I know that putting a resistor in makes things less efficient -- but what do you want? An astonishingly efficient oscillator that wastes some power, but still works? Or an oscillator that is is astonishingly efficient and useless?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

I doubt it. The whole thing is domnated by the Q of the tuned circuit. It's not a higher-order system. A cap in the base bias network could push it over the edge, like a super-regen receiver.

But why don't you build one and see?

John

If the behavior of this oscillator is a waste of time, give it up.

If it were interesting to me, I'd have one running in about 30 minutes, on a handsome, dremeled, labeled slice of copperclad.

John

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A bit extravagant for a 16kHz oscillator. If I get enthusiastic enough to build one I'll put it on a perforated prototyping board. I rather like the one Farnell sells that comes with a "collander ground plane" on the component side of the board. The copper rings around the holes on the track side can be chopped up to support surface mount parts if you want to mix and match.

It will take a bit longer than half an hour - winding transformers is tedious, even when you've got access to a coil winding machine.

-- Bill Sloman

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Doubt all you like. It squegs anyway, and always has, if you make the feed inductor too big

See Baxandall's footnote on page 752.

I've built several over the years. I've got no urgent need to build one now.

-- Bill Sloman, Nijmegen

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One in series with the feed inductor makes more sense than one in series with each of the switching transistors - see today's version of the circuit in my response to John Devereux. it doesn't help the efficiency.

-- Bill Sloman,Nijmegen

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Jim Williams' application notes AN45, AN49, AN51, AN55, AN61, AN65 make it perfectly clear that the circuit can be both efficient and useful. A bigger feed inductor would just reduce the (odd) harmonic content of the output. There's no perceptible even harmonic content.

It has been claimed here that using MOSFET switches rather than bipolar transistors lets you use a big feed inductor and still avoid squegging. My primary interest is trying to work out what causes the squegging, rather than in coming up with a cure for the squegging when it does happen.

If you can live with a slightly less efficient circuit, I think I've got a variant that keeps the harmonics some 80dB below the fundamental, which is better than you could do with any practical inductor (short of a high temperature super-conductor coil in liquid nitrogen).

-- Bill Sloman, Nijmegen

Can't the resistance be added in parallel with the inductor and achieve a lower Q?

You can wind a pot core in a few minutes, by hand.

ftp://jjlarkin.lmi.net/Z206_pcb.JPG

Or buy one of those dual-winding inductors for about a dollar.

Or not.

John

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It's no less inefficient. Any resistor carrying current is dissipating heat.

-- Bill Sloman, Nijmegen