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Old 19th March 2008, 11:20 PM   #101
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I just fixed up the Super Cuckoo converter diagram (for the Nth time).

The diode flux reset windings are now shown with less turns than the Mosfet drive windings for the faster reset time needed.

One note on the Cuckoo type converter(s) using 2 overlapped phases, the power transformers are only being 50% utilized as far as their power ratings go. This is likely a don't care issue for tube amps.

I would guess you can get 100 Watts out of a Cuckoo scheme using 4 xfmrs. and 4 Mosfets. Where as, a 4 phase scheme with the same 4 xfmrs. and 8 Mosfets (Berning, SCIC, SSC) can likely do 200 Watts.

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Old 20th March 2008, 02:11 AM   #102
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I've been doing a bit more research on the Cuk converter for use as an impednance converter, and its not looking so good.

It only maintains a low intrinsic (no feedback) damping factor up to a few hundred Hertz, due to resonances there between its L and C components. That rules it out for no feedback designs. Too bad, because I was looking forward to an impedance ratio control knob.

There are some ripple steering techniques that can be applied to other converters to make them ripple free, but they use L and C components too, so they probably have the same problem.

Well, on that note, I'm finished beating impedance converters to death, unless someone has some new ideas.

That leaves the Berning, SCIC, Super Cuckoo and SSC/Matrix, which all look pretty good.

Don
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Old 21st March 2008, 12:24 AM   #103
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Picture attached of the differential switched OTs and B+/2 xfmrs with HV windings on them for the SSC/Matrix 4 phase converter.

LV windings go on next. I used some fairly large ferrite cores as you can see, so that the HV windings would fit on in just one layer. I'm using thickly insulated wire to keep capacitance down. LV windings will be just a few turns, but will be a bunch of windings in parallel to fill the cores. Design flux levels have been kept conservatively low for operation up to 250KHz.

(a Pencil and a 9 pin tube are included in the picture for size comparison.)


Late note on the Cuckoo conv.:

On the Super Cuckoo converter, I earlier said that it didn't need an air gapped core with the flux reset windings added, but I see that Pressman recommends a 2 to 4 mil air gap still, for forward converter topologies (like the Cuckoo) in order to overcome core remanence. This would usually be done using a mylar film between halves of E cores, U cores or Pot cores. So don't try using a toroid for the Cuckoo design.

Don
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Old 21st March 2008, 01:53 AM   #104
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Default I give...

I havn't understood theory of operation in your last ten
or so design revs. Can't say for certain you havn't found
some strange way around the following concerns???

1) Flyback designs that completely reset the core by
forcing all the stored energy out. Through the tube
or to ground, giving it no-where else to go. Do not
present a variable to the loudspeaker. The speaker
side (if powered) sees only that it must charge a
completely reset core each cycle.

This might work OK for tube side power switching,
But the loadspeaker load is then completely hidden
from interactions with the plate. The tube sees only
a dumbed down completely reset core each cycle...
This could be good or bad, but it definately ain't
normal...

2) Designs that escalate to infinate stress if the tube
fails to conduct (perhaps if the cathode might be cold,
biased or driven to cutoff).

3) Designs that reverse or reset the core thru any
non-audio path (such as a freewheeling diode) other
than thru a tube (or mirror) or the loudspeaker load.
The relationship between tube and load is in error by
whatever flux the non-audio path may drain away...

--------------------------------------------------------------------

Variants (such as Berning) that reverse, rather than
reset the core seem relatively immune to all the above
problems. I am not too sure yet about some of these
other cycles... I want to understand, but I am lost.
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Old 21st March 2008, 03:03 AM   #105
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"I havn't understood theory of operation in your last ten
or so design revs. Can't say for certain you havn't found
some strange way around the following concerns???"

Oh..., sorry I lost everyone I guess. The flux reset thing is just for the Cuckoo converter. I sorta modded away until I ended up with the Forward Converter inadvertantly, so was a messy explanation I guess. Wasn't sure where it was going myself, just sorta stumbled into Forward Converters eventually. Main thing was getting the ON duty cycle a little past 50%, so only 2 phases are necessary, instead of 4 for continuous coverage.

The other ones (SSC) are just standard P-P converters like the Berning effectively. The Cuk is kaput.... N.G. The Matrix version of the SSC is just a Mosfet demod., instead of the cross conduction diode bridge demod.
--------------------

If you have a book on switching converters, the Cuckoo is just a standard Forward converter except fixed up to do the flux reset faster by lowering the primary turns for the reset diode. In Abraham Pressman's book "Switching Power Supply Design" 2nd ed. on page 76, he explains the trick. The secondary HV side diode disconnects the tube during the flux reset (which is really just a flyback pulse).

During the next Mosfet ON cycle, the the secondary reflects the same Volt.Seconds as the primary again x turns ratio of course (ie, transformer action, volt.seconds = change in flux) and powers the tube again thru the HV diode. It's standard Forward Converter operation setting the output voltages. But in any case, the Cuckoo is really just a distraction here, mainly for low parts count.
----------------------------------

I thought the switched differential Output xfmr design was OK with everone (the center tapped plate windings, multiplexed by pulling up the B+) (then I added the LV cross conduction diode bridge de-mod to get the audio back for the SSC version). Is there some question on this part?

I'll probably get the SSC/Matrix one up and running this weekend, so I can show some waveforms later if that helps. I'm pretty confident that this scheme is workable, just a matter of getting the design details all fine tuned.

Don

edit: Just read thru your #s again. The Cuckoo/Forward conv. brings the core back to the same conditions as a normal P-P scheme for each ON pulse, and the tube and output are connected by the turns ratio xfmr action the same. Just the reset pulse is altogether missing as far as the tube is concerned. But the other xfmr., running at 180 degrees fills in then, alternating back an forth between xfmrs. Really just the same as the Berning P-P case, but every other cycle is on a different xfmr, and the cycles are allowed to exceed 50% slightly.
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Old 21st March 2008, 05:02 AM   #106
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I just thought of an issue that is maybe part of what you are concerned about with the Cuckoo conv. The flux in the core does not reset to exactly the same condition, as after a reverse P-P type cycle. So with the next ON cycle it starts with some residual remanance not seen in the P-P case. The effect of this is to increase the magnetizing current during ON cycles, but the voltage transfer is still unaffected, since volt seconds convert into flux change and vice versa. After all, SE xfmrs work fine for voltage transfer. They just have high magnetizing current.

These magnetizing currents in the Cuckoo converter end up cross conducting between +LV and -LV since both sides are causing magnetizing current. The power actually gets returned to the power supplies via the diode/reset currents, so isn't wasted.

But the audio voltage for top and bottom converters is not the same generally, so the two magnetizing currents are not generally equal. It would seem to be depending on the voltage transfer action being a low impedance to avoid affecting the speaker. But since the windings have some resistance, there will be some effect, but might be just a linear effect with audio voltage, so un-noticeable.

To fix this, I'm thinking now that the reset/diodes should be connected to the speaker load instead of ground, so as to null out the magnetizing currents directly. Avoiding cross conduction currents. (there will be a half clock cycle delay of course, but likely not much change in the audio in half a cycle) Opinions?

Don
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Old 21st March 2008, 06:26 AM   #107
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Quote:
Originally posted by smoking-amp
"I havn't understood theory of operation in your last ten
or so design revs. Can't say for certain you havn't found
some strange way around the following concerns???"

Oh..., sorry I lost everyone I guess. The flux reset thing is just for the Cuckoo converter. I sorta modded away until I ended up with the Forward Converter inadvertantly, so was a messy explanation I guess. Wasn't sure where it was going myself, just sorta stumbled into Forward Converters eventually. Main thing was getting the ON duty cycle a little past 50%, so only 2 phases are necessary, instead of 4 for continuous coverage.

The other ones (SSC) are just standard P-P converters like the Berning effectively. The Cuk is kaput.... N.G. The Matrix version of the SSC is just a Mosfet demod., instead of the cross conduction diode bridge demod.
--------------------

If you have a book on switching converters, the Cuckoo is just a standard Forward converter except fixed up to do the flux reset faster by lowering the primary turns for the reset diode. In Abraham Pressman's book "Switching Power Supply Design" 2nd ed. on page 76, he explains the trick. The secondary HV side diode disconnects the tube during the flux reset (which is really just a flyback pulse).

During the next Mosfet ON cycle, the the secondary reflects the same Volt.Seconds as the primary again x turns ratio of course (ie, transformer action, volt.seconds = change in flux) and powers the tube again thru the HV diode. It's standard Forward Converter operation setting the output voltages. But in any case, the Cuckoo is really just a distraction here, mainly for low parts count.
----------------------------------

I thought the switched differential Output xfmr design was OK with everone (the center tapped plate windings, multiplexed by pulling up the B+) (then I added the LV cross conduction diode bridge de-mod to get the audio back for the SSC version). Is there some question on this part?

I'll probably get the SSC/Matrix one up and running this weekend, so I can show some waveforms later if that helps. I'm pretty confident that this scheme is workable, just a matter of getting the design details all fine tuned.

Don

edit: Just read thru your #s again. The Cuckoo/Forward conv. brings the core back to the same conditions as a normal P-P scheme for each ON pulse, and the tube and output are connected by the turns ratio xfmr action the same. Just the reset pulse is altogether missing as far as the tube is concerned. But the other xfmr., running at 180 degrees fills in then, alternating back an forth between xfmrs. Really just the same as the Berning P-P case, but every other cycle is on a different xfmr, and the cycles are allowed to exceed 50% slightly.

You lost me for a while on the flyback variations. Not that I couldn't follow technically but I have a slug of work from my day job and I have little interest in flyback or SE converter topologies for some reason...

Not sure I follow the anti-deadtime idea either unless it's the overlap between the quadrature drive phases.

What I like so far is the SSC on the tube side and a synchronous bridge, or multiphase half bridge using multiple windings, on the load side. I guess that's what you're calling the SSC/Matrix?

Your prototype should be very interesting. Do you care to share your operating parameters? What core material, cross section, mag path length, turns, frequency (you said 250 KHz?) Bmax (I guess could be calculated from the others)?

Do you plan to build it into an output stage or use a current source and resistive load for testing? It looks like your cores are plenty big depending on the permeability of the material. What's the B+ voltage and power envelope you're designing for?

Good luck!

Michael

PS
I'll be starting the build of my "big" 2A3-based 4X current boosted current mirror amp. Maybe one day soon we'll have a current mirror amp with a switched converter output.

Throw in a MOSFET source follower drive stage and you have hybrid SS technology used in three places where it's least likely to impart a lot of it's own character to the sound. We can build a 2 stage SE or PP character amplifier with plenty of gain and no grid current blocking (single stage drive + MOSFET), good power and damping factor (current mirror output stage), and a "perfect" output transformer (switched converter).
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Old 21st March 2008, 05:21 PM   #108
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"Not sure I follow the anti-deadtime idea either unless it's the overlap between the quadrature drive phases."

Yes, just overlapped phases, 50+% duty. But only two phases required per tube/converter then instead of 4 for 100% coverage.

"multiphase half bridge using multiple windings, on the load side. I guess that's what you're calling the SSC/Matrix?"

Yes, I managed to get rid of the diode (for blocking the reverse diode in the Mosfet) in series with the Mosfets that way.

"Do you care to share your operating parameters? What core material, cross section, mag path length, turns, frequency (you said 250 KHz?) Bmax (I guess could be calculated from the others)?

The larger cores I'm using for the differential P-P xfmr to the tube plates. It has two interleaved 32 turn, center tapped windings for the HV windings (using 600V insulated wire) that just fit on one layer. It uses two stacked Fair-Rite cores P/N 5977004203
( 2.7 in. OD, 1.5 in. ID, .5 in. thk per core) for a combined area of 3.87 cm2. Material #77 is a power matl. with Ui = 2000, Bmax = 4600, 100 OHm CM resistivity (measures 1000 Ohm across core).
Measuring across 32 turns gives 8.475 mH and .18 Ohm (on L/R meter) and between windings 200 pF (I think that means the tubes will see 1/4 of that between them, most of the interwinding cap. on both cores is just across the HF drive. Typical conventional OTs are more like 500 pF plate to plate). I'm assuming max acceptable flux density of 1600 G at 60 KHz, 1200 G at 125 KHZ and 800 G at 250 KHz.

Working this out at 125 KHz, 1200 G gives +/- 1009 V pk -pk on each 16 turn section, so that would be the maximum swing allowed for a tube plate. This clearly is well above what I need. So actual operation will be more like 400 G at 125 KHz. At 250 KHz, that would drop to 200 G being used. Should be nice low core losses.

The B+/2 cores are some unknown units I got from Electronic Goldmine. They measure 55 mm OD, 32 mm ID, 18 mm thk. Have a black coating on them. Area = 2.07 CM2, le = 136.6 mm. I measured .475 mH for 8 turns giving Ui about 3880. These have two interleaved 25 turn windings on them, filling a single layer.
Measure at 4.73 mH and .11 Ohm on L/R meter. Capacitance between windings is 113 pF.

Working out B+/2 at 1200 G and 125 KHz gives 310.5 Vrms or 439 Vpk. So max B+ would be 878 V which is well above what I plan to use, maybe up to 500V tops. So working Bmax would be around 683 G, 1/2 that at 250 KHz.

I plan to put together two more of the largest core units (identical to above) so as to have 4 of them available to do a Berning 4 phase setup. I will only be putting the audio AC pk-pk thru them, with a 150 V B+ DC (minimum voltage for plates) supply in series with the rectifiers. Don't have to put 2x peak plate voltage thru the xfmrs that way, although I guess my cores could handle it anyway.

For measurements, I think a simple test would be to just put delta DC on the tube side and load the output side to determine output Z.

Then I will test AC dist. (SSC conv.) with two SS audio amps to supply the "tube" signals (will have to bias them up +50V to allow HV diode multiplexing and reduce the B+/2 drive ) I guess the Berning conv. can be tested similarly. Can't really exercise the full voltage range that way however. Could try some DC power supplies and just measure loaded I/O voltages with the DVM if the dist. is large enough to see that way. Or run a SS amp backwards thru a tube P-P OT xfmr to simulate tubes. (maybe have to subtract dist. spectra then, won't work for freq. response though.)

Don
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Old 22nd March 2008, 03:16 PM   #109
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Default Update and new unexpected design issue

Well, I goofed on the center tapped plate xfmrs (SSC design) in a few ways, so I am rewinding them. Biggest goof, is not enough inductive reactance. These big cores make it too easy to meet the voltage specs with too few turns for the inductance spec.

As wound, I was going to use 2 turns for the primary (welding cable!) to get a 2000 Ohm p to p primary (500 Ohm per tube, for some horiz. type outputs), but this would give only 2.12 mH per tube, which translates to Xl = 1665 Ohm at 125 KHz, which is too low I think.

I say I think, because this magnetizing current is looking like DC current to the tubes due to the switching diodes demodulating it. (but see below) This seems to be just waisting power in the tubes. In any case it seems like a good idea to increase the primary inductance some, so I am going to rewind the cores with smaller wire, more turns. (this will further lower flux levels in the cores too, these big cores are really going to be loafing)

It's interesting that these switching designs not only get rid of the hysteresis, but even the primary inductance. But we still are left with DC current side effects.

This magnetizing current issue is different for the Berning design, since the power for the magnetizing current there comes from the LV side (burden on the Mosfets, not the tubes). An odd situation appears to occur in the Berning scheme though. The magnetizing currents are maximum there when the audio signal is zero or tiny. This means that reverse currents occur thru the Mosfets. So the reverse substrate diodes had better be matched or this stuff will cause effects in the audio during the quietest signal times. I'm wondering how well these substrate diodes are matched or controlled in production. I'm also a little uncomfortable now with the output signal having to jump from froward conduction thru the Mosfet to the backward threshold voltage of the diode, all during the quietest audio passages. Hmmm.


In the SSC case, the magetizing currents are proportional to audio signal level, so should never require backwards conduction thru the HV diodes, assuming they are smaller than the signal current. (more on this below) Providing that is the case, then the 90 degree phase shifted HF magnetizing current will end up adding and subtracting from the signal current during a clock cycle, with no net affect on the DC current. So no power will lost or waisted in the tubes fortunately.

But this brings up a new design issue:
It looks like the HV plate side primary reactance (at the switching freq.) must be at least greater than the reflected load resistance in all cases to keep magnetizing currents less than signal currents (or some reverse flyback currents will develop thru the opposite side HV diodes I guess). Bad speaker resonances could be an issue here. So I better get my xfmr. primary Xl up some more.

Also, I am re-winding the center tapped plate toroids as quadrafilar (there are two center tapped primaries, so 4 plate windings), not bifilar, this time. Each tube plate winding needs to cover the whole core.

This magnetizing current issue brings me back to the Cuckoo converter, which I was ready to forget about. It keeps the magnetizing currents out of the tubes, just like the Berning design does. And maybe even out of the Mosfets, due to the reset windings cancelling the magn. current for the opposite phase.

However, after winding some toroids, I have to say that # of xfmrs is really the criteria one wants to minimize, and the Cuckoo always requires 4 of them, and all of them are specifiic to the impedance ratio in case you want to change that. The SSC can get by on a single xfmr. if you really want to push things to the limit.

Don

note: I have decided that for testing purposes I don't really need to do the whole 4 phase setup, 2 phase should be sufficient. (4 phase is just two of these in parallel, so output Z will be 1/2, and input shunt C on the tubes will be twice) Will save some winding efforts till I feel comfortable with the final design. (although the controller will be full 4 phase capable from the start)
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Old 22nd March 2008, 04:22 PM   #110
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"I'm also a little uncomfortable now with the output signal having to jump from forward conduction thru the Mosfet to the backward threshold voltage of the diode, all during the quietest audio passages. Hmmm."

There will be a discontinuity in the output signal when the audio signal becomes large enough to eliminate the reverse magnetizing current thru the Mosfet substrate diodes. This is really NOT SO GOOD! If both tube converters go thru this transition at exactly the same time, then it would just cancel out in the output, but how likely are these to be that well matched. Probably one side goes thru transition then the other. The winding reactance of the xfmr LV windings will help leap the gap voltage-wise, so this is not going to be a 1st order problem. But it certainly would get my attention enough to put a spectrum analyzer on the output with an audio signal level just at the crossing threshold.

This is something that needs to be checked for the Berning type design and the Matrix design. I don't think this issue comes up for the SSC design except in the form of the primary reactance having to be larger than the load resistance (as mentioned in the above post).

------------------------------------------------------

Since we are now into design details, for any of these P-P converters ( Berning, SSC), DC magnetization of the cores is an issue that should be brought up. Normally, for switching power supplies, the use of an H bridge driver (used in the Berning design) is accompanied with a DC blocking capacitor in series with the winding. This keeps DC core balance. But a glance at the Berning schematics shows this capacitor is not used. Probably for "Audio" reasons, its not wanted in the audio path.

For the center tapped driven windings, that reduce the Mosfets from 4 down to 2 (that I have been suggesting), this capacitor is not an option anyway. So we are left in either case with the requirement that the Mosfets ON resistances must be matched (better than 10% recommended). Whether production lots of Mosfets are sufficiently matched is something that will need to be checked. One measures the match with a current probe on the xfmr winding leads. Opposite phase (alternating) current pulses need to be checked for match.

Don


error fix:
In post 109 above:

"As wound, I was going to use 2 turns for the primary (welding cable!)..."

this should read:
"As wound, I was going to use 2 turns for the LV winding (welding cable!)..."
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