Author Topic: SGTC MK1 - An Accomplishment in Progress  (Read 1695 times)

Offline Uspring

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #40 on: November 27, 2019, 02:03:02 PM »
Quote
I doubt frequency matters as long as it's in the 10+kHz range.  The ionized air path definitely decays significantly in 1-2ms, but I don't think it decays much in <100us.

I tend to agree with this. Plasma is caused by heat and that takes some time to cool off. But even during longer pauses hot air remains, which is a lot thinner than at room temperature. That reduces breakdown voltages along the hot path, which causes an easy reignition there. One can make nicely working Jacobs ladders even at line frequency.


Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #41 on: November 28, 2019, 12:44:10 AM »
My guess about insufficient voltage is that the MMC is charging to barely the spark-firing voltage.  When the voltage is marginal, a spark gap can fire or not depending on just how a corona streamer forms - what dust particles happen to be in the air, UV photons that happen to ionize some air, etc.  If the issue was limited current, the MMC would reach spark-gap firing voltage, just more slowly.

Another possibility occurred to me:  The flyback secondary winding has enough internal capacitance to resonate, and that is coupled to your primary ZVS resonant circuit.  That makes two resonant frequencies, one where the two winding voltages are in-phase and one where they are 180 degrees out-of-phase.  (The two resonant frequencies are discussed in some of the Tesla coil discussions, as the Tesla coil primary and secondary are two coupled resonant circuits.)  Perhaps the ZVS is occasionally locking into the higher-frequency out-of-phase mode.  I saw that occasionally a couple days ago in a ZVS-driven flyback experiment of my own - when the output was loaded more heavily.  The flyback is inefficient in that mode.

Concerning the nature of arcs, I'm just learning with my recent Jacob's ladder project.  My only previous experience was with spark-gap sudden discharges, not with continuous arcs.  Perhaps others here can assist.  If I had to guess, I'd say the fire-y arcs are higher current.  I doubt frequency matters as long as it's in the 10+kHz range.  The ionized air path definitely decays significantly in 1-2ms, but I don't think it decays much in <100us.

Many meter resistance ranges top-out at 20meg, so a 264meg resistor may have shown up as open.  An easy way to look for high resistances is with a DC voltage source and a volt meter.  Meters often have 10meg or 1meg input resistance on voltage ranges.  Apply a DC voltage to the HV output wire, then measure voltage on the pins.  If the DC voltage is negative and above ~30V, then the HV return pin can be found that way.  The internal HV diodes often have 20-30V forward drop, so at least that much voltage is required to see continuity from HV wire (positive output, which is the diode cathode) to the HV return pin.

If you want to head down the analytical path, I'd suggest measuring the flyback output turns.   There are a few ways to do so depending on what tools you have around.  Scope?  Probes good for a few hundred volts?  AC signal generator (some source of low voltage in the kHz range)?

That does make sense and I would agree that the issue would then appear to be voltage. I assumed 24V would be plenty to produce the energy needed. Without being able to measure the output, I couldn't be sure and didn't want to risk frying the little flyback.

It's very interesting that you mention two resonant frequencies. I'm not sure it's of note, but with a multimeter on either ZVS output, I was reading a relatively steady 70kHz. During a quick test the other night with a basic oscilloscope, I noticed the frequency would occasionally drop as low as 50kHz, but only for a very short time. Through my narrow perspective though, the waveforms appeared to be relatively as expected considering the crude Jacob's ladder I was using as a load, so I didn't think much of it. This may or may not be of any significance. We're getting into new territory for me now, and I'm learning more on every exchange. I need to widen my field of view when looking at these things and consider all variables.

I found it very curious that the two different flybacks had such drastically different arc formation. I might even speculate that my spark gap woes could be contributed to this. The arc formation of a typical Jacob’s ladder is exactly the kind of arcs this little flyback makes on its own and sustains them a considerable distance. It's quite impressive.

I experimented briefly with current limiting and was able to obtain a steady, controlled, arc across the spark gap (no capacitors connected). When I get back to it, I’ll try increasing voltage a bit while limiting available current and see what kind of results I get.

I did get ahold of another, larger, flyback today. Fortunately, I actually have the schematic for this one. (attached) I will do some comparisons and report the results.

Meter resistance range is also a good point. The one I'm using was actually a recent purchase and I can't recall the range off-hand. I'll have to double check. I hadn’t considered checking the voltage drop with an actual DC input, but that's a great idea.

I did put together a small signal generator that can output triangle, sine, and square wave up to 1MHz and can be finely adjusted. It can use any DC input. I usually just use a 9V battery.
I technically have a scope. It’s just a cheap, handheld, single-channel, bare-bones, oscilloscope that I purchased as a kit some time ago for very basic purposes. It gets the job done..mostly.
« Last Edit: November 28, 2019, 03:47:45 AM by jturnerkc »

Offline MRMILSTAR

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #42 on: November 28, 2019, 05:40:21 AM »
I have a Marx generator which is powered by an average-size flyback transformer driven by a ZVS driver. I use a 24 volt DC supply to power the ZVS driver. The flyback and ZVS driver have no problem with the 24 volts. Perhaps this will allay your concerns about the 24 volt power supply.
Steve White
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Online davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #43 on: November 28, 2019, 06:20:37 AM »
Your small flyback transformers may be designed for a bit lower voltage and higher current.  As Steve suggests, it's probably OK to run the ZVS input voltage a bit higher.  The biggest issue may be the unconnected focus pin, if it decides to arc inside the potted windings.  Those small flybacks look new enough that they are likely from a color TV or monitor, which suggests they'd be good for at least 20kV output.

Measuring the turns ratio would help decide what input voltage is safe.  One way is to feed your signal generator sine wave to the flyback input through a resistor, and measure the flyback HV output with your meter.  Hopefully your signal generator can product enough power to get at least 100-200V on the HV output with only the meter load.  Set the signal generator for something in the 20-50kHz range, lower if you can get enough output voltage.  Measure the voltage across the flyback primary with the scope, to see what actual voltage is achieved from the signal generator and resistor.  Adjust the generator and/or resistor to get 100Vdc on the meter.  Record the flyback primary peak voltage from the scope (or P-P and divide by 2).  Adjust the resistor or generator for 200V, and record that input voltage.  The turns ratio is then (200V - 100V) / (peak_input_for_200V - peak_input_for_100V).  The output diode forward drop cancels in the calculation by using deltas (changes in voltages).  (This presumes a 1.0 coupling factor.  The real turns-ratio will be higher by 1/K.)

An alternative method:  Charge a capacitor (0.1 to 5uF) to 100V.  Then connect the capacitor to the flyback secondary, negative to the HV output wire and positive to the HV return pin.  Measure the flyback primary waveform with your scope.  Repeat with the capacitor charged to 200V.  For the primary peak voltage, use the initial fast edge voltage, not the following ring-down.  Calculation is the same as above, except that the actual turns-ratio will be lower by a factor of K.  (It's worth repeating each capacitor discharge multiple times, taking the scope reading from the trace with the cleanest waveform - initial fast step followed by ring-down without subsequent fast steps.  Mechanical touching of wires often makes mechanical bounce.  The good traces will avoid that noise, or at least have it well past the initial step.  Or, use a TRIAC, such as BTA8 or BTA16 or BTB16 or ... as the switch.)

K (coupling factor) is probably between 0.8 and 0.85 based on the couple flybacks I've measured.  If you want to measure K, perhaps the easiest is to run your ZVS at as low a voltage as it can handle.  Measure the frequency with the flyback secondary open (not arcing), then again with the secondary shorted.  If it's like what I see, the frequency change will be ~2.5:1.

Your new flyback has focus taken from an intermediate stage instead of the HV output.  My larger flybacks are that way too.  I'd still suggest grounding that pin (pin 7) to be safe.  Does the specification list anything about output voltage or current?  How about the design input Vcc voltage?

For comparing the flyback arc characteristics, measuring HV return current would be informative.  A series resistor, perhaps 100 ohms to ground.  Measure the voltage with your scope, or add a parallel capacitor and measure DC value with your meter.

Based on how well you are understanding all the information, it's hard to picture that you are just starting.  Impressive progress!
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #44 on: November 30, 2019, 09:41:17 PM »
I have a Marx generator which is powered by an average-size flyback transformer driven by a ZVS driver. I use a 24 volt DC supply to power the ZVS driver. The flyback and ZVS driver have no problem with the 24 volts. Perhaps this will allay your concerns about the 24 volt power supply.

Thanks MRMILSTAR. The capacitors are rated at 630 VAC or 1200 Volts DC. The voltage measured across the capacitors when the flyback is operating and producing a spark is approximately 50-60 Volts, but a safe drain to source rating for the FETs would be 200 Volts, so I'm not particularly concerned whether my ZVS can handle over 24V, I was questioning myself whether I should push the tiny flybacks any harder, especially with the amount of current being drawn (over 10A peaks when not being limited).
The larger flyback I recently got ahold of is only pulling ~5A at peak (when not limited), with the same 24V input.

Your small flyback transformers may be designed for a bit lower voltage and higher current. As Steve suggests, it's probably OK to run the ZVS input voltage a bit higher. The biggest issue may be the unconnected focus pin, if it decides to arc inside the potted windings.  Those small flybacks look new enough that they are likely from a color TV or monitor, which suggests they'd be good for at least 20kV output.

Your new flyback has focus taken from an intermediate stage instead of the HV output.  My larger flybacks are that way too.  I'd still suggest grounding that pin (pin 7) to be safe.  Does the specification list anything about output voltage or current?  How about the design input Vcc voltage?

For comparing the flyback arc characteristics, measuring HV return current would be informative.  A series resistor, perhaps 100 ohms to ground.  Measure the voltage with your scope, or add a parallel capacitor and measure DC value with your meter.

Based on how well you are understanding all the information, it's hard to picture that you are just starting.  Impressive progress!

Another interesting note about the larger flyback: My ZVS driver uses two 0.33 uF capacitors. Each capacitor is connected from a FET drain to ground. With a primary inductance of 22 uH resonating with 0.167 uF, the resulting frequency is approximately 80 kHz (if I'm doing the math right). Observed operational frequency is closer to 70kHz (which is pretty much spot on for what I was seeing with my small flybacks). However, with the larger flyback, this drops to an average of ~40kHz. Unfortunately, I can't compare this to the first large flyback because I didn't take frequency readings on it.

I ran it on the crude jacob's ladder I use just as a load. After about 10 minutes, maybe, temp reading on the secondary/housing was 60C. The primary and core were noticably cooler. ZVS fets were cool as well. This is quite different from the results when running the smaller flybacks, where EVERYTHING was hot.
Grounding pin 7 on the larger flyback, probably contributed to this difference, I can imagine.
Speaking of pin 7 - were you recommending to ground pin 7 and leave the HV return pin un-grounded, or tying the two pins together and then grounding?
From what I was able to gather, I believe this flyback is rated for 24.2kV, but I don't have an exact number on the ouput current. I came across some information in a repair manual that seems to contradict that though, and implies a 30.5kV "limited EHT rating". An uneducated guess would tell me I could safely assume around 150W?

It may or not be of note, but figured I'd mention that I'm using a bifilar winding for the primary this time (still 5+5). I did the same with my first flyback, but not the smaller ones. The difference may be negligible, but I thought I'd give it a try again, thinking it may help, to some extent, reduce the resonance imbalance cause by leakage inductance, and perhaps help stabilize frequency fluctuations I was seeing in the smaller flyback tests. Waveforms appear to be effectively the same, both when drawing and arc and not. Although the frequency is lower, I'm not seeing the fluctuation I observed with the small flyback, but all-in-all it appears more likely that the alternative winding method is not making any noticeable difference.

As far as measuring turns in the secondary, my signal generator definitely can't put out that kind of voltage (i put it together to be pocket size and was really just using it as a trigger), so I would have to use the capacitor method. I will definitely make a note of that info and perhaps try it in the future, but I think I'll set that aside for the time being. I do think you're right on with the .8-.85 coupling coefficient though.

I appreciate the support and encouragement. I'm not super new to electronics, but still consider myself a novice. This is, however, my first foray into high voltage. Before even attempting any of this, I spent months reading and simulating, etc. Even with all that, I think I'm picking up things here, with you, that I never found mentioned in my research. I'm a quick study, so I always appreciate a challenge, so I'm loving this. I can't thank you enough for your time and sharing your experience!
« Last Edit: December 01, 2019, 01:14:57 AM by jturnerkc »

Online davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #45 on: December 01, 2019, 02:56:39 AM »
Bifilar winding shouldn't matter much.  For the ZVS, the leakage inductance just adds to the inductance from positive DC input, so is a slight advantage.

I'd missed your original note about the two 0.33uF caps being from drain-to-ground.  Most ZVS circuits I've seen have a capacitor between the two drains, but not to ground.  Having capacitors to ground causes the FETs to carry the resonant current in addition to the DC input current, so increases their power dissipation.  Having capacitors to ground as you do may have advantages, though.  I was thinking it might help keep the oscillation to the lower resonant frequency of the two for the case of coupled resonant circuits.  Attempted simulation of that topology a few days ago, but LTSpice didn't converge well, and I didn't take time to play with the algorithm parameters (charge tolerance, ...).

That's likely why you're measuring 70kHz instead of 80kHz - the HV winding capacitance scales by turns^2 and adds to the primary capacitance in the lower-frequency mode.  (The flybacks I've been playing with drop even more due to secondary capacitance.)

BTW, due to my not reading carefully about caps to ground, I'd been presuming 0.66uF primary rather than 0.167uF.  So, that may have messed up some of my previous-post calculations.  However, with 0.33uF from each drain to ground, the effective resonant capacitance is actually 0.33uF, not 0.167uF.  That's because at any given point in time, one of the two 0.33uF caps is shorted by a FET.  Which cap toggles, but only one is active at a time.  The higher FET current is due to that cap shorting, carrying the resonant current from the other cap.  You can see this easily in simulation - measuring frequencies and FET currents in both topologies.

How did you determine 22uH?  Was that something I calculated (from bogus assumptions)?  The best way I know is to measure frequency with a much larger capacitor (ie 3-30uF) so the secondary capacitance is less significant.  (For best accuracy, measure with two different large capacitors and do the algebra to calculate inductance and parasitic capacitance - two measurements and two variables.  That's what I just did for a flyback last week - one I intentionally fried the internal diodes to get AC output.)  Ring-down is the best method with large capacitors.  Running the ZVS will result in low frequency and high current and therefore core saturation, which lowers inductance.

A larger flyback is likely to have a bit more inductance (for 10-turns), but probably not enough to explain 70kHz to 40kHz (about 3x inductance).  Inductance could actually be 3x higher, or perhaps the resonant mode is different.

The HV return pin definitely needs to remain grounded, along with pin 7.  Otherwise the HV return current passes through the high-value resistors of the pin 7 internal network.

150W seems reasonable, 5-6mA at 24-30kV.   With ZVS sine-wave input, the current can probably be a bit higher, and the voltage a bit lower.  (Lower voltage because sine-waves are symmetric, while flyback waveforms have much less reverse voltage.  Higher current because the diode forward-conduction will have higher duty cycle with sine-wave drive, so lower peak current.)

For measuring turns ratio, the signal generator doesn't need to make more than 1-2V peak.  The secondary turn count is likely at least 1000, so at least 100x your 10-turn primary.  However, it needs to get 1-2V peak into a fairly low impedance, which it may not be capable of doing.

For your new flyback for which you attached a schematic, pins 2 to 6 is shown as 90Vpp.  That would be a great place to measure with your scope.  Ground pin 2 and measure pin 6 or visa-versa.  With no HV load, measure pin 2-6 Vpp relative to ZVS input voltage.  That will determine what input DC voltage is safe to run.

Fun project!  Thank you for the detailed information.
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #46 on: December 01, 2019, 07:32:44 PM »
Bifilar winding shouldn't matter much.  For the ZVS, the leakage inductance just adds to the inductance from positive DC input, so is a slight advantage.

I'd missed your original note about the two 0.33uF caps being from drain-to-ground.  Most ZVS circuits I've seen have a capacitor between the two drains, but not to ground.  Having capacitors to ground causes the FETs to carry the resonant current in addition to the DC input current, so increases their power dissipation.  Having capacitors to ground as you do may have advantages, though.  I was thinking it might help keep the oscillation to the lower resonant frequency of the two for the case of coupled resonant circuits.  Attempted simulation of that topology a few days ago, but LTSpice didn't converge well, and I didn't take time to play with the algorithm parameters (charge tolerance, ...).

That's likely why you're measuring 70kHz instead of 80kHz - the HV winding capacitance scales by turns^2 and adds to the primary capacitance in the lower-frequency mode.  (The flybacks I've been playing with drop even more due to secondary capacitance.)

BTW, due to my not reading carefully about caps to ground, I'd been presuming 0.66uF primary rather than 0.167uF.  So, that may have messed up some of my previous-post calculations.  However, with 0.33uF from each drain to ground, the effective resonant capacitance is actually 0.33uF, not 0.167uF.  That's because at any given point in time, one of the two 0.33uF caps is shorted by a FET.  Which cap toggles, but only one is active at a time.  The higher FET current is due to that cap shorting, carrying the resonant current from the other cap.  You can see this easily in simulation - measuring frequencies and FET currents in both topologies.

Thank you for pointing out the power dissipation! That wasn't something I even looked at when simulating. The difference seemed negligible, operation-wise, but I hadn't made considerations for power dissipation. I don't particularly recall the reason for doing that other than some proposed method I may have read and probably didn't understand. I can't exactly remember my thought process there.
Your note regarding resonant capacitance - I should have known better on that one. That's certainly quite clear to see in simulation.
I think i did originally misspeak regarding my ZVS configuration because I was not using the same configuration for my sim. That's my fault for the confusion there and now I understand that may have contributed to some of my other confusions.
Quote
How did you determine 22uH?  Was that something I calculated (from bogus assumptions)?  The best way I know is to measure frequency with a much larger capacitor (ie 3-30uF) so the secondary capacitance is less significant.  (For best accuracy, measure with two different large capacitors and do the algebra to calculate inductance and parasitic capacitance - two measurements and two variables.  That's what I just did for a flyback last week - one I intentionally fried the internal diodes to get AC output.)  Ring-down is the best method with large capacitors.  Running the ZVS will result in low frequency and high current and therefore core saturation, which lowers inductance.

The 22uH was based on some rough guesses entered into an inductance calculator and also seemed to match up closely to results another individual observed using effectively the same ZVS/flyback configuration. I made some other assumptions that were incorrect, as you pointed out, so this may or may not be near accurate. It made sense at the time as the results were close to my observed output when using the smaller flybacks.

Quote
A larger flyback is likely to have a bit more inductance (for 10-turns), but probably not enough to explain 70kHz to 40kHz (about 3x inductance).  Inductance could actually be 3x higher, or perhaps the resonant mode is different.

Right, I was not expecting that significant of a drop! I have that second large flyback, now, that I'll throw on there today and see what kind of frequency and other readings I get in comparison.

Quote
The HV return pin definitely needs to remain grounded, along with pin 7.  Otherwise the HV return current passes through the high-value resistors of the pin 7 internal network.

150W seems reasonable, 5-6mA at 24-30kV. With ZVS sine-wave input, the current can probably be a bit higher, and the voltage a bit lower.  (Lower voltage because sine-waves are symmetric, while flyback waveforms have much less reverse voltage.  Higher current because the diode forward-conduction will have higher duty cycle with sine-wave drive, so lower peak current.)

For your new flyback for which you attached a schematic, pins 2 to 6 is shown as 90Vpp.  That would be a great place to measure with your scope.  Ground pin 2 and measure pin 6 or visa-versa.  With no HV load, measure pin 2-6 Vpp relative to ZVS input voltage.  That will determine what input DC voltage is safe to run.

Excellent tip! Thank you for that! That will certainly help to know that value.
I should have time today to continue working, so will update with new results asap.
Thank you, again!
« Last Edit: December 02, 2019, 04:16:52 AM by jturnerkc »

Online davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #47 on: December 02, 2019, 04:09:37 AM »
When measuring pin 2 to 6 Vpp, there's no need to derate the 90 Vpp rating.  The diode reverse voltage is the output peak-to-peak voltage.  (The output peak voltage will be lower because of sine-wave drive, with as much reverse voltage as forward voltage.)

The inductance for a flyback I wound with 10 turns is ~60uH.  It's probably physically a bit larger than your small ones.

Can you provide any more details about your comment: "I found it interesting that the voltage across the caps was quite low when tied to ground"?  The peak voltage across each cap should be the same grounded or drain-to-drain.  Peak-to-peak will be half, because each cap sees a one-sided waveform, not going more than a diode-drop below ground.

Having some of the resonant capacitance to ground could have an advantage when using IGBT parts for a ZVS oscillator.  It will make the IGBT current negative just before switching off.  I haven't explored the effects yet, but it might be useful for IGBTs with slow turn-off.

Looking forward to your new update!  Hopefully it's a successful day of experimenting.
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #48 on: December 02, 2019, 05:49:05 AM »
When measuring pin 2 to 6 Vpp, there's no need to derate the 90 Vpp rating.  The diode reverse voltage is the output peak-to-peak voltage.  (The output peak voltage will be lower because of sine-wave drive, with as much reverse voltage as forward voltage.)

The inductance for a flyback I wound with 10 turns is ~60uH.  It's probably physically a bit larger than your small ones.

Can you provide any more details about your comment: "I found it interesting that the voltage across the caps was quite low when tied to ground"?  The peak voltage across each cap should be the same grounded or drain-to-drain.  Peak-to-peak will be half, because each cap sees a one-sided waveform, not going more than a diode-drop below ground.

Having some of the resonant capacitance to ground could have an advantage when using IGBT parts for a ZVS oscillator.  It will make the IGBT current negative just before switching off.  I haven't explored the effects yet, but it might be useful for IGBTs with slow turn-off.

Looking forward to your new update!  Hopefully it's a successful day of experimenting.

I tried to go back and correct my voltage statement, but you got to it before I did.
I was only reading half of the waveform and out of ignorance was not figuring peak-to-peak for both caps and confused myself a bit. As you pointed out, one is always shorted, so I was only reading the Vpp of that particular cap.

I'm not well versed in LtSpice. It's a little too deep for me at this point, but I've managed to make good use of the Falstad simulator. I've attached the circuits for the two different ZVS topologies. I haven't entered my 'real-world' values as this was just a base model for operational comparison purposes. I also still don't have accurate values on my flyback primary/secondary. These can be a pain to simulate, I've noticed. A rough guess has yet to get me any further in mirroring my actual circuit, at least as closely as possible, especially when trying to including the rest of the TC circuit.
*Note: the 1u resistors were used just to prevent capacitor loop error in Falstad. I modeled it this way simply because I wanted to show 2 capacitors. The values I used for the transformer are: 83mH primary, Ratio is 1:2, and K is .99 (again, just a working model). For clarification on the screenshot - the top two waveforms are each of the capacitors, the third waveform down is one FET, and the 4th waveform is the same FET's power consumption. The left column is the typical ZVS topology, and the right column is how I currently have mine configured.)

I noticed the frequency increase as well as a slight increase in power consumption, as you mentioned. The FET waveform changes and the current drops slightly negative, as you pointed out, as well as the voltage. I'm curious about that brief positive spike just before the drop and subsequent ramp up. FET voltage remains relatively unchanged (~1V), but current increases quite a bit. The rms voltage of the capacitors effectively match the FETs, which makes sense (i think). Frequency and current seem to be the most noteworthy.
Looking back, I believe this topology is proposed to be used in higher frequency applications than is needed here...which I suppose makes sense, as no components need to be added/removed to achieve a higher frequency. Regardless of its intended application, it would appear, to my eyes anyways, that I should go the drain-to-drain route with the capacitors.

I think i did originally misspeak, and may have gone back and corrected my claim, regarding my ZVS configuration because I was not using the same configuration for my sim at the time. That's my fault for the mix-up there and now I understand that may have contributed to following confusions.

After flipping through some old notes, the drain-to-ground implementation seems likely to have spawned from notes I made on a conversation regarding FETs burning out in an H-Bridge motor driver application a while back.

I spent most of the time I had set aside today practicing my soldering skills putting together 120, 100mH, inductors, so I don't have much to update on just yet, other than that I am glad it was not more! I used some small boards I had laying around and put together a little two-tiered stack of 60 inductors each. I didn't have time to wire it up in the circuit, but it's ready to go.
« Last Edit: December 03, 2019, 02:09:04 AM by jturnerkc »

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #49 on: December 07, 2019, 09:48:21 PM »

The small positive current spike is likely because the FET turns on slightly before its drain voltage reaches 0, so relatively-suddenly discharges its drain cap the last few volts.  BTW, the relative drain current between the two topologies depends on the relative resonant current to load current.  If you change the 1k load in simulation, it changes the load current, but not the resonant current.  A low-impedance resonant circuit would make the extra FET loss of the drain-to-ground topology worse.

Yes, the drain-to-ground topology smooths out the drain voltage during the transition points.  That may have value with slower switch devices (IGBTs).  In my bit of simulation, drain-to-ground caps made it more difficult to get the ZVS to oscillate at the lower frequency parallel mode of the input and output windings.  I'm not sure why.  I'll be experimenting with that for a DIY plasma ball eventually, but December is busy for me.

Coupling of 0.99 is high for your simulation transformer, but won't matter much until you add an output capacitor to simulate the flyback secondary's internal stray capacitance (wire and diodes).  With a realistic output capacitance and coupling (80-85%), you can experiment with the two resonant frequencies.


The parameters I used for the transformer are not representative of my actual application. I entered a higher inductance and just left the coupling parameter at default. In simulation, it made it easier to observe the waveforms, considering I can only increase the division so much. I found that just increasing the inductance would reduce the frequency and make those observations easier. This seemed like a simple enough solution at the time. Although it was not completely representative, it simply helped me visualize what is generally happening at different locations within the circuit, during different conditions.

The ZVS circuit can definitely be fickle in simulation.

A few updates:

The larger secondary and new topload that I was going to be using will be set aside for the time being, as I use the previous secondary and topload (since I already had that tuned for the most part). The calculated resonant frequency of the larger secondary (backed up by observations when testing), would indicate that I either need to increase the # of turns in my primary or double the capacitance of the MMC to reduce the primary resonant frequency. Simply adding another 20 caps in parallel would lower the frequency enough to come within ~2% of resonance (using JavaTC) I'm going to go ahead and wire up another 20 capacitors and set them aside until I decide what to do. That would be cheaper and less time consuming than purchasing more copper and re-'winding' the primary.
Decisions. Decisions.

In the meantime, I reduced the inductor string to 6H. Even after taking resistance readings, voltage drop, etc. the day before - the next day went to check everything again before connecting, and nothing. I ended up finding a single inductor reading as an open circuit. Odd, but I just need to swap that one out apparently. Each level of my 'inductor bank' had 60 inductors, so I removed that level and left myself with 60, so 6H.



During some simulation, however, I found that it may be beneficial to actually use 2 separate strings. When an inductor string is placed on both the HV output and return. It was important to ground the HV return and then attach the inductor between the ground point and the spark gap. Another observation I made, when simulating this configuration was that placing the MMC in parallel and the spark gap in series (swapping the components around) appeared to positively affect the circuit. I judge this from a certainly novice standpoint, but I see less noise and a more uniform operation when simulating in this way. The drain to ground implementaion also seems to aid in the scheme of things.

Here is how I have it laid out in simulation. (I know the flyback representation is probably not ideal, but I wanted to try and simulate the effect of grounding the focus pin with the HV return. I'm using three diodes due to 3 diodes being represented on the flyback schematic.



Testing last night was not entirely successful. I believe the issue is actually with my spark gap and crude adjustment method. I'm going to be working on improving that today and hopefully give it another run tonight with more thorough results. At this point, I'm just trying to get back to my previous results, before losing the first flyback, with these added stability improvements.

One note: the DC circuitry does not appreciate being tied to the same ground as is being used for the HV return (and pin 7). The first bang in the spark gap would immediately reset my regulator, and/or disrupt DC output of the Power Supply. I'm not exactly sure which, but I would lean towards the latter being the reason the regulator reset and shut the ZVS off. Then again, my oscilloscope reset itself as well and it was only connected to one of the zvs outputs. Either way, I didn't like it, so, for now, I'll be keeping the rest of the TC circuit isolated from DC and mains. Something odd like this is kind of what I expected and the reason I hesitated to use a shared ground, especially back to the mains ground.
The TC secondary is grounded to only the counterpoise, so I grounded the HV return (& pin 7) to the counterpoise for the time being and that did remedy the power disruption I was experiencing. I am considering running a line from the counterpoise to a nearby water pipe just for an added measure. In addition, I'm definitely noticing a difference in behavior depending on what I do with pin 7, but I can't determine what the best measure is. Performance seems better when it's simply left disconnected or grounded to the counterpoise, instead of grounding both the HV return and pin 7. Tying the HV return and pin 7 together seems to adversely affect performance.

Speaking of the counterpoise, I'm now using a 3'x3' piece of aluminum with a perforated pattern. I'm curious if it will work well.

 
« Last Edit: December 08, 2019, 05:01:14 AM by jturnerkc »

Online davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #50 on: Today at 12:53:57 AM »
Great detail.  Hopefully I can make a coherent response with all the thoughts it triggers.

For the inductor sets, I'd recommend mounting them side by side or end to end, which ever way extends the rows or columns in the same sequence they are wired within each board.  In other words, input to the inductor string on one edge and output on the opposite edge.  With the inductor boards stacked, the capacitance between boards bypasses inductors for the highest-frequencies of the spark gap.  (This may be less important if driving the MMC directly instead of the spark gap - see farther below.)  My other possible concern is the etched copper on the breadboard used for inductor support.  Is the copper just at the fingers and plating through holes?  Or, does it have any larger traces or planes (intended for distributing power or ground in a typical circuit)?  Such traces could bypass the start-to-end of each inductor array with close-enough gaps to potentially arc over.  Even if not arcing, they would add capacitance that bypass inductors much as the board stack.  6H vs. 12H isn't an issue.  It's that with 6H, the voltage is split among only 60 inductors, so higher voltage per inductor, so higher risk of breaking down the inductor's internal magnet wire enamel.

For the larger lower-frequency secondary, the trade-off is of energy-per-spark vs. spark repeat frequency.  Twice the capacitance is twice the energy, which will take twice as long to recharge for given ZVS input power.  I don't have any particular recommendation.  Others here are more likely to have experience with which would produce a more impressive system.

For simulation, note that the flyback schematic had diodes between secondary winding sections, and the focus tap came after the first diode.  Focus is a DC tap, not AC as in your simulation.  Unrelated thought on focus:  since this flyback has focus coming from a tap rather than the full HV output, there's a better chance of keeping it from arcing when open.  It will be at only 1/3rd of your output voltage.  I'd still recommend tying it to the HV return pin, but perhaps not critical.  (I suspect your small flybacks take focus from the full HV output as do the smallest ones I have.)

For grounding, I'd suggest a ground plane under the entire "low" voltage side, the front half in your image.  Given the relative proximity to the higher-voltage spark-gap components, I'd recommend that the ground plane include a vertical part between the two halves.  Ie. a piece of aluminum bent into an L, with horizontal leg under all the LV circuitry and vertical leg separating the two halves, with some cutout or pass-through for the flyback primary leads.  I'd ground this sheet-metal piece to the line ground at the input to the power supply (at the point the line cord enters your system), and to the negative supply output terminal.  (For added protection against supply confusion, run the supply output wires (both + and -) for a few turns through a large ferrite bead or other common-mode choke.  Ground the - side after the choke.)

Concerning power-supply confusion due to sparks, that's my guess.  However, if a ZVS circuit stops oscillating for whatever reason (usually a low-Q resonant circuit), it's supply current ramps up "indefinitely", to the short-circuit capability of the FETs.  That would shut down the supply (of course).

The larger counterpoise should help.  Is it on a concrete slab or other surface built directly on the ground, or on a typical indoor raised floor?  The former will have better capacitance to ground.  If the latter, then I think some connection to a water pipe or line ground would be wise.  For the middle section (flyback secondary/MMC/spark-gap circuitry), I'd still recommend some form of grounding of the HV return pin.  Otherwise stray capacitance could cause a voltage spike on that pin when the spark-gap fires, eventually breaking down insulation within the flyback.  If the power supply still has issues after adding a ground plane under it, perhaps grounding the HV return down to the counterpoise would be less problematic.

Wiring the flyback HV output to the MMC instead of the spark gap isn't normal (to my knowledge), but could have advantages in your case.  It does make the inductor string critical.  After spark-gap firing, the MMC voltage resonates negative almost as far as it was charged positive.  Negative voltage on the flyback could be problematic depending on it's internal leakage inductance, as the diodes would all be forward-biased.  However, as you pointed out, the MMC doesn't have the sudden voltage step of the spark-gap.  That makes the slew rate more manageable, lowering the influence of stray capacitance.  So, it's a trade-off of gentle slew rate for higher peak-to-peak voltage across the inductor string.  Thus, if you feed the MMC, having all 120 inductors is more important, to share the higher peak-to-peak voltage, but the physical layout of the inductors isn't quite as critical.  (I'd still avoid the stack configuration, however.)

If I missed anything for which you wanted feedback, please let me know.
David Knierim

High Voltage Forum

Re: SGTC MK1 - An Accomplishment in Progress
« Reply #50 on: Today at 12:53:57 AM »

 


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