Kaizer DRSSTC IV - Poor mans QCW

[Mads Barnkob]

DO NOT REPLICATE THIS PROJECT, HARM TO NEARBY ELECTRONICS IS A HIGH RISK

The idea to this coil came with Steve Ward showing off his first QCW DRSSTC that used a buck regulated DC supply to ramp up the supply voltage along with a long on-time to grow straight and very long sparks compared to the secondary coil length.

I thought it could be done simpler, yet with a lot less control over spark growth, by using the rising edge of 50 Hz mains supply voltage. From start of the sine wave to the top it corresponds to a on-time of 5000 uS and to be able to use large IGBT bricks the frequency would have to be kept down. Sword like behaviour of sparks is however mostly seen at above 300-400 kHz, where as lower than that results in more branched sparks.

Considerations
A high impedance primary circuit is needed to keep peak current at a level that the IGBTs can handle to switch for very long pulses, for a DRSSTC, up to 5000 uS.



To use 3 IGBT bricks in parallel it is important to ensure as even current sharing as possible, this is done by mounting them close to each other on the same heat sink, drive them from the same gate drive transformer with individual gate resistors matched as close as possible.

Steve Wards universal driver 2.1b only has a robust enough 24 VDC section to run up to about 300 uS on-time with large gate capacitance, when trying to run with longer on-times than that, the 24 VDC 1.5 A regulator is now longer enough to supply the needed current. A external 26 VDC 8 A power supply is used instead and output stage will have to be reinforced to conduct higher currents and dissipate more heat.

Specifications
Bridge: 3x  SKM125 IGBTs in a parallel half bridge configuration
Bridge supply: 0 - 260VAC through a variac
Primary coil: 21 turns of 8 mm copper tubing in a up-side-down U shape
MMC: 10 strings in parallel of 10 in series (Kemet R474N247000A1K capacitors for 0.047 uF at 9000 VDC, 280 A peak and 40 A rms rating)
Secondary coil: 160 mm diameter, 330 mm long, 1500 windings, 0.2 mm enamelled copper wire.
Resonant frequency: Around 100 kHz.
Topload: 100 x 330 mm spun aluminum toroid.
Input power: ?   
Spark length: ?
 
Schematic
Not yet

Bridge section
Not yet

Driver section
Same as Steve Wards universal driver version 2.1b. Just made on single sided PCB without SMD components and the 24VDC part has traces reinforced, MOSFETs heat sinked and uses a external power supply.


Construction

31st October 2011
I put the bridge together on a heat sink with 3 phase rectifier used with all inputs in parallel for 1 phase supply and connected all 3 half bridge IGBT bricks in parallel with 3 straight bus bars. All recycled components from a DC link inverter.



Designed staccato PCB as the old layout used in my VTTC I was on a veroboard. A optical output was added to use the interuptor with a standard DRSSTC driver.

Started construction of the secondary coil.

2nd November 2011
Etched PCB board for staccato controller.



Finished winding the secondary coil. It was made with a total of 1500 turns of 0.2 mm wire and dimensions 160 x 330 mm. Varnished the secondary coil with polyurethane varnish.



3rd November 2011
The secondary coil was given a second thick layer of varnish, not the most pretty job as I tried to pour as much varnish on as possible and let it rotate and settle it around the coil by itself. As the secondary coil was not completely in level it result in a little running, but overall a fair result of adding a lot of thin varnish.

Finished assembly of the staccato controller and bench tested it.



The project got shelved due to starting on a new job, after a long rest of over 3 years the box with parts was once again brought out in the light and construction could continue.

7th March 2015
Etched driver PCB and started populating the board with all passive components.

6th October 2015
Construction of a very cheap MMC from capacitors that was bought from a 10$ ebay auction for 100x Kemet R474N247000A1K, rated for 900VDC 28 A peak and 4 A rms. A easy and uniform construction, with current sharing in mind, is to construct it around a round piece of wood or plastic tube.

The resulting MMC has a capacitance of 0.047 uF at 9000 VDC, 280 A peak and 40 A rms rating. Which is spot on for this coil to be running with design goal primary inductance of around 100 uH, 300 A peak, 5000 uS on-time and maximum 10 BPS.




11th October 2015
Construction of acrylic primary supports, that has 21 slots and is formed for a up-side down U shape primary coil. A way of getting a large primary inductance and still maintain a certain distance to the topload as the secondary coil is very short.

The supports are made by hand using a saw, file and drill press.



16th October 2015
Getting the coil winded from the inside and out was no easy task, the whole large roll of copper tubing is heavy, easily bends too sharp and is like a spring. It will lock itself in the wrong slots and it can be a very frustrating piece of work. The complete result is however worth the effort, it looks smooth and even.

As the slots was not made to snap the copper tube in, from shear fear of cracking the acrylic, I made a small hole behind each slot that made it possible to tie each turn at each primary support, with a little piece of copper wire it is secured from deforming the coil.





As water cooling of the primary coil is going to be a must with the long on-times, simple clamps was made from copper sheet and two screws the fastens the 4 braided copper flexible wires to the tubing. The same 4 braided copper wires was soldered to the MMC terminals, as even as possible distributed around the circular copper wire terminal.

Having made the MMC on a wooden stick makes it easy to mount with two wooden blocks with holes in for the extra length of the round rod.



29th December 2015
Driver board populated with all active components. 24 VDC regulator is left out as this will be supplied externally from a 26 VDC, 8 A power supply. All traces related to the 24 VDC is reinforced by soldering a 0.5 mm2 copper wire along them. Four 2200 uF 35 VDC capacitors was added to the underside of the board, one at each N- and P-channel MOSFET. All output stage MOSFETs have heat sinks mouted.

All these precautions are hopefully enough to ensure no under voltage or over heating situation is possible when running at 5000 uS on-time.

25th July 2016
The coil have been put together with power supply, bridge, driver, platforms, secondary and topload. Two fuse holders for large bussman fuses was used at the end of the flexible copper braids for primary tap.



7th October 2016
First test run where a normal interruptor and the staccato for synced mains was tested. Blue and yellow is the top and bottom gates of the half bridge. The first oscilloscope shots are from the normal interruptor, showing find rise and fall items.



The next images shows the gate waveform when using the staccato interruptor, for very long ontimes and it can be seen that the power 200 Watt power supply can keep the gates running.



Now here is where it gets a little weird and where I am stuck at today, the coil refuses to oscillate, despite having tried many different setup changes, tapping the primary for only half its value, adding voltage splitters to the half brigde or just run it with one end of primary circuit to negative rail.

yellow trace = voltage
blue trace = current

With interruptor turned off, there is no driving signal to the driver


With interruptor turned on and at 300uS ontime

[Max]

Hello Mads,


Nice coil! Athough it's only a detail, I like your simple yet effective primary tap.

I guess the current curves of your last pics are the primary current. But the voltage? Is that the bus voltage?

Anyway, there are a few points which are not clear to me or might pose problems:

• Of course you have no bus capacitors, as they wouldn't make any sense here. So your primary current is directly drawn from mains/variac - which is surely not designed for low inductance... The variac will probably limit the primary current and in that way prevent the coil from any reasonable output. If not, won't you get heavy inductive spikes in your grid at home (possibly damaging other devices)?
• At which voltage does your interrupter enable the coil? The UD needs from the first moment on a sufficient feedback - otherwise it won't work. But you probably know this
• How does a halfbridge work without capacitors? One leg of the primary circuit to the halfbridge and the other one?

Kind regards,
Max

[Mads Barnkob]

Hello Mads,


Nice coil! Athough it's only a detail, I like your simple yet effective primary tap.

I guess the current curves of your last pics are the primary current. But the voltage? Is that the bus voltage?

Anyway, there are a few points which are not clear to me or might pose problems:

• Of course you have no bus capacitors, as they wouldn't make any sense here. So your primary current is directly drawn from mains/variac - which is surely not designed for low inductance... The variac will probably limit the primary current and in that way prevent the coil from any reasonable output. If not, won't you get heavy inductive spikes in your grid at home (possibly damaging other devices)?
• At which voltage does your interrupter enable the coil? The UD needs from the first moment on a sufficient feedback - otherwise it won't work. But you probably know this
• How does a halfbridge work without capacitors? One leg of the primary circuit to the halfbridge and the other one?

Kind regards,
Max

The voltage is the output voltage from the inverter, so it is the voltage across the primary circuit.

1. It would draw a high unsymmetrical current and it is properly one of the worst ideas ever The variac is however not even pegging or making any sound, no current is drawn.
2. This is most likely the course, no proper start oscillation, the interrupter is on from a detectable rising edge of the 50 Hz mains, and remains on for the on-time setting, 0 to 5000 uS.
3. A half bridge without capacitors can have LC circuit connected in the middle and then tied to negative rail. I tried both with and without splitting capacitors, so that is likely not the problem.

[Max]

Hello again,


1. Okay. I'm curious which current the variac will allow the coil to draw once it runs.
2. My coil needs about 10V to start to oscillate.
3. You're right, sorry. Didn't think well enough about that 


Kind regards,
Max

[Mads Barnkob]

I just remembered that I did try to add 6000 uF on the DC bus to try to run it as a regular DRSSTC, which did not give any different results than those shown in the last two waveform oscillocope screenshots.

I think my next step is to take the coil apart and test the IGBT bricks, the drive signals look fine at the gates, but I will have to see if they actually conduct through the C-E junction when driven.

I could also take a driver out of one of my other coils and cross check if its a problem with the driver, phase lead drivers can have bigger problems starting up than the UD1.3 drivers.

[Steve Ward]

Nice work so far.

Can you run the system off the signal generator to check the IGBTs out?  Just keep the voltage low, and keep the drive frequency above resonance so you never have body diode recovery.

What turns ratio did you use for the current feedback?  Perhaps it would help to reduce the turns ratio so you have a higher loop gain, it should make it easier to start oscillating.

It looks like you get some ringing current on the primary (which is a good sign), but is the amplitude significant?  And is the frequency right?

[Mads Barnkob]

Nice work so far.

Can you run the system off the signal generator to check the IGBTs out?  Just keep the voltage low, and keep the drive frequency above resonance so you never have body diode recovery.

What turns ratio did you use for the current feedback?  Perhaps it would help to reduce the turns ratio so you have a higher loop gain, it should make it easier to start oscillating.

It looks like you get some ringing current on the primary (which is a good sign), but is the amplitude significant?  And is the frequency right?

I will try to run the bridge directly from a signal generator and see what I can get from primary current/inverter voltage.

CTs are regular 1:33:33, I could try to wind a new one, something like 1:20:20 maybe? Or do you suggest even higher gain?

I did not note down how much it was, but I am pretty sure that the primary current measured on the scope shots are only in the 10's of Ampere range.

[loneoceans]

Nice job on the build. This idea was exactly what prompted me to build my ramped SSTC http://loneoceans.com/labs/sstc3/ a couple years ago and the results were quite pleasant, achieve cute small ‘QCW-like’ sparks about 18” or so. The reason for not making it a full DRSSTC was in fact to ensure that I -wouldn’t- draw too much current since I would be running it from simple rectified mains.



As for long pulse operation on the UD2, I typically run mine via 24VDC input. In my QCW1 which uses a UD2.7, as you have found (and since you are driving 3 half bridges), the gate drive uses a lot of power. The solution was to simply add a large capacitance on the 24V rail, in my case just a 5600uF cap I had lying around.

Another thing which I think will be fun to study would be your 100kHz intended operating frequency. In tests myself and others have done, operating the coil <300kHz or so seems to lead to less and less straight sparks, so it may become difficult to achieve the ‘sword sparks’ the QCW coils do.

Some people who used some of my leftover boards also achieved some pretty spectacular results: http://loneoceans.com/labs/sales/sstc4/ (scroll down to community builds)

Finally, you’ll need a small amount of ‘startup’ capacitance to allow the coil to begin oscillation, e.g. maybe a 1uF cap. In addition, you’ll likely want to reduce your CT from 1:33x33 to something like 1:50. I like to aim for about 1A of peak current across the burden resistor of the UD2 feedback. 

[Mads Barnkob]

I reinforced the UD2.1 driver with solid copper wire rails for the output stage, a 3300uF capacitor underneath each MOSFET pair, heat sinks on the MOSFETs and the 200 Watt 26VDC power supply for it. It is properly able to drive a bridge directly by now

I was forced to use a low resonant frequency, as I was using brick IGBTs, that is the only reason. But yes, I do expect a lot of branching.

The small capacitance for start up, is that DC bus capacitance? So that a snubber capacitor across the DC bus would be sufficient for that?

I will wind a new 1:50 CT for the feedback and adjust resistor accordingly, that sounds like a easy and good first step to take before taking much more apart for single component checks.

[loneoceans]

I reinforced the UD2.1 driver with solid copper wire rails for the output stage, a 3300uF capacitor underneath each MOSFET pair, heat sinks on the MOSFETs and the 200 Watt 26VDC power supply for it. It is properly able to drive a bridge directly by now

I was forced to use a low resonant frequency, as I was using brick IGBTs, that is the only reason. But yes, I do expect a lot of branching.

The small capacitance for start up, is that DC bus capacitance? So that a snubber capacitor across the DC bus would be sufficient for that?

I will wind a new 1:50 CT for the feedback and adjust resistor accordingly, that sounds like a easy and good first step to take before taking much more apart for single component checks.

Sounds like a plan ^_^. In the naive version of my RSSTC, a small capacitor was necessary since I simply triggered near the start of rising mains line when the voltage was very low. You could also try starting up with an offset from the actual zero voltage crossing. However it seemed easy just to add some small capacitance (which if you have the typical 'snubbers' across the bus on the IGBT) should be sufficient. This is due to the fact that the UD2.x requires feedback to work and doesn't have it's own starting frequency driver. So you'll need an initial kick to the primary resonant system to generate a sufficiently large signal for feedback. Likewise having a low-ratio CT helps in a stronger feedback signal especially since the start of the ramp will have lower primary currents. Good luck!

[Mads Barnkob]

The only capacitance on the DC bus was a 0.68 uF snubber capacitor, which was a lot less than I thought I had put on these. This was increased to a 10 uF capacitor bank of MKP film capacitors, one that I initially made as a voltage splitter for the half-bridge, but later left out during the fault finding.

First light was achieved with some 20 cm long sparks, remember that the coil is far from tuned for maximum performance and the input voltage was only 200 VAC.

So far the prototype has worked and shown that the concept works.

The spark formation is more straight than first anticipated, as most QCW coils operate above 300-400 kHz to get long sword like sparks. It is however clear that the sparks produced by this coil is swirling a lot. I do think that it will branch much more at higher voltages.

Proper tuning and more testing is needed!

As you can guess from the video, I am happy

[Mads Barnkob]

First some more pictures from the bridge, driver, IGBTs and various connections between the parts.


I made a wide range of secondary and primary circuit measurements to find the resonant frequencies, as the U-shaped primary makes it difficult to simulate with tools like JavaTC.

The measurements was done with secondary in place inside the primary. But secondary ground was unconnected and primary circuit was left open loop by removing the tapping point. I am not sure if this is the correct method, as resonant frequencies are much different when measured with secondary ground or primary loop closed, this is because energy is then transferred between the two resonant systems. The results could however vary with 10-20 kHz compared to the open loop measurements...

Secondary circuit test results
Setup with a 80 cm long wire with 3 bend wires hanging over and pointing down to be "branches"
Signal from signal generator connected to ground terminal on the secondary coil, ground left floating.
Signal into oscilloscope captured from open loop probe hanging next to secondary coil.


Unloaded result: 101 kHz


80 cm wire result: 91 kHz


80 cm wire with branches result: 88 kHz


Primary circuit test results
Setup with signal generator and oscilloscope connected across the primary LC circuit and with a jumper across the IGBTs to have a closed loop. Signal generator is connected through a 10K resistor.

Primary resonance with secondary ungrounded - 7th turn from bottom 102kHz


Primary resonance with secondary ungrounded - 6th turn from bottom 96kHz


Primary resonance with secondary ungrounded - 5th turn from bottom 90kHz


Primary resonance with secondary ungrounded - 4th turn from bottom 86kHz


Rest of the measurement results did not have individually saved oscilloscope shots, so here is a overview of the primary tapping frequencies.


I recorded video from oscilloscope and spark formation (dark dark video, blerg, sorry). There are 4 tests where primary is tapped at 96 kHz, 90 kHz, 86 kHz and last at 65 kHz, where it for reasons I still do not fully understand, performed the best! This is a huge detuning compared to the loaded 88 kHz secondary measurements. I also tried all the taps between 86 kHz and 65 kHz, with only increasing performance until I could detune it no further.

The staccato interrupter is not particular stable and does not really give a good clean 5ms on-time, it starts conducting before the zero crossing, possibly due to non-adjusted phase correction on the driver, this will get looked into next time its running. Waveforms are highly distorted, peak currents are low in the magnitude of 50 A peak. (Blue 100V/div inverter output - Yellow 100A/div current - 5-6 ms on-time)


and also some close up pictures of the beautiful sparks, still much shorter than what I expected, but maybe the very high impedance primary circuit just limits the current way too much, future experiments would be with a step up transformer for a higher primary peak voltage.

[futurist]

Nice results so far

High primary impedance could be the only thing limiting performance, check this coil

/>4 turns primary and higher primary tank (if the original onetesla's is used)

[Uspring]

Hi Mads,

like futurist says, high primary inductance might be limiting primary current. A large inductance is by itself not a problem, since you can get primary current up again by detuning (as long as you have taps for this). A large inductance implies many turns, though, and copper losses for a given current will be larger.

Greets, Udo

[Mads Barnkob]

A 21 turn primary of 8 mm tube, I think its 0.5 mm wall, is 26 mm^2 and with 15 meters of that, the resistance is 0.01 Ohm.
The inductance of this helical coil is 61 uH, resonant tank is 50 nF, this gives me a primary impedance of 33 Ohm at 100 kHz.

The small coil in the youtube video, he listed the specifications in another video.

/>
15 turn primary, #14, higher resistance than my tubing, it got an inductance of 41uH, 7.5 nF resonant tank which gives a primary impedance of 74 Ohm at 350 kHz.

So when comparing those two, I do not see where I am wrong

I think I will dig out a step up transformer and see what a higher bus voltage will do, fiddle with zero crossing phase lead, to see if its the driver or the staccato that is off.

Or maybe I have just not had any luck with tuning it yet? Maybe the coupling is not high enough?

[Uspring]

Do you know, how much the coil in the video was detuned? You did see the best performance at the largest inductance tap. It still might improve beyond. I'd test with a somewhat larger MMC.

[Mads Barnkob]

Do you know, how much the coil in the video was detuned? You did see the best performance at the largest inductance tap. It still might improve beyond. I'd test with a somewhat larger MMC.

My coil was tuned at 65 kHz and my best guess at loaded secondary is 88 kHz, the small one in the video I linked in last post, he wrote "Oscillating at Upper Pole (365kHz - 352kHz)".

I got a old MMC I could drag out, 8 parallel strings of 6 in series CDE 942C capacitors, I can quickly mod it to be half of that for 0.1 uF.

I just had another thought, I am not sure how to verify it without just plugging the coil straight in the wall, but could the inductance of my variac be the limiting factor? It is not really clucking hard from abrupt current, so I do not think that's it, but wanted to mention it...

[futurist]

Check the videos, the one I posted shows different coil, mains ramped DRSSTC vs async. buck QCW DRSSTC you posted

[Mads Barnkob]

From last nights experiments, I think that this idea might work on a small scale as demonstrated by the modified oneTesla coil.

I tried tuning the coil at 120 kHz and 130 kHz, way above the estimated loaded secondary Fres of 88 kHz. When I ran the coil and zoomed in on the current waveform, it was oscillating at 175 kHz, I am not even sure what is going on anymore

Nevertheless, it performed better than ever before at 120 kHz tap, which properly makes a little sense compared to the better performance at 65 kHz, so maybe it would go nuts at 175 kHz? It certainly does seem to be a harmonics pattern here. But I do not think I can tap it that far down on the inner side of the primary coil right now.

There was however also much higher current draw, loud clunks from the variac and lights dimming! The voltage spikes on the mains supply are at levels where my voltmeter was damaged in my variac. This is also why I call quit on the project as it is, its future will be rebuilding it to a conventional, properly PWM controlled, QCW.

I had sparks fly out to about 50 cm as it can be seen in the video

/>

DO NOT REPLICATE THIS PROJECT, HARM TO NEARBY ELECTRONICS IS A HIGH RISK

[Uspring]

I'm sorry to hear about the demise of the project.

I tried tuning the coil at 120 kHz and 130 kHz, way above the estimated loaded secondary Fres of 88 kHz. When I ran the coil and zoomed in on the current waveform, it was oscillating at 175 kHz, I am not even sure what is going on anymore

Given enough coupling that is what you'd expect. The problem with running at the upper pole is, that arc loading will move the secondary res frequency away from the operating frequency thus increasing primary current, which will lengthen the arc. That is a positive feedback situation. Running at the lower pole the feedback will be negative, stabilising the primary current.
I don't think the variacs inductance plays a role as long as you have a large enough cap on the DC bus. Large enough means, that the voltage drop on the DC bus cap due to the primary current (i.e. using primary current frequency and amplitude) is much less than the bus voltage.