Author Topic: Portable Q(uarantine)CW Tesla Coil  (Read 941 times)

Offline Weston

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Portable Q(uarantine)CW Tesla Coil
« on: April 05, 2020, 10:47:07 PM »
I have been sporadically working on a new QCW tesla coil over the past few months, now with the lockdown I have been making more progress and have decided to make a project thread for it.

The goal is a portable QCW tesla coil with something like 2'-3' long sparks. This is building off my earlier work on my "CuteQCW" and largely shares the same FPGA based controller architecture https://highvoltageforum.net/index.php?topic=587.0

My original plan was to have the coil be mounted on the front of a bike to bring to burning man. Due to storage constraints and the fact I found some cheap electric chainsaws with 120V batteries when looking for a power source for the coil I am now planning on mounting this on the chainsaw frame and having the secondary replace the chainsaw blade.

Due to wanting to mount the entire coil on a bike/chainsaw the electronics are designed to be compact. All of the electronics except for the MMC fit in a 4"x8" footprint, which was what I measured I could fit inside the frame of a bike and still have my legs clear while peddling.

The architecture is a CCM boost converter off a 120V battery pack to supply a bus voltage of ~450V. The power stage is a single SiC MOSFET fullbridge module, F423MR12W1M1B11BOMA1. Its a 1200V 50A module and switches fast enough where I can do phase shift modulation for a full ~20ms pulse and have sufficient thermal margin. Gate drive is provided with isolated DC/DC converters and isolated gate drive ICs, which provides a bit faster switching and allows me finer control over the switching patterns than would be achievable with a GDT.

The github repo is here: https://github.com/westonb/biwheel-coil

Right now I have the boost converter working in constant off time peak current control, which I chose for its simplicity. It also allows me to easily set the maximum current draw from the battery which is the real constraint in charging the bus capacitor. I do not intend for the boost converter and the QCW power stage to run at the same time. They share a single ADC for the boost current mode control and the QCW over current protection.

The QCW driver is based on a digital PLL and seems to be working pretty well with digitally controllable phase lead. Here is a ~1.4ms current ramp during testing, driving a RLC load at low power on the testbench. You can see


Here is a photo of the driver electronics during assembly from a few months ago. You can see the bulk energy storage capacitor sits directly over the control electronics to save space.


Bringup of the verilog modules for the boost converter and QCW driver are mostly done, right now I am mostly working on integration of everything. All the modules are sequenced by a RISC-V softcore which I can program in C and the coil is controlled by a fiberoptic link. For a portable / handheld coil I am going to have to make some sort of control board. 

In the next two weeks or so I hope to have integration of the verilog modules done and then I can move on to some more testing. Right now I am using my MMC from my old coil and a roller inductor to emulate the tesla coil. It might be hard to test with the secondary due to the lockdown but I am sure I can figure something out. I have only brought the power stage up to ~60V but all the waveforms seem to be good. I am really hoping I don't blow up anything because it might be a bit difficult to assemble a second board during lockdown.




Offline davekni

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #1 on: April 06, 2020, 12:43:50 AM »
I like the overlapping copper sheets for your bulk capacitor!  That's a great low-inductance design, better than side-by-side bus bars.
David Knierim

Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #2 on: May 02, 2020, 10:36:44 AM »
It took a week longer than I planned but I finished all the verilog. I ended up not moving everything to a AXI4-lite bus because of the added complexity.

Due to timing constraints the RISC-V softcore is running ay 80MHz while the control state machines and all the other peripherals run at 240MHz. I have realized I never should be buying the lower speed grade FPGAs, the extra time spent trying to resolve possible timing issues (and extra time having the synthesis tool trying to route the FPGA with worse timing constraints) is not worth the marginal dollars saved.

Today I got my first sparks with the old secondary / MMC from my previous QCW coil. Small sparks, but sparks never the less! Attached photo did not capture the spark at the full size, but its not that much bigger. A fair amount more work until I am pumping out 3'+ sparks like the previous coil, but progress never the less.

I am having an issue with EMI crashing the FPGA. I am not sure what exactly is causing it but I suspect its related to the programmer remaining connected. I am going to remove the un-needed test leads I had for debug and load the bitstream on the flash memory later this weekend and see if that helps. For a future design I think I am going to try to put all the control electronics under a shield can.

On the theory side of things, I am seeing almost a constant current as I ramp my phase shift after the coil has breakout, its almost acting as a current source. I seem to remember discussion about this before? It also matches what I saw with my previous coil.  It does not match what Loneoceans saw in his QCW coil https://www.loneoceans.com/labs/qcw15/ https://www.loneoceans.com/labs/qcw15/23MayFirstLight.png . That coil used a buck converter modulator, but that should not impact it.






Offline Uspring

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #3 on: May 02, 2020, 04:31:23 PM »
A very neat little coil  ;D
The shape of the primary current envelope depends much on the tuning. There are 2 opposing effects: If the driving frequency is very different from the secondary resonance frequency, the lowering of secondary Q due to the larger arc loading later in the burst will widen the resonance curve and increase the power transfer to the secondary. That will keep primary current low.
If driving frequency and secondary res frequency are well matched, then the dominant effect of larger arc loading will be the reduction of secondary current. That will decrease power transfer to the secondary. Both effects are captured in the equation:

Qpri = (Qsec/k^2) * (1 - f^2/fsec^2)^2 + 1/(k^2 * Qsec)

Qpri describes, how far primary current will rise for a given bridge input voltage. Depending on whether f is near fsec or not, the primary Q will either rise or drop with dropping Qsec. I suspect, that you're running the coil a bit distant from fsec (=secondary res frequency). How does your feedback work? If I understand this right, you are hard switching the primary for power modulation. The current zero crossings, which define the pole frequencies, might be modified.

« Last Edit: May 02, 2020, 04:44:48 PM by Uspring »

Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #4 on: May 02, 2020, 11:34:04 PM »
I ran some more tests today. I added some shielding and loaded the bitstream on to the configuration flash so I can disconnect the jtag programmer. No more crashes! With that I have brought things up to 90V input and got a somewhat bigger spark. Its hard to take these spark pictures, I need to set up a camera tripod...



90V is the limit of my two lab power supplies in series. For higher power I am going to have to enable the boost converter on the input. This will require writing a command parser so I can set the bus voltage over the fiber optic serial link. Eventually I am going to be running the entire coil of a 120V lithium-ion chainsaw battery.

Beyond that, everything seems to be running decently smooth. Here is a scope capture of one of the pulses with a 90V bus voltage:



I am not getting a monotonic ramp current because I a slightly oscillating around the resonant point. A lower Q with larger sparks / higher power may dampen this, otherwise I am going to have to tweak my loop filter. The high peak current at the beginning before breakout is also a bit concerning. I probably have the thermal margin for it, but I should be able to eliminate this by starting sufficiently above resonance and then backing down once things stabilize.

Looking at a close up of the switching waveforms it seems like I might want to reduce my phase lead by ~100ns or so, better to switch too early than too late though. The ringing looks a bit ugly, but I think a lot of that is due to my probing technique, a switching transition on one node should not induce that much ringing on the other node.



My peak primary current is already at ~32 amps for a 90V bus voltage. It seems to be rising very sub-linearly with bus voltage (Almost the same at 60V, ~22A at 30V) but it looks like I might need to reduce my MMC capacitance and use a higher turn count primary for the final primary/secondary configuration. I can push up to a 500V bus voltage and want to keep peak primary current under 60A or so. The SiC brick I am using (datasheet: https://www.infineon.com/dgdl/Infineon-F4-23MR12W1M1_B11-DS-v02_00-EN.pdf?fileId=5546d462689a790c01690e9d9fb63802 ) has very low switching loss so I should have a large thermal margin, but I really don't want to blow this up. BOM cost was high and its all integrated, so one failure and the whole PCB is toast.

Uspring:
Thanks for providing the explanation. Is "f" the primary resonant frequency? I am running on the upper pole with the secondary tuned above the primary, so based on that formula spark loading should pull things into better tune and reduce the primary current. This primary / secondary + MMC is from my previous QCW coil which was detuned a fair amount to account for loading from ~3' sparks.

My driver uses a digital PLL which allows me to lock on to the upper pole when primary is tuned below the secondary if I supply a small number of fixed frequency starting cycles to energize the correct pole.

One side of the bridge switches on the zero crossings of the tank current with an adjustable phase lead. The other side switches with an adjustable phase offset (aligned such that I get inductive loading on the hard switched leg). This configuration does mean that the effective phase of the applied voltage and the current varies as I sweep the phase offset.
« Last Edit: May 03, 2020, 01:30:39 AM by Weston »

Offline Uspring

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #5 on: May 03, 2020, 05:29:44 PM »
I am sorry about the missing explanation of the equations terms. f is the bridge frequency and fres the one of secondary resonance.
Since you are running at low power, Qsec is probably large, making the first term of the sum

Qpri = (Qsec/k^2) * (1 - f^2/fsec^2)^2 + 1/(k^2 * Qsec)

dominant even if tuning is good, i.e. f near fsec. As you increase bus voltage, the increase in arc power will lower Qsec and possibly keep the primary current at bay.

Another maybe minor issue is the effective phase difference between primary voltage and current you mention. Since you are on the inductive side, the bridge frequency f might be a bit higher than the upper pole frequency. The difference f - fupperpole is probably small due to the high Qpri, but it increases f/fsec since f > fupperpole > fsec. That causes somewhat high primary currents than expected, when the bridges are phased for low power output. At max power, i.e. 180 degrees between your bridge outputs you'll be back to f=fupperpole, though.

Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #6 on: May 04, 2020, 10:10:57 AM »
Ah, and for a normal coil the bridge frequency is the frequency of the dominant pole. I have been delaying but I am going to have to go through and derive some of the equations myself. And also possibly figure out a more optimal ramp pattern than a linear phase ramp. The relation between power delivered and phase is not linear.

If Lone Ocean's write up is accurate, the waveform of his linked is with a different tuning, most likely  f_pri > f_sec, driving the upper pole. It makes sense that the difference in tuning would account for this behavior, I am just surprised to see such effects in my own waveforms with such small sparks. Perhaps I am going to be able to stay under ~60A or so at full bus voltage. Its interesting to think about how a different driver can allow you to utilize your switches a lot more effectively. Anyone have any estimates of loaded secondary Q? I might try to calculate the difference in switch utilization you get for different tuning conditions.


In project log land, I spent a long time this weekend battling with software libraries and trying to build toolchains from source. The FPGA has everything sequenced by a RISC-V softcore (its open source! https://github.com/cliffordwolf/picorv32 ) and all the tesla coil control stuff connected as memory mapped peripherals. This is how I structured my previous tesla coil driver and also a setup I have been using for some of my research projects. In the past, library support has been pretty much non-existent, which makes basic stuff that would be accomplished by the C standard library pretty painful. Now that I want to have a full serial command interface I need string parsing, which is implemented in the C standard libraries.

My project is now set up to use picolibc ( https://github.com/keith-packard/picolibc ) which provides a light weight implementation of the C standard library for embedded devices. To support an system for basic input/output commands you only need to provide basic putchar and getchar functions, which is nice. Getting it to build was another issue and took up most of my day. I had to update my compiler to a more recent version of the GCC RISC-V toolchain to build the library. I was having issues getting the library to build with the riscv-gnu-toolchain that I was building from source. It turned out to be something related to the compiler build process producing a compiler with issues and switching to the toolchain provided by SiFive worked. Apparently RISC-V support is now in the mainline GCC releases, so I might investigate that in the future.

Now that I have basic library support for the processor it should not be that difficult to write a serial parser and control the boost converter over the fiber link to do higher power bringup. I am hoping to have that done and do higher power tests by next weekend, I might have to order an extension cord to do testing in courtyard of my apartment complex, the current setup is in my apartment bedroom and its pretty crowded.

No project update is complete without a photo, so here is a photo of my current test setup. Its a bit tight, this photo guest stars my bathroom towel and the kitchen chair:




Offline Uspring

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #7 on: May 04, 2020, 05:18:46 PM »
It looks like Loneoceans had fpri > fsec on his first version, but later fpri < fsec on the "Fat toroid" model. That can, as you say, account for the different behaviour. Loneoceans has quite long arcs as compared to the toroid size and that implies markedly changing tuning conditions during the burst. If you want to keep primary currents small it is important, that secondary resonance stays above primary resonance. If this is not the case the upper pole frequency won't follow the secondary res as it is going down. A short illustration: (fupperpole is also the running frequency f)

---flowerpole----fpri-----fsec----fupperpole----      ; initially
---flowerpole----fsec-----fpri----fupperpole----      ; medium power
---flowerpole----fsec-------------fpri----fupperpole----      ; high power

Better is:

---flowerpole----fpri---------------fsec----fupperpole----      ; initially
---flowerpole----fpri-------fsec----fupperpole----      ; medium power
---flowerpole----fpri-fsec----fupperpole----      ; high power

It is a bit weird, but in upper pole operation, with the primary tuned low, it does not really matter at which frequency the primary tank resonates, as long as it is below the secondary fres.

My best guess for Qsec comes from Wards top voltage measurements with his sword sparc QCWs. Typical voltages were around 70 kV peak and only weakly dependent on length of the arc. You can derive a ballpark arc loading resistance by using your power level and this voltage value.
« Last Edit: May 04, 2020, 06:49:46 PM by Uspring »

Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #8 on: May 10, 2020, 09:32:43 AM »
Time for a project update!

I am planning on deriving the tesla coil transfer function and seeing how that behaves with a load that clamps the voltage, which should help me optimize my new secondary / primary. However, I have been putting that off as I can always work on theory while waiting for new parts / PCBs if I blow something up  ::)

In order of progress, I:

Changed the picorv32 soft core configuration / makefile from RV32I to RV32IM instruction set (soft core now supports multiply and divide instructions!)

Wrote a serial parser that can support arbitrary commands and values, its loosely based on the SCPI command syntax set used for test equipment. This allows me to call functions / set register values for things like burst length and boost converter target voltage from my laptop, which is really helpful for bringup, it allows me to test things without having to generate a whole new FPGA bitstream. This it the first time I have had arbitrary bidirectional communication with the FPGA, exciting! I should be able to backport this setup to my PhD research projects which will be helpful in the future.

Brought the boost converter up to 300V output and made some tweaks. The boost converter runs in constant of time peak current control because that was the quickest thing to write a controller for in verilog. I fixed a minor issue with my minimum on time being too low, causing runt pulses. Related to this and also fixed was issue with inductor current exceeding my set point during start up when Vin is close to Vout. This was caused by the minimum on time and fixed off time, I fixed it by keeping the FET off until the current goes below the limit.

With constant off time I enter discontinuous mode at higher conversion ratios, shown here (yellow gate drive, orange inductor current, green switch node voltage, blue output voltage):


I might want to keep things at critical conduction mode to reduce switching loss, but this constant off time control is leading to poor utilization of my semiconductors / inductor and causing more ripple on the DC input. Its not that critical now as I am not trying to achieve high BPS at this point in the bring up, but at some point I want to rewrite the controller to operate in CrCM or CCM. The inductor current is sampled with the same ADC I use for over current protection (80MSPS) so I have a lot of control flexibility.

This bringup also allowed me to verify the switching waveforms of the boost conveter. I plan a maximum DC bus voltage of 500V and the boost converter uses a 650V super junction FET. The To-247 leads are at their maximum length due to the standoff from the heatsink imposed by the SiC MOSFET module so I was a bit worried about stray inductance and voltage spikes during switching. Scoping things out shows minimal ringing during turn off (Yellow: gate, orange: inductor current, green: drain):



I also brought up the coil to 300V DC bus voltage. Large sparks are damping the secondary Q and eliminating oscillation of the PLL loop filter, but the discrete nature of the oscillator is still evident. I have a 240MHz clock frequency for the PLL module on the FPGA which gives me ~ 660Hz resolution at 400KHz. Looking at my current waveforms, it seems this is leading to quantization error which is showing up as slight steps in the current envelope. These are small so it does not seem like it would be worth spending effort to rectify, but its interesting to observe. I should be able to get a higher frequency resolution with dithering.



In bringing the coil up to 300V I have new longest sparks from the coil! I apologize for the blurry photo, I need to set up a tripod. This exact spark / photo also marks the point at which I realized that any higher power testing will need to be done outdoors:



Lastly, I also tested the coil with the battery for the first time, with everything powered directly from the 120V power tool battery. given the high current rating of the battery, I was a bit nervous plugging it in, but everything seems to run fine! This should make outdoor testing easier, I only need to run a power cord for grounding and the scope.

Now that the power electronics have mostly been brought up its time to focus more on repurposing the chainsaw enclosure to house the electronics, making a stand along interrupter type controller for the coil, and doing enough theory work to optimize a new primary / secondary for the coil.   

Also, as a question to the readers, does anyone have advice on grounding for a portable (hand held) coil? I have been connecting the secondary into mains ground and also have a few square feet of foil on the chair right now to act as a sort of counterpoise. I had the mains ground get disconnected during testing and the over current detection would trip (55A right now) and I would get a very small spark. I assume I will have more capacitance that a few sheets of foil and I can connect the ground to my shoes to hopefully get a ground return to whatever I am standing on, but it has me a bit worried. 

Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #9 on: May 16, 2020, 09:16:15 AM »
I have some more testing planned this weekend (which will test predictions of my recent modeling) so I figured I should make a pre-update.

This week I took apart one of the electric chainsaws I have and decided that it was going to be too much effort to modify the existing housing to fit my driver and the coil itself. I decided it would be easiest to 3d print one and bought the biggest/cheapest 3d printer I could find, the ANYCUBIC Chiron. It has a 400x400x450mm bed size so I should be able to print the enclosure in the minimal number of pieces (the original chainsaw plastic housing is ~18" long). I am also upgrading the hotend to the E3D volcano to get a faster extrusion rate and not have to wait forever for the prints to finish. I think it would be cool to print a housing out of translucent plastic and insert some LED strips. I am planning on having a ESP32 based controller board internal to the case with its own battery and connected over the fiber link (its really the only interface I exposed....) and some switches + a display for control. It should be pretty cool looking! I just need to decide what plasma/energy weapon from scifi/video games I want to base the enclosure design off of  ;D

My main achievement this week was spending a lot of time  working on tesla coil modeling. I really want to optimize the utilization of my power electronics and achieve maximum primary current at maximum bus voltage. Given that I only want to make one new primary/secondary, this is going to require an accurate model of the primary impedance under spark loading. My old primary / secondary used on my cuteQCW coil seems to have a good impedance match for keeping tank currents at ~ 50A or so, but that was really just luck. Further complicating this, I am considering using a ferrite core in my coil which javaTC can not model. I have been testing my coil with the primary/secondary from my cuteQCW system which serves as my reference point for testing my modeling. The modeling is split into two parts: accurately modeling the primary / secondary and accurately modeling spark loading.

For modeling the primary / secondary itself I am using FEMM to model the magnetics. I have wrote a script which creates my primary, secondary, and topload in FEMM using the octave scripting interface (the UI for manual entry is a pain)  https://github.com/westonb/biwheel-coil/blob/master/modeling/cuteqcw_secondary.m . I can directly verify the inductance of my primary generated from this model, can sort of verify the inductance of the secondary by measuring the resonant frequency, and measure the coupling between the two by looking at the measured frequency difference between the upper and lower poles. For the capacitance of the secondary I am using the Cl-DAE formula (which is derived from the medhurst self capacitance formula) from this very excellent paper: http://g3ynh.info/zdocs/magnetics/appendix/self_res/self-res.pdf and for the capacitance of the topload I am using the deep fried neon toroid capacitance calculator (I really need to find out where this formula comes from) http://deepfriedneon.com/tesla_f_calctoroid.html .

This is only for N=1, but so far my model seems to be more accurate than JavaTC. JavaTC predicts my secondary resonant frequency to be 301KHz, my modeling predicts it to be 285KHz and my measurements put the resonant frequency at 285.1KHz. That match is way closer than the uncertainty of the empirical  formulas for capacitance but hopefully I did not just get super lucky in this one case and its actually accurate! Java TC predicts my coupling at 0.433, FEMM predicts the coupling as 0.426 while fitting the rest of the modeled system to my measurements gives a coupling of 0.47. I believe this discrepancy is due to the self resonant effects of the secondary. When running close to a waveguide resonant mode the secondary winding current is non uniform. In the quarter wavelength resonant mode the current at the top drops to zero. Based on this, windings closer to the base are going to have a higher effective coupling factor. No easy way to incorporate this into my model but the effect should reduce with added capacitance from  larger sparks / topload which moves the secondary away from the quarter wavelength resonant mode.

One interesting thing from my modeling is that I can model the impact of the topload acting as a shorted turn. At least for my system (which has a relatively close topload) the topload has a relatively minor impact on the impedance and loss of the secondary. At 380KHz the secondary with the topload has an impedance of 148.569+I*52889.5 Ohms (thats a 22.15mH inductance) and the secondary without the topload has an impedance of  146.103+I*54592 Ohms (thats a  22.9mH inductance). In reality the effect should be even less due to the previously mentioned non-uniform current distribution. Here is a  plot from the FEMM model (which is axisymetric, hence the missing half) showing how the topload distorts the magnetic field when the secondary is excited with a current of 1A:



Once I had a decent model of the static tesla coil system I moved on to the modeling of the impedance of the system under spark discharge. Based on my previous measurements of relatively constant tank current and other peoples reports of the streamers clamping the secondary voltage to a ~ constant value I hypothesize that the primary + secondary acts as an immittance converter, which converts a voltage on one port (the secondary) to a current on the other port (the primary). As the spark grows and adds capacitance but the voltage (in theory) stays relatively constant the characteristic impedance of the immittance converter changes and leads to a change in primary current. With my current spark length it seems that Cspark << Ccoil+Ctopload so the characteristic impedance stays relatively constant, hence the constant current.

I tried modeling my coil with the spark model provided by Uspring here https://highvoltageforum.net/index.php?topic=1073.msg7715#msg7715 but it leads to signifigantly higher primary currents than I have measured (~70A vs ~43A measured). Based on the start of the pulse it seems that the initial topload clamping voltage (~38kv) leads to a correct initial current, but the streamer model adds too much capacitance / the topload voltage rises too much, leading to an increased current beyond what I observe. I made my own simple model that uses a voltage source to clamp the topload voltage to a constant value of ~35kv. For streamer capacitance I measured my coils spark loading by looking at the frequency difference between the start and end of a burst and calculating the change in capacitance that would be required to achieve that (~2pF for a ~2' spark). I ramp this capacitance from zero the sqrt of the modulation ramp. Its a predictive model of spark loading, not reactive, but it seems to give decent results. Here is the current envelope I showed previously for a 300V bus voltage and here is the model, it seems to be pretty close.





This weekend I am planning to test my coil outside for the first time and run it up to a bus voltage of ~450V to verify the power electronics work at full power and  collect more data to validate my (simple) spark model. I also modified the verilog for the controller of my boost converter to run it in something close to critical conduction mode and lead to faster charging of the bus capacitor. Once I have more data on spark impedance at this frequency I can work on designing a more compact primary / secondary for the chainsaw coil along with an enclosure.
« Last Edit: May 18, 2020, 05:05:04 AM by Weston »

Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #10 on: May 19, 2020, 07:55:54 AM »
This week I validated my boost convert up to 500V and tested the coil outside for the first time.

In validating the boost converter I found a minor (but potentially destructive) issue where the current sense was mux'ed away when the last boost switching cycle was still occurring, leading to a loss of current limiting. That was a relatively simple fix. I also added a switching threshold for turning the FET back on after inductor current drops to zero to keep the system in something like boundary conduction mode, which will allow for faster charging of the bus capacitor for a given peak inductor current.

After working on the boost converter I tested out a "burst" QCW mode where I had multiple pulses in quick succession and ended up destroying my nice DC load and almost destroyed the driver  >:( . The ADC on the FPGA that I was using to sense the bus voltage cut out and the boost converter did not shut down when it should have, eventually suppling something greater than 500V and dumping 500+j into the DC load, completely destroying it. It seems all the pass transistors in the load are blown and the event blew the DC load fuse and my breaker (which I am quite confused about). The DC load seems like a total write-off. On the bright side it protected my driver from a similar fate, which would have only been a little less expensive and a lot more difficult to replace. Here is the most impressively blown FET, but all 8 in the load suffered similar fates. My coil also killed a USB hub so it now has a 2/0 K/D ratio  ::)



I believe this failure was caused by leaving he JTAG programmer connected to the FPGA during testing. It was previously causing crashes when left connected at higher powers but I had not previously had issues at lower powers. I believe the previous crashes were due to the programmer cable picking up EMI which was interested as JTAG commands and causing general weirdness / resetting the FPGA (the FPGA is protected with a level shifter so I don't think it was EMI directly causing problems). The XADC on the FPGA I am using is also directly accessible from JTAG. I think an invalid JTAG command may have reset / halted the XADC and lead to the failure to regulate the boost converter output voltage. After this incident I have added some sanity checks on the ADC output, disabled JTAG access to the XADC once the bitstream is loaded, and I will no longer be testing anything with the programmer left connected.

Today I tested outside for the first time! The coil was running off the battery but I had to run an extension cord for my scope and some supplementary grounding. Results were decent, it looks like my longest spark is ~3' so far. I ran these tests at a ~10ms burst length and a 450V bus voltage. The peak tank current is 80A, which is a bit higher than I would like. Prior to testing I reran the switching loss figures and I should be able to run up to 80A peak tank current but it makes me a bit nervous, especially if I want to go up to 20ms. Also, it seems my OCD is off by ~ 10%. I swapped out the current transformer and need to adjust the gain.

Here are photos of a decent spark and the tank current envelope:

 




I also recorded some 20M point data sets I can look at to determine how much the coil detunes during operation and use that data to test the spark models.

The sparks are more branched than I would like, I think that is due to some combination of ramp rate / pulse time and weather. I am going to mess with ramp rate and go up to 15ms+ in the future.

Over all, this testing was a success! Limited by the rating of the DC bus capacitor, I can bring the coil up to 550V but I am limited by peak tank current right now. This new data will help me better design a secondary / primary for the final chainsaw tesla coil.
« Last Edit: May 19, 2020, 07:58:27 AM by Weston »

Offline SteveN87

  • High Voltage Enthusiast
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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #11 on: May 19, 2020, 02:08:20 PM »
Quote
I believe the previous crashes were due to the programmer cable picking up EMI...

I've seen a case where some FPGA-containing equipment failed EMC radiated immunity at 3V/m due to a 3.3V rail being gradually "pumped up" by a parasitic charge pump formed by a cable (connected at the equipment end only) and the external (and presumably internal) clamp diodes of an interface IC. A TVS on the 3.3V rail solved it.

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #11 on: May 19, 2020, 02:08:20 PM »

 


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