Author Topic: A three-phase "tree phase" Tesla coil  (Read 335 times)

Offline Steve Ward

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A three-phase "tree phase" Tesla coil
« on: June 27, 2020, 09:28:56 PM »
I've been on somewhat of a 3-phase kick in my career, focusing primarily on multi-phase permanent magnet motors and inverters. I had, long ago, seen a picture of a set of 3 SSTCs, by experimenter Duane Bylund:

http://www.duanesradios.info/html/tesla_coil_photos.html

It was 3 of these units, with their bridge switching synchronized and phase shifted, which is essentially the approach with my 3 phase machine. 

Cutting to the chase, you can watch a few videos of this little monster:


The power stage is 3 half-bridges.  With only a half-bridge feeding each Tesla coil through a "larger than resonant" (LTR) primary capacitor, power is controlled/modulated by  excitation frequency, particularly above the secondary resonance (upper pole freq).  By switching further above resonance, power is reduced, and switches of the mosfet type, operate essentially in zero-volt turn-off (ZVS) mode with low switching losses. This is because the mosfet can switch off quickly - limited mainly by gate driver - but the voltage rises as a function of the total output charge of the half-bridge and the current at the output during the transition.  So the voltage across the mosfet junction while it transitions from on to off is less than the total supply voltage, and switching losses are reduced.  The truth is, the switch off loss is still significant at the operating frequency range of 540khz to 375khz.

With the primary capacitor set "very much larger than resonant", each of the 3 coils looks electrically like the classic SSTC with a "dc blocking" capacitor on the primary side.  A SSTC self-oscillates at a frequency above the secondary resonance, just like a QCW-DRSSTC, at the "upper pole" frequency.  Because the "dc blocking" cap sets the primary side resonance to a frequency far below the operating frequency, its typically ignored, however the interaction of the mutual inductance and the secondary self inductance fields are still "fighting" each other so that the effective inductance is less than the secondary on its own and so the system oscillates at f>Fsec.   In my system the primary side cap is only marginally LTR so Fpri < Fsec, initially, but ideally Fpri = Fsec_loaded.  This condition, i think, minimizes stored energy (lowers Q) which is considered extra "cost" as it doesn't directly contribute to the energy in the spark.   In practice, Fpri < Fsec_loaded in my setup because I still wanted to limit the power as the inverter comes to switch near ZCS.  I find this is a practical compromise when its not easy to re-wind the primary with more turns to raise the impedance.  Also, keeping the primary a little detuned helps linearize the power vs frequency transfer function.  You can think of it as just partial "power factor correction" by nearly cancelling the primary leakage inductance with the series cap.

The first experiments utilized a semi-retired prototype from work which featured GanSystems 650V mosfets, 12 of their largest 25mOhm fets ganged up in parallel per switch location for ultra low Rds and reasonable charge losses for motor inverters (and softer switching for ZCS/ZVS tesla coil driving as the junction charge is relatively benign if not slightly helpful).  This early stage experimentation was just a hacked version of a motor controller I'm developing.  It was also lack luster performance because I was too shy to stress the inverter as it was particularly expensive in terms of part cost.   

After the GaN switches, I tried some of the new 4-lead TO-247 style Si MOSFETs from fairchild.  This showed a lot of promise up until its ultimate demise, of which im not sure what the cause actually was.  I suspect a bug in my controller allowed switching below the resonant (zero phase) frequency of the system, giving hard body diode recovery at voltage slew rates outside of the SOA triggering their prompt failure.  Disappointingly, all 3 half-bridges failed so energetically that they were a complete loss, recovering only the heatsinks for future use. 

I decided to move on with SiC devices instead, figuring they should be perfectly fine with body diode recovery if it happens, but also their lower gate charge makes switching them faster, more realistic.  Also, they are widely available in the 4-lead TO-247 design that i had my boards laid out for.  I chose the C3M0021120K devices, but if i were to re-do it, i would consider the of United SiC cascode mosfet, especially their new 650V parts in a TO-247-4L.  The switching losses seem to go up disproportionately for the 1200V devices, but its hard to resist the extra power density if you can still dissipate the heat. 

To help with heat removal, I've chosen a "live heatsink" design so that each device has 1/2 of a small CPU cooler directly mounted to it with a small amount of thermal grease, but no isolation pad which usually has 3-4X the thermal resistance as the internal resistance of a high-end 247 device.  Direct heatsinking alone should result in significantly lower junction-sink temperature difference.

Gate driving is handled by a simple boot-strap type driver.  Gate drive transformers simply have too much leakage inductance and also supply inappropriate negative drive voltage for Wide Band Gap (WBG) semiconductors like SiC or GaN (with some exceptions like cascode SiC which actually has a Si mosfet control input).  My design uses a 10A gate driver IC (NCP81074B) located immediately next to the G-S pins on the TO-247-4L fets.  The 10A driver ICs are fed isolated switching signals from yet another gate driver IC which has built in dead time generation (Si8231).  There exist better options for driver ICs and isolation if taking a fresh start, but the architecture would look mostly the same, locating the driver ICs as close as possible to the mosfets, which in my case required individual drivers for high and low side fets.

The switch timing is generated by a STM32F405 microcontroller, using its timer/counter peripheral which has 3 phase output capability.  The 3 square wave drive signals for the 3 half-bridges are phase shifted by 120 degrees (or close to it, depending on quantization limits of the timer/counters).  The primary current of each coil is monitored by a CT, checking the current amplitude with a ADC, and also checking the zero cross timing with a high speed comparator feeding into a timer-capture peripheral that is synchronized with the gate switching signal.  The MCU is constantly watching how much "phase margin" there is between mosfet switching and primary current zero crossing.  The goal is to never switch later than the current zero crossing, and ideally keep switchings slightly ahead of it so that switching losses are reduced by ZVS conditions.  While the MCU approach cannot keep up with every zero crossing at 500khz, it only has to "spot check" the phase margin every few cycles (at 20uS update period) because of the quasi-steady-state nature of the system.  That is to say, there should be no fast changes to track, and we can generally assume its "safe" to raise the driving frequency if necessary as it will reduce the power throughput and also give safe switching conditions (no body diode forced recovery). 

The remote control "ramp generator" is essentially commanding the operating frequency of the inverter.  Lowest power sets the frequency to 475khz, while the highest power setting is 375khz, provided the zero cross detector sees enough phase margin for near ZCS, and also that the ADC reports primary current within limits.  If the commanded frequency is "too low", the controller will just track the coil's natural oscillation frequency via zero cross measurements and slow adjustments of the timer/counter period.  As the loaded coil frequency drops, the primary tuning boosts the power, helping feed the sparks.  The ramp generator i use features a 5-point piece-wise linear waveform generator so that i can experiment with shaping the ramp function to optimize spark growth.  While this functionality proved useful for a single coil making long sword sparks, it doesn't really matter much for the chaotic, short sparks created by the 3-phase setup.

The start-up of the system works at a much higher frequency still, 540khz.  This is to gracefully bring the system into stable oscillations without seeing the "transient" response that would otherwise show up when driving closer to resonance, which generates too much voltage for the coils to stand.  For the first 400uS or so, the drive is sweeping from 540khz down to 475khz.  At 475khz plasma is just starting to form, which quickly brings down Qsec and brings stability to the system.  From here the ramp generator is setting the frequency command, and the zero cross detector is checking if theres enough phase margin to allow it (generally, if there is not, it means the spark load has not grown enough, but given time it will usually get there).

The machine has a built-in active rectifier (PFC).  The PFC includes a number of nice features.  Inrush/pre-charging is handled by the full-wave bridge rectifier common for a single-phase PFC.  The bridge rectifier uses SCRs for the two "upper" diodes feeding the positive voltage input to the boost circuit.  The SCRs are phase-angle controlled by the microcontroller (which is also watching AC line zero crossings).  Therefore, no external resistive/capacitive charging element is needed, and no extra relays/contactors are needed.  The pre-charging profile could be fine tuned to maximize charging current, but i just use a linear adjustment of phase firing angle over time until reaching full 180* conduction, at which point the PFC boost action can begin.  The precharge current pulses are on the order of 15A peak.

Since the PFC is controlled via MCU, it is easy to build in voltage setpoint (380V to 800VDC), which is a slider on my remote control box.  I decided not to bother with variable input current setting, however this would be easy to add.  Instead, if the MCU detects 120V line level, it sets the PFC max current to 15A RMS, but if 208/240V line is detected it allows up to about 57A RMS, or something over 13kW output from a very compact design weighing maybe 5lbs excluding the large output capacitors.  Of course, i can't claim it could reliably operate at this power level continuously, it probably would not be very reliable, but it seems to work OK for short term duty cycles.

PFC current regulation is also handled inside the MCU.  Once per PWM cycle (at 50khz) the line voltage is measured, the PFC boost current is measured, and a new duty cycle is calculated to maintain sinusoidal input current.  The voltage control loop, which drives the current setpoint, updates at every AC line zero crossing to avoid distorting the AC line current.  This is similar to what an analog PFC chip would do by low pass filtering the voltage loop to avoid any response at line frequency. 

Speaking of power... one significant issue was over-heating of the secondary coils.  Eventually i added an acrylic "chimney" around the coils and used 150mm fans to blow air up past the primary MMC, through the primary winding, along the secondary coil, and venting out of the toroid (which can also get warm!).  I have a few destroyed coils as the PVC form expands from the heat, stretches the copper wire and then retracts, leaving the wire loosely hanging on the form.  The fan cooling seems to handle this problem, just fine.

I plan on taking a deeper dive into more details of the design and operation of this machine, this was already getting to be a long introductory post.  Please note that some of the pictures are a little outdated with respect to current hardware (like, the half-bridges shown are my Si versions but the SiC version looks almost the same).

I also have plans to power this setup with a 444V lithium ion battery, capable of supplying enough current to run all 3 phases CW.  More to come...
« Last Edit: June 28, 2020, 12:05:55 AM by Steve Ward »

Online Hydron

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Re: A three-phase "tree phase" Tesla coil
« Reply #1 on: June 28, 2020, 12:57:53 PM »
Very interesting, thanks Steve. Looking forward to further info.

Regarding the heatsinking, did you make any measurements of the thermal performance of isolated vs non-isolated parts? I've gone a similar way in my coil design, using one of these guys per IGBT: https://uk.rs-online.com/web/p/heatsinks/0203599/ (will have forced air cooling, so ultimately will be substantially below 5K/W, and I'm spreading the heat over a number of devices). Can't remember the original inspiration for doing it, but may have been due to some ancient post from you complaining about the interface material actually.
I'm particularly interested in whether direct heatsink contact helps with the delta-T of the die during a burst - at 10-20ms I suspect that the case-heatsink thermal impedance starts to come into play (rather than just heating up the copper plate under the die); the transient thermal impedance graph is pretty close to the ultimate junction-case value in the 0.01s area for both the FGY75N60SMDs I have and the FGH60N60SMDs that seem to be the most popular TO-247 choice.

As for the gate drive, I'm surprised that bootstrap works so well - I was expecting separate supplies for each high-side device, which is a major pain once you have lots of them (and the main reason why I'm still using a GDT, albeit with somewhat heroic efforts to reduce leakage inductance). Have you got a separate 12V supply to each half bridge, or is everything running off a single supply?


Online Mads Barnkob

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Re: A three-phase "tree phase" Tesla coil
« Reply #2 on: June 30, 2020, 01:04:30 PM »
Thank you for sharing such a complete design walk-through, when I first saw the videos back from your first tests, I was not sure if it was a new topology, so interesting to see its a SSTC driven much like a modern X-Ray transformer with a large dc block capacitor and variable frequency to determine the output voltage.

After the GaN switches, I tried some of the new 4-lead TO-247 style Si MOSFETs from fairchild.  This showed a lot of promise up until its ultimate demise, of which im not sure what the cause actually was.  I suspect a bug in my controller allowed switching below the resonant (zero phase) frequency of the system, giving hard body diode recovery at voltage slew rates outside of the SOA triggering their prompt failure.  Disappointingly, all 3 half-bridges failed so energetically that they were a complete loss, recovering only the heatsinks for future use. 

Please tell me you have this on video!

The switch timing is generated by a STM32F405 microcontroller, using its timer/counter peripheral which has 3 phase output capability.  The 3 square wave drive signals for the 3 half-bridges are phase shifted by 120 degrees (or close to it, depending on quantization limits of the timer/counters).  The primary current of each coil is monitored by a CT, checking the current amplitude with a ADC, and also checking the zero cross timing with a high speed comparator feeding into a timer-capture peripheral that is synchronized with the gate switching signal.  The MCU is constantly watching how much "phase margin" there is between mosfet switching and primary current zero crossing.  The goal is to never switch later than the current zero crossing, and ideally keep switchings slightly ahead of it so that switching losses are reduced by ZVS conditions.  While the MCU approach cannot keep up with every zero crossing at 500khz, it only has to "spot check" the phase margin every few cycles (at 20uS update period) because of the quasi-steady-state nature of the system.  That is to say, there should be no fast changes to track, and we can generally assume its "safe" to raise the driving frequency if necessary as it will reduce the power throughput and also give safe switching conditions (no body diode forced recovery). 

The remote control "ramp generator" is essentially commanding the operating frequency of the inverter.  Lowest power sets the frequency to 475khz, while the highest power setting is 375khz, provided the zero cross detector sees enough phase margin for near ZCS, and also that the ADC reports primary current within limits.  If the commanded frequency is "too low", the controller will just track the coil's natural oscillation frequency via zero cross measurements and slow adjustments of the timer/counter period.  As the loaded coil frequency drops, the primary tuning boosts the power, helping feed the sparks.  The ramp generator i use features a 5-point piece-wise linear waveform generator so that i can experiment with shaping the ramp function to optimize spark growth.  While this functionality proved useful for a single coil making long sword sparks, it doesn't really matter much for the chaotic, short sparks created by the 3-phase setup.

The start-up of the system works at a much higher frequency still, 540khz.  This is to gracefully bring the system into stable oscillations without seeing the "transient" response that would otherwise show up when driving closer to resonance, which generates too much voltage for the coils to stand.  For the first 400uS or so, the drive is sweeping from 540khz down to 475khz.  At 475khz plasma is just starting to form, which quickly brings down Qsec and brings stability to the system.  From here the ramp generator is setting the frequency command, and the zero cross detector is checking if theres enough phase margin to allow it (generally, if there is not, it means the spark load has not grown enough, but given time it will usually get there).

The machine has a built-in active rectifier (PFC).  The PFC includes a number of nice features.  Inrush/pre-charging is handled by the full-wave bridge rectifier common for a single-phase PFC.  The bridge rectifier uses SCRs for the two "upper" diodes feeding the positive voltage input to the boost circuit.  The SCRs are phase-angle controlled by the microcontroller (which is also watching AC line zero crossings).  Therefore, no external resistive/capacitive charging element is needed, and no extra relays/contactors are needed.  The pre-charging profile could be fine tuned to maximize charging current, but i just use a linear adjustment of phase firing angle over time until reaching full 180* conduction, at which point the PFC boost action can begin.  The precharge current pulses are on the order of 15A peak.

Since the PFC is controlled via MCU, it is easy to build in voltage setpoint (380V to 800VDC), which is a slider on my remote control box.  I decided not to bother with variable input current setting, however this would be easy to add.  Instead, if the MCU detects 120V line level, it sets the PFC max current to 15A RMS, but if 208/240V line is detected it allows up to about 57A RMS, or something over 13kW output from a very compact design weighing maybe 5lbs excluding the large output capacitors.  Of course, i can't claim it could reliably operate at this power level continuously, it probably would not be very reliable, but it seems to work OK for short term duty cycles.

So the ramp generated for the PFC is sinusoidal for it to work in quasi-resonant mode?


I also have plans to power this setup with a 444V lithium ion battery, capable of supplying enough current to run all 3 phases CW.  More to come...

After seeing what LOD could do with battery packs, electrical vehicle inverters on his large trailer packable coil, this is a interesting approach and for sure a advantage over having front-stages and PFC.
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Offline Weston

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Re: A three-phase "tree phase" Tesla coil
« Reply #3 on: June 30, 2020, 08:22:33 PM »
Thanks for taking the time to create a thread on this coil! We were talking about it on IRC a while back when you first posted a youtube video.The power electronics are surprisingly compact.


Due to the coupling between the coils does the power drawn by each inverter remain matched despite mismatches in spark loading? Does it matter which primary you monitor the phase from?
 

Does this system, similar to a bipolar coil, have reduced ground currents / reduced grounding requirements?


Is there some advantage in controlling the switching frequency instead of the phase to control power?


SiC FETs seem pretty ideal for these QCW / hard switched type applications. Given the relatively poor performance of the transphorm GaN cascode parts and the weird snubbers required I am a bit suspicious of the cascode based United SiC parts, but based on the datasheets they seem to have and Dr. Kilovolt's coil they seem to work well.

Offline Steve Ward

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Re: A three-phase "tree phase" Tesla coil
« Reply #4 on: July 01, 2020, 02:43:34 AM »
Heatsinks: I don't have any measurements to back my predictions, just going off of what the insulator manufacturers claims are for conductivity.  I do agree that at ~10mS the transient die heat is starting to become dependent on the tab-heatsink resistance,  It matters more and more the longer the "pulse" duration, of course.

Quote
As for the gate drive, I'm surprised that bootstrap works so well - I was expecting separate supplies for each high-side device, which is a major pain once you have lots of them (and the main reason why I'm still using a GDT, albeit with somewhat heroic efforts to reduce leakage inductance). Have you got a separate 12V supply to each half bridge, or is everything running off a single supply?

Originally, all gate drive power came from a single supply.  The problem with this is when you have high current on the negative bus rail, and some inductance/resistance due to some wire length, so that the negative rail of the PFC was different potential than the negative rail of the half-bridges.  The result can be loss in gate drive voltage depending on bus current, as well as some current passing through the wires supplying gate drive power to the modules.  For good margin, i decided to use independent 12V (really 16V i think) supplies for each of the 3 half-bridges and the PFC gate drive power and completely avoid this power loop issue.

Quote
Thank you for sharing such a complete design walk-through, when I first saw the videos back from your first tests, I was not sure if it was a new topology, so interesting to see its a SSTC driven much like a modern X-Ray transformer with a large dc block capacitor and variable frequency to determine the output voltage.

Yeah, its a pretty common scheme found in high voltage supplies because usually any transformer capable of generating high voltages will have a resonance near the practical range of operating frequencies, so you may as well use it to your advantage for power control.

Quote
So the ramp generated for the PFC is sinusoidal for it to work in quasi-resonant mode?

I should have explained that the PFC just charges up the large 400V 12mF (x2 in series) bus caps that the 3 phase RF inverter runs from.  The PFC just does the best it can to maintain a set DC voltage, but the power demand from the 3p inverter can greatly exceed the capability of the PFC, and so the DC bus voltage rapidly drops down, in fact, during the "QCW ramp".  Far from ideal, but it does "work".  This is part of the motivation to switch to battery power, and perhaps even keep the PFC's boost section only, since it can handle about 50A RMS, at 450V.  Or i can ditch the boost and the battery can supply >150A, but only at 400V or so.

Quote
Is there some advantage in controlling the switching frequency instead of the phase to control power?
Ehh, I'd have to think about what the difference is here, i think it comes down to stability. I wondered the same thing about controlling phase instead of freq. maybe it would be better to try a phase regulator, making it more like a classic 4046 PLL chip (phase comparator drives operational frequency).  However, the real machine produces primary currents that are far from simple sinusoids at times, so phase comparison can be corrupted.  In particular there is a strong 3rd harmonic present.

Quote
Due to the coupling between the coils does the power drawn by each inverter remain matched despite mismatches in spark loading? Does it matter which primary you monitor the phase from?

There's some "funny stuff" going on that tends to throw the 3 phase system out of balance (probably different spark loads).  Sometimes 2 coils will have 180* phase between primary currents, and the 3rd phase is somewhere inbetween.  You cannot count on them being 120* phase shifted, at all.  So my controller watches all 3 primary currents for zero crossing events, compared with all 3 switching commands.  This means, if one coil has a higher resonance frequency than the others, it will cause the inverter to switch faster, holding back performance.  I can generate much longer sparks with just 1 coil running instead of 3, i think due to this compromise. 

In fact, when i first built this thing my quality control was poor and my coils were not exactly tuned.  The resulting current waveforms were really something, especially because this early iteration had the 3 coils very closely coupled.  Since then, ive burned up the original coils, giving me opportunity to re-do with my new winding machine i got from ebay with a turn counter built in. Once i actually tried, the coils match rather nicely.

Despite this, there is still a "power struggle" between coils.  Over time there is a tendency for primary currents to oscillate between the 3 phases, in magnitude. 

RF envelope 1 and 2 show some examples of what can happen.  Definitely no guarantees of balance or phase relationship that i can see.

One experiment i have in mind is to tie all 3 primary returns together (in a "star" or "wye" or "Y"), rather than returning them to the DC bus as i have now (which keeps them more isolated).  This essentially gives a floating "neutral" point, which may serve to balance the power to the 3 coils?  It might actually make balancing worse, if the coil with the weakest spark has the highest Q and lowest imepdance.  I haven't made up my mind on how this could work out, yet :P.

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Re: A three-phase "tree phase" Tesla coil
« Reply #4 on: July 01, 2020, 02:43:34 AM »

 


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[Light, Lasers and Optics]
davekni
July 08, 2020, 04:39:10 AM
post Measuring LED efficiency
[Light, Lasers and Optics]
TMaxElectronics
July 08, 2020, 12:46:23 AM
post Re: Configuration of a half bridge.
[Electronic Circuits]
hammertone
July 07, 2020, 10:28:37 PM
post Re: Configuration of a half bridge.
[Electronic Circuits]
ElectroXa
July 07, 2020, 10:09:22 PM
post Re: Configuration of a half bridge.
[Electronic Circuits]
hammertone
July 07, 2020, 09:36:53 PM

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