Author Topic: Portable Q(uarantine)CW Tesla Coil  (Read 2770 times)

Offline Steve Ward

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #20 on: June 12, 2020, 05:18:09 PM »
Quote
I have a strong belief, that arc length (for a given frequency and ramp up speed) only depends on power and power is given by Ipri^2 * real(Rinp). So he must have had a larger Rinp to balance the effect of a smaller primary current. I've no idea how he could have increased Rinp without the corresponding loss of primary current such as you have seen, when you added some capacitance to your MMC.

I think the apparent performance gain here was partly under-estimating the cost of branches in the sparks, and the tuning change promoting branchless sparks.  I'm assuming whatever claim of 20% less primary current for same spark length was from tuning the primary resonance below secondary and using the starting oscillator to force the mode to secondary resonance (upper pole).  This likely resulted in a straighter spark with less branches that made better use of the power, is my guess at this point.  There is some transient nature to these sparks, afterall, they behave "poorly" if not fed at an optimal rate.  Once the ramping has reached its saturation level, the spark might have nearly the same input power, but will transition from a long spike into a short serpentine with thicker "arc" channel.  My guess is that tuning alters the "impedance trajectory" that a coil operates at over the ramp duration, in ways that can be more or less beneficial for growing long branchless sparks. 

Offline Uspring

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #21 on: June 12, 2020, 07:23:47 PM »
Hi Steve, it's certainly a pleasure to hear from you and draw on your expertise here.
Quote
I'm noticing that you model your spark as a parallel RC, but as far as i know, everyone else is assuming a series RC model, so any numbers based on my postings are always series RC.
I've used a parallel circuit, since it allows me to lump the arc capacitance into the top load capacitance. That simplifies the equations a bit. The series circuit is somewhat more intuitive as it reflects the arc being a resistive medium feeding a space charge. The difference between both types of circuit is small, though, if the frequency range over which the coil operates is limited.

Thank you for your measurements. The voltages seem rather low. Are that peak or RMS kV? The supply voltages you quoted seem more or less constant. Did you also use a phase shifted bridge? Otherwise, how do you control power input?

Quote
I think the apparent performance gain here was partly under-estimating the cost of branches in the sparks, and the tuning change promoting branchless sparks.

The upper pole operation with the primary tuned low shows a much flatter response of the input resistance when arc load increases. The other tuning often leads to a steep drop in input impedance at the end of the burst, so that power input suddenly rises. That could cause branchings.

Offline Steve Ward

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #22 on: June 12, 2020, 08:57:35 PM »
Quote
Thank you for your measurements. The voltages seem rather low. Are that peak or RMS kV? The supply voltages you quoted seem more or less constant. Did you also use a phase shifted bridge? Otherwise, how do you control power input?

I'm usually more of a "peak" value kinda guy. I'm assuming those are the peak voltages i recorded.  And yeah, they do seem low.  Maybe its worth taking another shot at calibrating the probe - i noted my last check was 758:1 where i think the nominal ratio is supposed to be ~1000:1.  However, i did record the secondary coil base current, and when i run my spice model, i see pretty close agreement to the measured primary current, secondary current, and top voltage.  I'll update the attachment in the previous post with base current measurements.

Yes, this setup used phase shifted bridge control.  The bus voltage reported is likely recorded at the very end of the ramp.

I do recall something like 70kV maximum for my first QCW project (because i recall pushing the 60kV rated probe to the point that it would occasionally emit a glow discharge internally) which did make larger and more branched sparks at full power relative to the measurements reported on the TeslaGun V1 system.

Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #23 on: June 12, 2020, 09:21:43 PM »
Uspring: I dont have any more data from that rub beyond that I started with a DC bus voltage of 450V. I will have to start measuring the input voltage too. Its a bit of a pain to set up everything for testing because I have to do it in the open courtyard of my apartment complex.

Hi Steve! Thanks for dropping by.

I think I am pretty close to having the design for my new primary / secondary complete. USPS seems to have lost the package with my coilform though....

One QCW theory question:

Whats the deal with coupling? From theory and my simulations changing coupling alters the impedance of the system and the ratio of power circulating the in the primary and secondary. It seems that aside from practical / standoff voltage constraints a higher coupling would be desirable  Your first QCW coil had very high coupling but your tesla coil gun only has a coupling of 0.31. In some old 4hv posts you commented that the system seemed to perform better with a lower coupling. Dr. Killivolts coil uses ferrite to get K=0.55 https://highvoltageforum.net/index.php?topic=1073.0 .

Is there some optimization or some factor I am not considering or something? Once tank impedance is normalized is the coupling actually not that important as long as it is not excessively low? Given the high Q of the primary it seems that for most tunings the power dissipated in the primary + mmc will be minor compared to the power delivered to the spark.


I have been thinking about the QCW tuning and the d(Impedance)/d(Power) due to spark loading. When the secondary is tuned above the primary the impedance at the upper pole increases with spark loading, when the primary is tuned above the secondary the opposite happens. If impedance decreases with power you have a positive feedback effect that makes it difficult to control your rate of spark growth. I assume this can make it difficult to grow straight arcs.

Offline Steve Ward

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #24 on: June 13, 2020, 07:26:46 PM »
Coupling is sort of confusing to me, too.  I think about it as controlling the how much of the spark load we want reflected onto the primary, which should increase real(Rinp) relative to Zpri.  I think the answer is right there in Uspring's post on may 30? 

I think it comes down to your assumption that the component losses are negligible.  If that is indeed the case, then the payoff for higher couplings is less and less (i think this is probably right, but if its trivial to raise the coupling, its like free efficiency boost, even if its small).  In the case of my teslagun V1 i had flashover issues and also the spark growth was branchy and inconsistent with the higher K.  So i still think there is maybe something to this tuning stuff, which is only important with respect to spark growth/power control behavior, and not at all important in terms of power efficiency.
« Last Edit: June 13, 2020, 07:30:53 PM by Steve Ward »

Offline Uspring

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #25 on: June 13, 2020, 07:35:52 PM »
Steve, thank you for the update with secondary base currents. A quick look at them seems to confirm, that your top load voltages indeed are peak voltages. I'm working at a more rigorous check. In the back of my mind I always have an improvement of my arc model. QCW data is particularly interesting as these arcs are easier to understand than the short lived usual DRSSTC bursts, since there the warming up of the arc channel has to be considered. Steady state QCW arcs avoid this problem.

Weston wrote:
Quote
Is there some optimization or some factor I am not considering or something? Once tank impedance is normalized is the coupling actually not that important as long as it is not excessively low?
A big coupling constant makes the system less sensitive to arc detuning. Consider the 2 tanks at the same resonance frequency i.e. fsec=fpri. The the poles will be at fsec/sqrt(1+k) and fsec/sqrt(1-k). A large k will move the poles (i.e. operating frequencies) away from the secondary resonance frequency and will actually detune the system. But this is compensated by the fact, that a large k will also increase the primary impedance. In many cases, these effects will more or less cancel each other in terms of input impedance. Look also at the equation here: https://highvoltageforum.net/index.php?topic=1024.msg7498#msg7498

But: Since the coil with the large k is already detuned considerably, additional detuning due to the presence of the arc doesn't matter as much anymore.
This is a bit of a ballpark estimation, since input impedance depends much on the location of primary and secondary resonance frequencies and the pole you're running at.

« Last Edit: June 13, 2020, 07:41:23 PM by Uspring »

Offline Steve Ward

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #26 on: June 14, 2020, 07:52:40 PM »
On the topic of switching losses, I'm interested to know what your gate driver solution looks like, if you've opted for negative bias and what the effective Rgon and Rgoff is.  One thing that i wonder about is why SiC datasheets seem to imply a minimum external gate resistance without specifying it.  What i mean is, all of the data graphs wrt gate resistance stop at some fairly high value, on the order similar to the internal Rg_int.  Seems to me there is still potential to switch off twice as fast as the datasheet gives data for.

On my 3 phase SiC Coil (which, I should make a post about) I'm using boot-strap gate drive at ~20V with no negative bias.  My gate drive IC can sink 10A even down to a few volts, and i use no external gate resistance on the C3M0021120K transistors, which are quite similar to the module you use.

Another idea to consider is a small amount of capacitive snubber across the switches, as this can greatly reduce switch-off losses at the cost of requiring a minimum current/deadtime for ZVS turn on.  If you check the spec sheet for Eoff vs Id, you can see the effect of the junction capacitance as a snubber- instead of a linear increase in switch off loss versus current, its more exponential since low current switch off is effectively snubbered by Coss.  So even a relatively minor boost in Coss can have a nice improvement in Eoff, so long as we dont have to pay for it with Eon (and with ZVS, that is zero).  In the scenario of using extra capacitor snubbers, it may make sense to keep all the hard switching to 1 bridge leg instead of alternating, so that the ZCS leg doesnt have any extra cap, which limits ZCS performance somewhat.

Since you estimate half of your losses are from switching, these might be nice options.


Offline Weston

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #27 on: June 17, 2020, 01:56:22 AM »
I am using UCC5390SC isolated gate drivers, which are a 10A minimum, 17A typical gate driver, with a 1 ohm turn on and 1 ohm turn off gate resistor. Its powered by a +15/-3V isolated converter. So it seems I should be getter pretty close to the datasheet switching speeds.

I dont think its quite likely with the expected switching speeds, but I am a bit worried about exceeding the CMTI of the isolated gate drivers, which could cause things to blow up in a hurry.

The negative gate bias helps prevent dv/dt induced turn on. In the module there is a bit higher gate inductance due to the packaging so the effect clamp impedance is higher. I don't think its much of a worry for these applications and newer devices are better in this respect, but the SiC MOSFET gate threshold can shift down with aging. I think on some early devices it could even make it to 0V.... So I suspect thats the main reason for the emphasis on negative gate bias. Either way, a +15/-3 isolated converter was only a bit more expensive than a +15V converter, so I figured why not.

I considered adding a capacitive snubber for turn off but it just seems to increase the potential for things to go wrong. Conduction loss goes as I^2, so even if I were to substantially reduce the switching loss I would only get a bit more current out. Alternating the switching leg is just some more verilog and would half the per FET switching loss without any hardware changes. Anther avenue would be to have some sort of adaptive over current protection based on the total switching losses. Right now my OCD is set assuming hard switching at 80A over the entire burst, which is not a realistic assumption. Davekni was discussing an analog implementation of this a while ago, I have  ~unlimited DSP power on the FPGA so it would just be a matter of coding it.

I have been a bit busy with the impending end of the spring term at school. I just ordered some PCBs to serve as FR-4 endcaps for my new secondary ( I am going to try the whole fiberglass secondary thing...) and my secondary MMC PCBs. Hopefully I will be making more progress soon-ish, term ends this week.


Steve: you should create a post on your 3 phase coil, I saw the youtube video a while ago and it seems pretty cool!

Also, do you have any guidance on the maximum per pulse junction temperature rise? I was reading some app notes related to it, but it seems most guidance is guided towards longer time period thermal cycles and infineon does not actually provide any of the required values to calculate thermal aging for the SiC parts. I arbitrarily chose 50C for my max delta T, mostly based on maximum junction temperature and some worst case assumptions on heatsink temperature. It seems like it might be high, but its also driving the calculations for my assumption of hard switching at maximum current for the entire pulse, which is also a bit unrealistic .



Offline Steve Ward

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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #28 on: June 18, 2020, 02:14:42 AM »
Sounds like a nice driver setup, no room for faster switching there!  One thing i might warn about is taking care with fast turn on, in the event the body diode does have reverse recovery, you might consider a bigger Rg-on to reduce the di/dt of such an event.

I ran into a few problems with my system, one of them somewhat related to CMTI (common mode transient immunity - how fast can you switch the high voltage side of a gate driver before causing it to malfunction, in possibly catastrophic ways), but really more of a input noise coupling issue that was solved by boosting my PWM input signal filter cap from 150pF to 1nF.  After resolving this (and other issues), I checked things out by switching 900V @ 175A, which produced about 100V/nS slew rate, double the "typical" rating on the silicon labs SI8231 that i use for isolation/deadtime generation.  I also checked for dv/dt induced turn on, looks ok, even with my 0V turn off.  However, dv/dt induced turn on shouldnt really be a thing (i mean, yes, it slows down the turn off) if you're switching ahead of the current zero crossing.  I also check the hard diode recovery - even though it shouldn't happen, it is important to be capable of handling it.  No noted problem with 900V 100A hard switching, but I do use 4.7 ohms for Rg-on to tame things a little.

I've had noise coupling issues on a number of SiC and GaN inverters i've developed, so to me this stressful switch testing seems crucial to weed out noise feedback problems that only happen when the voltage or current is high enough. If i can, i like to go about 20% over normal voltage and up to 2X the normal switching current.  Of course, if i blow up some parts it's not coming out of my pocket, so its a little easier to go forth with such tests.

Pulse die heating is probably a big driver of unreliability, but i dont have a good feel for what the MTBF versus delta T looks like for any of this stuff, and i think no device manufacturer will confidently post specs on this kinda thing.  When i used to work at Fermilab, which has lots of pulsers, the guidance was to limit the pulse delta T to just 3 degrees C!  However, those machines are typically pulsing at 10s of hz for ideally... 10..20...30 years and its really expensive to shut stuff down to fix it.  When i work out the delta T for the IGBT tesla drivers its not uncommon to see a worst case of 30-50*C, however, thats also under some of the assumptions you made, like hard switching the maximum current for the entire ramp, so really its maybe half of that delta T.

Aging of semiconductors just seems weird, and beyond my 1 semester of semiconductor physics understanding. I was completely unaware of this Vgs-threshold change with age.  I did run into a reference on SiC body diode Vf increase with age, but as far as i can tell that's probably not an issue with recent SiC devices.


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Re: Portable Q(uarantine)CW Tesla Coil
« Reply #28 on: June 18, 2020, 02:14:42 AM »

 


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