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Registered Member #2694
Joined: Mon Feb 22 2010, 11:52PM
Location: Russia, Volgograd (Stalingrad).
Posts: 97
Just watching this stuff fall me into deep delight! Thanks, Steve that you make this stuff be real. It is absolutely amazing.
Many things are hard to understand yet, but one of them is which mode you are use in H-bridges? Does they drives in phase-shifted mode or in regular like your first qcw with buck?
Look like with new MCu driver many features possibly are done in software (Buck modulator, Buck Snubber driving, bridges driving) and does this software-part could be replaced by analog-based electronics or some features won't properly work without MCu in prototype this high-power kind of QCW? Simply, if H-bridges work in regular mode, then if I try to make this stuff according to qcwinfo from, for example, Gao site for my home bass fun, then will or won't work it with his(Gao), for example, buck modulator circuit design?
could you please tell something more about driving buck snubber circuit algorithm? It's a little bit difficult to understand how exactly it get stored and get back to bus the energy.
You wrote about duty cycle are 75%. Does 13kVAC RMS MMC are enough for polyphonic operations like bass guitar fun? Did you saw any MMC overheating playing bass?
"8x H-bridge, each drives its own transformer" - means 8 GDT cores, right?
If so, then 2 MCu drivers are works synchronously for multiplying H-bridges gate power or by another reason?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Uspring:
I did some quick calculations from the specs you provided. It seems, like you didn't use a secondary MMC, which implies (by some more calcs), that arc loading would increase top capacitance by a factor of 2 or 3. How did you deal with that amount of detuning? My guess would be upper pole operation together with some heavy primary low tuning, e.g. 30 or 40%.
I chose to not use a MMC for the secondary due to their limited reliability and cost. I think the Csec = 44pF. My estimate for C of a 11 foot arc (by extrapolating linearly from previous guess of 2.5pF/ft) is 27.5pF which is a bit over half Csec.
It does have upper pole operation, and the primary is detuned, but im not sure its anywhere close to 30-40%. From my FEMM model and Java TC i get a Fpri of 260khz, Fsec of 270khz, K = 0.33, Fupper = 325khz. Im not sure how to define the % of detune, but this looks like just a few %. I wanted to try detuning further but somehow my driver could not cope and the coil would more often than not, end up at the lower pole mode which performed very very poorly (required too much current).
Comparing arc length to peak power relationships with your previous coil (20kW, 5-6 footers), there seems to be a deviation from the square root law, i.e. arc length ~ sqrt(P). Any idea about this?
First off, i mis-calculated the buck power (i did it based off the peak inductor current, but thats not right, it should be average inductor current during a switch cycle). So real max power so far is 460A*350V = 160kW. The tesla gun setup is my other benchmark for the 5-6 footers it uses about 100A*350V = 35kW. So doubling the spark length has called for ~4X the power, which seems to roughly follow a squared/square root law as anticipated, no?
Intra:
Many things are hard to understand yet, but one of them is which mode you are use in H-bridges? Does they drives in phase-shifted mode or in regular like your first qcw with buck?
The bridges are ZCS only, no phase shift modulation.
Look like with new MCu driver many features possibly are done in software (Buck modulator, Buck Snubber driving, bridges driving) and does this software-part could be replaced by analog-based electronics or some features won't properly work without MCu in prototype this high-power kind of QCW? Simply, if H-bridges work in regular mode, then if I try to make this stuff according to qcwinfo from, for example, Gao site for my home bass fun, then will or won't work it with his(Gao), for example, buck modulator circuit design?
I chose to do things digitally, but the same performance could come from a bunch of TTL chips and analog circuits. I think you have the right idea in mind.
could you please tell something more about driving buck snubber circuit algorithm? It's a little bit difficult to understand how exactly it get stored and get back to bus the energy.
The snubber consists of a diode feeding into a capacitor bank (2x250V lytics in series). This bank has a boost circuit (series inductor, IGBT to bus -, diode to bus +) that will take the 475V-500V level of the snubber caps, and send that energy back to the 700V main bus via boost circuit. The voltage on the snubber capacitor is monitored by the MCU, if the level exceeds 490V the buck converter is forced to shut down. If the voltage is > 475V, the booster circuit is enabled to bring the snubber voltage down. Fairly simple stuff to implement in a MCU, which is why i prefer this system, but the start up time to design it is much higher.
You wrote about duty cycle are 75%. Does 13kVAC RMS MMC are enough for polyphonic operations like bass guitar fun? Did you saw any MMC overheating playing bass?
"8x H-bridge, each drives its own transformer" - means 8 GDT cores, right?
If so, then 2 MCu drivers are works synchronously for multiplying H-bridges gate power or by another reason?
When playing music through the coil, the peak power is usually limited because the average power is much higher and so the silicon is running hotter. The MMC has been very impressive, i havent noticed it getting warm at all after many minutes of making huge arcs .
The transformers are on the output of the H-bridges, this is not typical, but unique to my design for current sharing among all IGBTs.
The IGBTs are still GDT driven. Technically you only need 1 GDT with 32 secondary windings to drive all the IGBTs, but for ease of construction i used 4 GDTs total.
The 2 MCU-based controllers/drivers control the buck stage and the tesla coil stage separately, but share communication over rs-422. I could likely make 1 controller do all of this instead of 2, but this was the fastest way forward for now. There is a fiber optic link from the buck driver to the human controller, which generates the modulation signal.
From Cpri * Lpri = Csec * Lsec, Lsec should be around 8 mH. For some reason Javatc gave me a Lsec of about 3 mH, so I assumed Csec to be a lot higher, than your value of 44pF, which made be believe in a secondary MMC.
My estimate for C of a 11 foot arc (by extrapolating linearly from previous guess of 2.5pF/ft) is 27.5pF which is a bit over half Csec.
I've estimated arc capacitance differently, i.e. by its power consumption. In the corresponding TCML thread, you mentioned an estimate of 100kVac top voltage. I assumed this to be peak voltage. From this and 160kW power, the arc resistive load would be 0.5 * V^2 / P = 31kohm. From my measurements, I've found Conners hungry streamer model to be approximately accurate, which states, that capacitive and resistive loads are similar, i.e. R = 1 / (2*pi*f*Carc). Carc would then be about 20pF. That's a bit lower than your estimate, but the same ballpark.
So doubling the spark length has called for ~4X the power, which seems to roughly follow a squared/square root law as anticipated, no?
Registered Member #1637
Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
Why don't you just shut down buck in overcurrent condition and let groundstrike consume ~40 J of energy that is left in LC filter?
I dont think that could harm bridge. From my experementation, such an long streamer has considerable impedance and primary current rises by only ~100Ð. So I think it's safe to disable buck and let system rampdown by itself.
Registered Member #2694
Joined: Mon Feb 22 2010, 11:52PM
Location: Russia, Volgograd (Stalingrad).
Posts: 97
Steve, thank you clarify all the details. It very helpful for me.
Steve Ward wrote ...
Intra:
Many things are hard to understand yet, but one of them is which mode you are use in H-bridges? Does they drives in phase-shifted mode or in regular like your first qcw with buck?
The bridges are ZCS only, no phase shift modulation.
Look like with new MCu driver many features possibly are done in software (Buck modulator, Buck Snubber driving, bridges driving) and does this software-part could be replaced by analog-based electronics or some features won't properly work without MCu in prototype this high-power kind of QCW? Simply, if H-bridges work in regular mode, then if I try to make this stuff according to qcwinfo from, for example, Gao site for my home bass fun, then will or won't work it with his(Gao), for example, buck modulator circuit design?
I chose to do things digitally, but the same performance could come from a bunch of TTL chips and analog circuits. I think you have the right idea in mind.
could you please tell something more about driving buck snubber circuit algorithm? It's a little bit difficult to understand how exactly it get stored and get back to bus the energy.
The snubber consists of a diode feeding into a capacitor bank (2x250V lytics in series). This bank has a boost circuit (series inductor, IGBT to bus -, diode to bus +) that will take the 475V-500V level of the snubber caps, and send that energy back to the 700V main bus via boost circuit. The voltage on the snubber capacitor is monitored by the MCU, if the level exceeds 490V the buck converter is forced to shut down. If the voltage is > 475V, the booster circuit is enabled to bring the snubber voltage down. Fairly simple stuff to implement in a MCU, which is why i prefer this system, but the start up time to design it is much higher.
You wrote about duty cycle are 75%. Does 13kVAC RMS MMC are enough for polyphonic operations like bass guitar fun? Did you saw any MMC overheating playing bass?
"8x H-bridge, each drives its own transformer" - means 8 GDT cores, right?
If so, then 2 MCu drivers are works synchronously for multiplying H-bridges gate power or by another reason?
When playing music through the coil, the peak power is usually limited because the average power is much higher and so the silicon is running hotter. The MMC has been very impressive, i havent noticed it getting warm at all after many minutes of making huge arcs .
The transformers are on the output of the H-bridges, this is not typical, but unique to my design for current sharing among all IGBTs.
The IGBTs are still GDT driven. Technically you only need 1 GDT with 32 secondary windings to drive all the IGBTs, but for ease of construction i used 4 GDTs total.
The 2 MCU-based controllers/drivers control the buck stage and the tesla coil stage separately, but share communication over rs-422. I could likely make 1 controller do all of this instead of 2, but this was the fastest way forward for now. There is a fiber optic link from the buck driver to the human controller, which generates the modulation signal.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Why don't you just shut down buck in overcurrent condition and let groundstrike consume ~40 J of energy that is left in LC filter?
That is the main mode of operation for my system, except the buck tries to maintain constant output current during the ground strike instead of simply shutting down. This gives much more spectacular ground arcs, especially during high rep-rate operation.
Overcurrent on the bridge is set pretty high to avoid ever tripping it, but some ground strikes have done it, and i see my "snubber over voltage" indicator come on when it does. It would be nice to have less complexity and not need the snubber, but i think there should always be some over-voltage protection in place since you have no guarantee the tesla coil driver might not suddenly shut off for any number of reasons, allowing the buck output to over-voltage.
Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Very interesting coil.
Does the micro controller form part of the feedback path? If so, does it sense the tank current with a comparator on the zero crossings or an ADC?
If clocked logic or a micro controller is used in the feedback path it seems like you would get a lot of jitter in the gate drive waveforms and fall into hard switching some of the time. Does this not end up being an issue?
Registered Member #2922
Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226
Hello guys!
That is the main mode of operation for my system, except the buck tries to maintain constant output current during the ground strike instead of simply shutting down. This gives much more spectacular ground arcs, especially during high rep-rate operation.
Can you explain it better? When over current is tripped the coil driver start to skip cycles and the buck goes in a "constant current mode" to help the cycle skiping thing to work better?
Registered Member #2694
Joined: Mon Feb 22 2010, 11:52PM
Location: Russia, Volgograd (Stalingrad).
Posts: 97
Steve Ward wrote ...
The snubber consists of a diode feeding into a capacitor bank (2x250V lytics in series). This bank has a boost circuit (series inductor, IGBT to bus -, diode to bus +) that will take the 475V-500V level of the snubber caps, and send that energy back to the 700V main bus via boost circuit. The voltage on the snubber capacitor is monitored by the MCU, if the level exceeds 490V the buck converter is forced to shut down. If the voltage is > 475V, the booster circuit is enabled to bring the snubber voltage down. Fairly simple stuff to implement in a MCU, which is why i prefer this system, but the start up time to design it is much higher.
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The "Fat Coil" QCW Tesla Coil
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