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Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Well, the start of one, anyway. I've decided to jump on the bandwagon and build one of these. Blog post here: Currently tested at low powers on a resistive load; whether the controller can deal with real Tesla coil load conditions remains to be seen.
Registered Member #2922
Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226
Hello man. I'm making one too, and what I learned is that you need less LC capacitance to get a more fast response of the output. Linear control will not work too, now I'm using a delta modulator same of hysteresis modular, bang bang modulator inspired on steve/eric modulator. Good look!
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Gregory wrote ...
Hello man. I'm making one too, and what I learned is that you need less LC capacitance to get a more fast response of the output. Linear control will not work too, now I'm using a delta modulator same of hysteresis modular, bang bang modulator inspired on steve/eric modulator. Good look!
I don't see why less capacitance will improve response times; so long as your switches can cope with charging the output cap through the inductor that quickly it should be fine, especially with delta modulation. I was designing this for a coil that would pull 600Apk, hence the air-cored inductor and huge output cap. Has it been experimentally confirmed that QCW's do not work at <300KHz? If so, I guess I need to scale things down a bit.
Registered Member #2922
Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226
as it been experimentally confirmed that QCW's do not work at <300KHz
Well, I think that people have already tryed to use a < 300khz ressonator at vttc or half retified sstcs and what they got is a "normal spark" no the sword like spark.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
About bandwidth and power supplies using LC output filters. The LC resonance frequency (called a pole, actually its a double pole at that freq) limits the bandwidth of the power supply, after the Fres, you got a -20dB per decade drop off, unless you compensate for it some way. So this is mainly how i'd look at sizing my LC components, or at least their Fres. The other thing to consider is the output impedance of the power supply vs the input impedance of the tesla driver. You dont want the bus voltage to sag too much based on the intverter current, so you need some minimal amount of capacitance.
About control schemes: you can do a more classical power supply control (a "PWM control"), but the compensation for the resonant output filter is required. Look for documentation on forward and buck converter controls from TI, fairchild, etc... There are always short comings of this type of control with relation to the filter, and you may just need to avoid feeding the system any control signal with frequency near the Fres as this could make the thing go unstable. This is why i opted for non-linear control laws for a high speed buck conversion. The hysteretic control is both unstable (it oscillates at its own, much higher carrier frequency, which is not fixed but controllable) and very stable (as in, tracks a reference signal) at the same time. It merely turns the output on and off so that on average it meets its goals, but at any instant in time, its either too high or too low. If you make it self-oscillate faster, the regulation is improved for a given output filter. Loading the thing can alter the self oscillation frequency too, but i dont think ive ever seen it do anything undesirable. This kind of control will gladly accept references that contain frequency content above the LC resonance, and the frequency response of such a controller is simply dominated by the LC network, except its a roll off instead of a big peak at the Fres, so we say its stablized.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I wanted to add a little of my experience in as well.
<300KHz operation will be non-sword sparks.
For me 320KHz seemed to be the magic frequancy where sparks became nice swords.
The output capacitance is important in a couple of ways, it dose have to be sized properly because it plays a larger part in the performance of the buck to regulate. As Steve said the cap should be big enough not to have the inverter sag, but at higher power levels the use of a feed-forword capacitor in the feedback network will probably be needed to speed up the switching frequancy to the correct level. However the cap doesn't have to handle supper large amounts of current, not as much as the inductor at least.
Lately however I have been playing around with a new kind of delta modulation that uses internal integration rather than relying on the integration of the output capacitor of the buck. I'm hoping that this helps with both noise immunity and also making the switching frequency controlled internally rather than being dependent on the output C and load.
I would also like to point out that the origonal BB modulator I used was Steve ward's design, and not mine. Steve's deserves the credit for a well desighned modulator.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
happy new years all, isn't there a balence of how much capacitance you can hang on the output inverter like Gregory specified? if there is to little capacitance it won't it won't filter the PWM and to much will be a burden and may afect the rate of rise that the inverter can handle? I would think that you wouldn't want any sort of resonance in the filter.
Dr. Isotop I noticed that you said that you are using a MBED to generate the controle hows that working out? I know that it has a 100Mhz core and I would think that it would be a exalent controle device. that is a interesting method of modulation that you have used to track the input with the output VS standard delta modulation. hope it works.
I to want to try building the ilusive QCW and I have gone with the SLR it is still in the consrution phase. Eric gave me the idea through this simulation inadvertantly.
I to want to rectifie and filter the output. I'm hoping 70uf will be enough to filter but not alow the output to be the ramp wave to track the input. right now it is open loop but may have to closed to controle nonlinearity like you have pausibly done with the MBED.
Registered Member #2922
Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226
You need some closed loop control. The bang bang control is the bestest for that situation because is very simple to implement, very stable, and will do a good job with the fast transients of the qcw bridge. For SLR maybe a PD control will work very well
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
teravolt wrote ...
happy new years all, isn't there a balence of how much capacitance you can hang on the output inverter like Gregory specified? if there is to little capacitance it won't it won't filter the PWM and to much will be a burden and may afect the rate of rise that the inverter can handle? I would think that you wouldn't want any sort of resonance in the filter.
Dr. Isotop I noticed that you said that you are using a MBED to generate the controle hows that working out? I know that it has a 100Mhz core and I would think that it would be a exalent controle device. that is a interesting method of modulation that you have used to track the input with the output VS standard delta modulation. hope it works.
I to want to try building the ilusive QCW and I have gone with the SLR it is still in the consrution phase. Eric gave me the idea through this simulation inadvertantly.
I to want to rectifie and filter the output. I'm hoping 70uf will be enough to filter but not alow the output to be the ramp wave to track the input. right now it is open loop but may have to closed to controle nonlinearity like you have pausibly done with the MBED.
Happy new year! With an LC filter like on the buck, too much capacitance can be compensated for with less inductance. I use 100 uH + 400 uF so the inductor has less wire and less Vdrop, but as Eric pointed out on my blog, too much capacitance causes an impedance mismatch with the coil, reducing the effectiveness of regulation. The Mbed has worked well so far but this has only be with a resistive load. The only thing I don't like about it is that its rather fragile and expensive to replace. Also, it has the risk of latching up around Tesla coils and dying...optocoupling the feedback signal would probably be a good idea. A good middle ground would probably be analog comparators to do the comparisons with the bounds, and a micro to do the actual control loop. A nice thing about micros when fiddling with control loops is that you can change the parameters or the control scheme by just changing a couple of defines in the code; no need for pot-twiddling or new boards
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
hi Gregory did you mean PID controller? I was thinking that it may be possible to measure the vout and feed it back through a op amp circuit to my VCO. it also may be possible to do this with Dr. Isotop's delta modulator. I thought about using a sg3525 then realized that I need a vco for the type of modulation I want to do even though both would work. I set the SLR up so that I am sending energy packets into a resonant circuit at a rate of my choice then rectifying and filtering it to average the total. it will be hard to tell how well it will track with a dynamic load like a tesla.there still is a lot of tweaking. A thought, since the modulator what ever type it is, is supposed to ramp the current or energy to a tesla. How about instead of treating the QCW as two separate systems treat it as one system and make it closed loop. Would it be possible to enter a voltage at the front end and make the secondary current track so a sparks current can be controlled in a more linear manor? would it mater?
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QCW coil
