This is my most complex project to date! I Started this project after seeing Steve Ward's brilliant design at the 2010 WWT. As far as I know I was the first to duplicate Steve Ward's results.
My design was based heavily on Steve's original coil specs. I did this for two reasons, one I happen to have the secondary and primary on hand and also to make my first attempt some what familiar to a working system, considering there were no other ones to base my design off of at the time.
The QCW works on a very similar mode of operation to what the VTTC dose. The system power supply is a ramp or half sine (in the case of a VTTC) and this is fed to a high Z high fres Tesla coil usually in the order of 20 ohms+ and 300KHz+ This coupled with the slow ramp up time in the mS range give the sword like appearance of the sparks.
The QCW can be broken into 3 main parts.
The modulator is one of the most important parts of this entire build. My modulator is a class D delta modulator. The class D amplifier is a half or full bridge feeding a low pass LC filter. The bridge is drive with a PWM signal that chops up it's 380VDC bus and send that through it's low pass LC filter. The filter gets rid of all the high frequency switching hash and leaves only the average voltage of the PWM. The output LC has a low impedance making it's slew rate rather fast giving the converter the ability to change it's output voltage level in a matter of uS.
The control method I use to drive the class D is known as delta modulation. Delta modulation is a much better control method than a proportional control loop. This is because it's stable under almost any condition. Unlike a proportional controller that can oscillate out of control or have low bandwidth if not tuned right. Delta modulation is also referred to as Hysteresis Control or Bang Bang control.
The control loop has an integrating op amp that takes the error signal and integrates it before feeding it to a comparator to square it up and turns it into a suitable signal for gate drive. At low power the switching frequency will be low, but as power increases the switching will speed up to compensate and add more power to the output. The faster the op amp integrates the signal the faster the overall the switching speed will be, so the integration speed must be set correctly to suit the output filter and switches used for the half bridge.
The output LC is another part of the system that needs special attention. Keep in mind that because there is no bulk capacitance for the main inverter to pull from the peak current that will be in the tank LC of the Tesla coil will also be pulled from the modulator. This current can be as high as 250Apk in extreme cases. So the half bridge switches and inductor have to be rated to handle the peak tank current. My QCW doesn't run any higher than 120Apk, so the output inductor of the class D had to not saturate at 120Apk or less. It also had to have an inductance of 250uH to match up with the 10uF C to make a low pass filter with a pole at 3KHz. (switching freq ~20KHz). For this I used a stacked ferrite E core core with a 3/8 inch air gap.
Ok so the modulator basically takes the 380VDC bus that we create for it from a large storage cap (this should be a big cap say 10mF) and converts it to a signal of our choice that can swing from 0 to 380VDC. The signal that works best for long strait sparks in the QCW is a long linear ramp 5mS to 20mS long with a 2mS to 5mS ramp down time. The ramp down is there to bring the voltage back down slowly after a burst so that you don't get a large "pop" in the output. I also usually bring the voltage back down to 30VDC and float it there so that the DRSSTC drive can start up oscillating correctly at the beginning of a burst. It's very important that there is not a large voltage sitting on the inverter input when it starts up because this can blow the inverter to kingdom come.
The left photo is on a resistive test load, yellow is the inverter output, and the blue is the half bridge switching waveform. Notice how the frequency gets higher with higher output power. Right photo is the QCW in operation, yellow is inverter output notice how it's floating at 40VDC and blue is primary current about 28Apk in this case ( that's about 1 foot of sparks maybe 2).
On the left is the class D delta modulator in all it's glory driving a resistive test load. On the right is the delta modulator wired into the system in a lash up setup.
High Z Tesla Coil resonator
Well the hardest part was getting the modulator working. It's one tough SMPS to get working. The other big thing that sets the QCW apart is it's high Z high frequency tank. If you have ever done a VTTC this should be nothing new. The idea with the QCW is to have a high impedance tank circuit that helps keep the primary current low, this is essential because the pulse length is so long 25mS in some cases. The high frequency is important because it is required to keep branching to a minimum. Fres <300KHz tend to branch more while >300KHz tend to stay strait, getting straighter as the freq increases.
The primary configuration for my system was based on the tank capacitor I had on hand. The tank cap I used was a 10nF 10Kv MICA given to me byDr.Spark (Thanks Dr. Spark!) Unlike a regular DRSSTC the peak current is not all that high and as a result tank voltages are also rather low. The only rating that has to be carefully looked at is the RMS current handling rating of the capacitor. This is very important because of the high duty cycles the QCW runs at.
I have tried many different types of tank capacitor in my QCW and I will give the results of my experimentation with them:
The Teflon -> They work and they work HOT!!! They suck plain and simple... (but they do work... sorta...)
The primary was designed for high coupling with the secondary. Because this system has very low output voltage (<70Kv in most cases) the primary and secondary can be coupled very tightly without flash over. This results in two things, the upper pole of the secondary is moved way up and the energy transfer is very fast. The coupling of my system is 0.369K
One other important note for the secondary circuit is the use of a good breakout point. Small wire will quickly melt away leaving molten blobs of wire on your toroid! Thick steel or tungsten should be used if possible because those sparks are hot!
For the most part the this is just a regular DRSSTC driver with primary current feedback and a full or half bridge of IGBTs switching at the zero crossings. There are however a couple things to take note of that impact this significantly.
I also found that a good gate drive setup is requested for reliable operation. I used a large GDT to drive my full bridge of 60N60s.
On the left is the gen one water cooled 60N60 full bridge. On the right is a lashed up test setup.
And now some photos of the QCW in action!
This project is still a work in progress and as being so will be updated as work progress on it. Some things that I will be trying and/or implementing in the future:
I want to give a big thanks to all the people that have helped me along the way with this project: