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Messages - Weston

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1
Cool coil! Its always nice to see people coming up with their own controller instead of directly copying the UCD design. I have a FPGA based controller for a QCW coil and I was considering implementing a similar limit on the total IGBT heating, its nice to see that someone else is already doing it.

Why did you choose to use so many paralleled TO-247 IGBTs? Surplus IGBT bricks are pretty cheap nowdays so I assume they would be cost competitive in addition to simplifying the bridge layout.

How much modeling have you done on the the concept of switching in additional capacitance with TRIACs? I would be worried about dv/dt induced turn on in the TRIACs beyond the general added complexity. QCW based coils typically get around the detuning issue by tuning the secondary above the primary and then having the controller run on the upper pole, so the added capacitance from the arc pulls the coil better into tune. 

2
Dual Resonant Solid State Tesla coils / Re: QCW Tank Capacitor Choice
« on: October 07, 2019, 09:50:20 AM »
Good catch, it looks like there are two versions of the datasheet online. I used the one archived on digikey, which was published in 2015 https://media.digikey.com/pdf/Data%20Sheets/TDK%20PDFs/B32641B-B32642B.pdf My values are correct for the 2015 version (table on page 10), you observation is correct for the updated datasheet published in 2018.

I really wonder what went wrong there, with TDK changing the datasheet values so drastically. Either way, the capacitors seem to work well. I wonder how conservative the updated ratings are. The only reason for the change I can think of other than massive error in the original is that the updated datasheet lists the capacitors as automotive qualified, which may have required some derating to meet the more rigorous specifications. If I have time I might have to set up some resonant circuit and take a thermal camera to one of the capacitors.

Given that discrepancy, the B32672L1333K000 (sold on mouser, not digikey) is probably the next best capacitor, costing ~$30 for a similar MMC. Its the same lead spacing and only slightly wider than the B32642B0333J and has a higher maximum temperature and slightly higher RMS current / voltage rating than the values I originally had for the B32642B0333J. 

3
Dual Resonant Solid State Tesla coils / Re: QCW Tank Capacitor Choice
« on: September 29, 2019, 02:02:09 AM »
Doorknob capacitors are aesthetically pleasing and come with nice screw terminals (which I suspect are the real reasons why they are used so often), but there is no need for them in a QCW coil. A MMC of correctly chosen film capacitors is a lot cheaper, allows you to tweak the capacitance value, and is a lot easier to source.

I made a small-ish QCW coil last year and spent some time sorting through digikey for the best QCW MMC capacitors, ultimately compiling this spreadsheet comparing all the relevant capacitors I could find: https://docs.google.com/spreadsheets/d/142e_12Ojj_ahDvq699jDEijQQiF_3dN0BCUUxEYnwjM/edit?usp=sharing

I ended up using B32642B0333J capacitors for my MMC as they were the most cost effective I found https://www.digikey.com/products/en?keywords=B32642B0333J . The whole B32642B series compares pretty favorably to other lines and around 33nf per capacitor seemed to be the sweet spot of maximum reactive power at 400KHz within the voltage / current ratings for most varieties of capacitors I looked at.

The resulting MMC was 30 capacitors, which cost $18.  Its 10KV 10nF and rated for 15Arms with a 10 degree C temp rise based on the datasheet for the caps. Extrapolating from that, you get 30Arms with a 40 degree temp rise. The peak current rating based on the capacitor dv/dt limit is 300A. The 30Arms is a continuous current rating so you would have to be running at a pretty high bps to exceed that.

You can see a picture of the MMC here: https://www.flickr.com/photos/63341195@N08/47089852332/ .  I was using it at 80 Amp tank current with no issues. You would most likely have thermal issues with your switches before having thermal issues with the MMC.

Gerbers for the MMC are here if anyone wants to use it: https://github.com/westonb/cute-qcw . It fits on a 100 x 100mm two layer PCB.


4
Electronic circuits / Re: Si-wafers, ideas?
« on: June 25, 2019, 07:07:57 AM »
Assuming you lack access to a clean room, I have heard that simple bipolar transistors are possible to make at home with a diffusion process. However, actually making any active device is going to be difficult, obtaining wafers is the absolute easiest part of the process.

You can have fun cleaving the wafers. If you apply pressure with a hard point in the right orientation relative to the flat of the wafer you can split the wafer with atomic level precision.

Other than that, silicon has pretty good transmission in the IR region. A thermal camera should be able to see through it and a CO2 laser should be able to somewhat shine through it.





5
Dual Resonant Solid State Tesla coils / Re: cute-qcw Tesla Coil
« on: March 18, 2019, 07:55:52 PM »
Some mechanism is needed to provide phase lead and allow for zero current switching.

In some designs inductive loading is added to the CT so provide phase lead in the analog domain. The UD 3.0 driver adjusts a timer to switch the inverter based on the timing of the last ZCS transition, which is similar to a phased locked loop approach. Other FPGA based controllers have implemented phase lead in the digital domain with a FIR/IIR filter, but this requires more logic resources and a high speed ADC.


The two scope shots are with the same system parameters. I suspect that some noise is causing the PLL to lock on to some other operating frequency that leads to a different voltage distribution on the secondary and leads to the flashover between the middle of the secondary and the primary. I think that the flashover itself is leading to some system parameters changing (secondary effectively partially shorted),  and the PLL tracking this new resonant mode causes the sudden increase in current.

I suspect by altering my PLL loop gain and adding a bit more digital filtering of the ZCS signal I can eliminate this issue, but I will see if I can get higher resolution scope shots showing that frequency the PLL is locking on to.

6
Dual Resonant Solid State Tesla coils / Re: cute-qcw Tesla Coil
« on: March 16, 2019, 08:04:52 PM »
I forgot to say, but the coil uses a phase shifted fullbridge for modulation. Right now I am running at a 10ms burst and hitting a peak tank current of ~60A.


The FPGA logic runs at 240MHz. It may have been due to some other factors which I eliminated in debugging, but I was having issues with PLL stability which I thought may have been due to the limited frequency resolution of the counter generating the gate drive signals, so I bumped it up to 240MHz from 160MHz. 

I think my PLL may occasionally still be mis-locking, causing the coil to operate in some mode that leads to primary - secondary flashover. I managed to capture waveforms of both a normal discharge and a flashover (yellow: inverter voltage, blue: current), when it flashes over you can see that there is a high current pulse, but from the start of the pulse the current is lower, implying that something is wrong with the pulse from the start.

Normal Pulse:

Flashover Pulse:




7
Dual Resonant Solid State Tesla coils / cute-qcw Tesla Coil
« on: March 16, 2019, 06:01:44 AM »
Hi All,

I have been working on a FPGA based qcw tesla coil driver for the past 3 months, documenting photos on flickr and posting all the PCB layouts / code on github. Right now the secondary is a bit big compared to many QCW builds (12" tall), but I intend for the coil to be small eventually, so I called the project cute-qcw. The end goal is a compact version of the coil to mount on the front of a bicycle for Burning Man.

I had first light earlier this week and I ramped the coil up to full power for the first time earlier today.

Now that everything works I figured its time to share the project.

The secondary is 12" tall and I am running at the upper pole of ~390KHz, with the secondary tuned above the primary. This puts the gain of the lower pole higher than the upper pole, which can make it difficult to drive with a direct feedback controller. My driver uses a digital PLL and forces ~20 RF cycles at a fixed frequency at the start of the burst to lock on to the upper pole.

Here is a photo to catch your attention:


Github repo is here: https://github.com/westonb/cute-qcw
More photos here: https://www.flickr.com/photos/63341195@N08/sets/72157678478822788/with/47390843571/

I am going to be documenting everything a bit better on github in the next few weeks. Please ask questions if you have any.


8
So this controller regulates the average tank current by skipping pulses? Is this for QCW operation?

Do you have any photos or waveforms of the controller in operation? In the past, people have talked about using a pulse skipping controller for QCW coils but the consensus was that loaded Q is too low to effectively regulate the tank current.

I see a crystal oscillator on the board, are you using the FPGA to control the phase lag digitally to get more accurate zero voltage switching?

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