ZVS DRIVERLESS DRSSTC

[radbloke]

I made a PCB based ZVS oscillator which I’m directly driving the primary coil of a DRSSTC at resonance with resonant primary tank tapped to close to the resonant frequency of the the secondary coil. I originally posted this as an answer to a question regarding ZVS oscillators. It was suggested  by a forum member that this would be a more appropriate place for this post

The primary tank oscillated close to 300kHz using a single 100 nF CDE cap. When 1/2 wave rectified mains from a variac is fed into the positive side, we get great ramped output of several inches. The output shown in the picture is from 55V AC (closer to 40V AC with voltage drop from the small variac) !

Please see posts by  Davekni where’s he’s pushed the power envelope on these type of coils. The main goal with this post is to use the simplified version of the driver at lower power levels to easily build a double resonant driverless coil.

I have worked on this with Jerry Miller of ArcAngel Tesla Coil fame and we have put together a PCB for which I can give you a link to the Gerber files. The PCB has been tested and works great. The primary resonance starts when 12V gate voltage is applied. Then the ramps start when mains is added via a variac. Here is a link:
https://teslascience.wordpress.com/cw-tesla-coil-running-directly-off-a-zvs-driver/

For those interested in the PCB and Gerber files along with BOM:

Gerber files for making your own board from JLCPCB can be downloaded here:
https://drive.google.com/file/d/1LT99xMJM3GokFdwl1bjIvzCh1FI0mYt3/view?usp=sharing

Bill of materials (BOM) can be downloaded here:
https://drive.google.com/file/d/1e315jqtna1GlXcUyhYC_6J7t62630wV5/view?usp=sharing

The two IRFP260 MOSFETs are not included in the BOM and would have to be manually added. Screw terminal mounts make for easy replacement. I’m sure this would also work with other MOSFETS such as the IRFP460 (500V 20A). On testing the boards, the current in the primary was too high using a 150 nF capacitor as the primary only had 2 turns to resonate the secondary. I soldered up a second board and used  a 100 nF capacitor instead. The results were much better with less current and capacitor/wire heating and the primary turns were closer to 3.25.

We plan shortly to replace the IRFP260N MOSFETs (200V at 50A) with the more robust
APT94N65B2C3G Microsemi N-Ch Power MOSFET 650V 94A 30mOhm 833W TO-247 (T-MAX)
These have identical switching characteristics to the IRFP260N MOSFETs but with much more power handling.

It’s interesting to note that the ZVS DRSSTC Tesla coils are full bridge operation.

[davekni]

I originally posted this as an answer to a question regarding ZVS oscillators. It was suggested  by a forum member that this would be a more appropriate place for this post

Thank you for this new topic.  More appropriate place to discuss your coil.  (ZVS circuit is appropriate for previous thread.  Just not a good place for continuing discussion about your coil.)

I made a PCB based ZVS oscillator which I’m directly driving the primary coil of a DRSSTC at resonance with resonant primary tank tapped to close to the resonant frequency of the the secondary coil.

Any scope measurements of operation?  Starting at equal primary and secondary frequencies, I'd guess operation is at upper pole frequency.  That would solve some of the issues I had with my lower frequency 8kW coil.  Avoids rapid frequency transition between poles.  Also moves farther away from tuned as arc grows.  That should minimizing the issue I have due to arc voltage being roughly constant.  As your coil's arc grows and input voltage increases, tuning change counteracts rapid input power increase.  Would be fascinating to see this in scope captures.
A JavaTC model of your coil would also be of interest if you get time to play with that.
BTW, for my ZVS coil, running upper pole frequency required more resonant energy for given power level.  That's not likely a significant concern for lower power coils.

Please see posts by  Davekni where’s he’s pushed the power envelope on these type of coils. The main goal with this post us to use the simplified version of the driver at lower power levels to easily build a double resonant driverless coil.

Great info and circuits.  There's always interest here for more accessible projects than some of my obsessions.  And my version is much lower frequency so does not make such nice sword-like arcs.  BTW, a direct link in case anyone wants to check out my low frequency version:
    https://highvoltageforum.net/index.php?topic=3057.msg21792#msg21792

The primary resonance starts when 12V gate voltage is applied. Then the ramps start when mains is added via a variac.

Yes, that is a key to reliable ZVS oscillator startup.  It's the technique I use in most of my recent ZVS projects.  It is possible to start by switching gate drive instead (as in my 8kW coil), but that may not work in every case.

We plan shortly to replace the IRFP260N MOSFETs (200V at 50A) with the more robust
APT94N65B2C3G Microsemi N-Ch Power MOSFET 650V 94A 30mOhm 833W TO-247 (T-MAX)
These have identical switching characteristics to the IRFP260N MOSFETs but with much more power handling.

I'm guessing you may find issues with this FET replacement.  Gate capacitance (gate_charge / gate_voltage_swing) is about 3x higher.  Will require 3x gate current to switch at same speed.  (Gate drive current is why I added emitter-follower buffers to gate drive in my 8kW coil.)
On-resistance is ~35% lower.  This may allow 1.5x current.  On-resistance and Vgs threshold are usually what limits ZVS current rather than FET's rated current.  At high current, FET Vds-on increases enough that diode to opposite gate can't keep opposite FET off.  (Which is why I added level-shift in series with gates and increased gate supply voltage by same amount.)
This on-resistance and Vgs threshold issue isn't too bad when FETs are cold (room temperature) as in low duty cycle staccato operation.  As FETs heat up, Vgs threshold drops and on-resistance increases.  Both make issue worse.  That's one reason you'll find large fans over higher power ZVS induction heater units from China.  Only way to keep them running is to keep FETs well below max rated temperature.

[曹靖]

It’s interesting to note that the ZVS DRSSTC Tesla coils are full bridge operation.

A few years ago, I also tried ZVS direct push SSTC. What I did was in continuous mode, but the effect was not good. In the case of secondary faults, the frequency always shifted, causing serious ZVS failure. The hard shutdown generated a lot of heat

[radbloke]

Yes exactly. I saw a frequency pole shift in 2 instances:

1) if the primary was tuned to the same frequency as the unloaded secondary, then with loading of the secondary and spark growth, there was a sudden jump in frequency of the system to a new higher frequency with little or no plasma output. This jump was hard on the switches. If the primary was detuned a good bit with respect to the secondary to a lower frequency then with spark growth, the frequency jump did not occur.

2)  grounding the secondary output by touching the spark resulted in rapid pole jump to a higher frequency of the entire system. 

[davekni]

A few years ago, I also tried ZVS direct push SSTC.

ZVS coils are generally dual-resonant (DRSSTC).  Unlike an SSTC, ZVS circuit operates by resonance of circuit capacitor (MMC) with primary coil.  If secondary frequency is far enough above primary, it could be insignificant.  Normal bridge-driven SSTCs run at secondary frequency.  A ZVS driven coil will run at primary frequency if secondary frequency is high enough to have insignificant effect on primary.

In the case of secondary faults, the frequency always shifted, causing serious ZVS failure.

Could be similar issue to what I saw with my 8kW ZVS coil when switching pole frequencies.  Or could be just a frequency increase or Q decrease enough to cause issues.

Images suggest a reasonably high frequency coil.  Gate pull-up resistors look to be ~450 ohms.  I'd guess switching times are very marginal (too slow) for high frequency.  Perhaps a frequency shift was just enough (if towards higher frequency) to make switching times become more catastrophic.

Yes exactly. I saw a frequency pole shift in 2 instances:

1) if the primary was tuned to the same frequency as the unloaded secondary, then with loading of the secondary and spark growth, there was a sudden jump in frequency of the system to a new higher frequency with little or no plasma output. This jump was hard on the switches. If the primary was detuned a good bit with respect to the secondary to a lower frequency then with spark growth, the frequency jump did not occur.

Thank you for these details!  I'd mistakenly thought you continued to keep primary and secondary tuned to same frequency.  Now I understand that you are running at lower pole frequency as my 8kw coil does.  Would be interesting to see a scope plot of line current vs. time to see if it looks anything like my measurements.  Perhaps your variac is acting as a ballast much like the resistor I added to line input.

Yes, frequency jump causes a jump in load (drop in Q).  Makes a spike in current draw or could even cause oscillation to drop out if Q gets low enough (or current is high enough that FET Vds is too high to keep opposite FET off).

2)  grounding the secondary output by touching the spark resulted in rapid pole jump to a higher frequency of the entire system.

Shorted-secondary frequency is between lower and upper pole frequencies.  It is an increase from lower pole frequency operation.

[radbloke]

All very helpful info! I will try and get some scope shots in the next few days before during and after a pole jump event.