ZVS driver based DRSSTC with pictures and details - Student project

[CwhyteKGMS]

My students are just ending a unit on LC circuits for Grade 11 Electronics. As a summative project several of the students fell in love with Tesla Coils and decided to create a smaller version of Arc Angel's ZVS-based coil running an old  48.5V ~10A PSU from the 1980s.  The coil is wound on a 1.5" white PVC pipe used for Vacuum systems, so acc. 1.7",  ~1400x of 30ga wire wound to approx 29cm. The ZVS driver is running ~82.5nF for tank capacitance with a 3" dia  1/4" copper refrigerator tubing primary with 6.5 turns tapped to 2.5 turns.  Output is 15-20cm long, sword-like sparks from a small toroid made from an aluminum coke can with a 14ga soil copper wire discharge lead.  The output from this coil lights fluorescent tubes 3m away with ease.  The coil can be run for up to 2 minutes with little heating of the MOSFETS.  Secondary res. freq. is ~700Khz, ~200m winding length and the res.frequency of the ZVS is ~350Khz.
The res. frequency of the secondary drops with the top-load into resonance. Sparks greatly increase as I put a conductor nearby.  They will easily jump to hand a good 20cm above the discharge lead at full power.  Without a grounded object nearby the sparks are 10-15cm in length. 
   

Question - It would be nice if we could interrupt this coil with another MOSFET connected to a PWM driver. I know Arc Angel (of youtube fame) was doing this, but I am unable to get in contact with him  Does anyone have info on doing this? It was suggested that we use a TC4420 MOSFET driver driven by a PWM generator circuit that might work. Does anyone have opinions on this   


Pictures below -     

[davekni]

Looks like a successful project already.

Question - It would be nice if we could interrupt this coil with another MOSFET connected to a PWM driver. I know Arc Angel (of youtube fame) was doing this, but I am unable to get in contact with him  Does anyone have info on doing this? It was suggested that we use a TC4420 MOSFET driver driven by a PWM generator circuit that might work. Does anyone have opinions on this

The closest thread I'm aware of on this forum is about interrupting a ZVS oscillator feeding an HV transformer.  This should be similar.
https://highvoltageforum.net/index.php?topic=1391.msg10563#msg10563

You'll need a FET to switch input current as you mentioned.  Will also need a catch diode after the FET to handle ZVS input inductor current after FET is switched off.  The FET and diode need to be close to the power supply or preferably have a filter capacitor on the input (power supply side) in close proximity.  Minimize wiring inductance of these three parts (filter capacitor, FET, and diode) to minimize FET voltage spike at switch-off.

It will also help to add a power resistor bypassing the FET, so that the OFF state is not fully off.  ZVS oscillator circuits tend to have startup transient issues (high initial current spike from supply, then high voltage spike on ZVS switching FETs).  There are many posts on the forum about ZVS issues (and successes).  A resistor keeping the ZVS weakly oscillating makes startup more gentle.

Good luck!

[CwhyteKGMS]

Thank you for your comment. I was thinking this as well, but never having tried building a ZVS driver-based DRSSTC before, the interrupter points you make seem logical. 

I am more of an SGTC person with large builds to 9" dia coils at 8-10KW input power.  I am just getting back into coiling because it is useful for teaching about LC circuits. My Ss. like building them very much.