HFSSTC not working well

[toooldforthis]

Hi all,

I am not sure what I am doing wrong here. I built the standard HFSSTC circuit with the capacitor voltage divider gate drive circuit that every one uses. I am seeing some very poor gate waveforms and I am not able to achieve breakout on my secondary. Perhaps I have made a stupid mistake that someone can spot? Too many paracitics somewhere?

I'm obviously not achieving class-e switching here. I must need more loading, however I am not able to achieve breakout. I have tried changing the coupling between primary and secondary which did not help.

Do I have to tune the resonance of the secondary to match the primary side tank? I would like to do it the way Steve Ward did it which is why I made the secondary resonance about twice that of the primary side.

I made the tank cap out of polystyrene. It is 115pF and should survive up to 10kV I would guess.

[Kevindk9]

First off, your secondary coil (60 turns) should not be grounded.  It should be connected right on the corner of your schematic between the primary coil (2.15uh) and the 115ph capacitor.

5nf may be too much gate-source capacitance, i use 2.5nf but it varies from transistor to transistor (don't ask..  ) . Im using SiC mosfets, but the IRFP460's liked ~8nf capacitance.

I think you should make your primary coil tunable, try 5 turns of 12 gauge wire, 3 inches wide and 3 inches tall.  use a wire with an alligator clip to move it up and down the last winding.

I also have my primary coils frequency about half of the secondarys frequency.  I might be wrong here, but the theory is that the plasma flame will reduce the resonant frequency of the secondary coil, the bigger it gets, the lower the frequency drops.  My coil will not start by itself, but if I bring a piece of metal near the secondary coil (reducing its resonant frequency), a flame will break out. As I turn the pot up, I bring the metal (screwdriver) away until it can self sustain the flame. 

So if you still arent getting a break out, you must be way out of tune.

[radbloke]

I made the tank cap out of polystyrene. It is 115pF and should survive up to 10kV I would guess.

Polystyrene is a poor material for RF impulse rated caps and would be lossy or not work at all  Better to use mica or polypropylene film caps such as the CDE series.

[Anders Mikkelsen]

Did anybody successfully make a HFSSTC with a superjunction FET? Your named part number is a superjunction MOSFET, and these behave very differently from conventional MOSFET transistors. At least this is something to look into if the circuit doesn't behave as expected.