This is the subject that kills more QCW drivers than any other, and almost all of it is about not connecting two things that look like they should be connected.
Three grounds in the gate drive
- GND2 is the high side device's emitter. It floats with the device. Do not join it to PGND.
- PGND is the low side's emitter, which is fixed.
- GND1 is logic ground. Join it to PGND at one star point, Kelvin.
The point of the star is that power currents should have no physical path through the logic ground.
The buck's gate supply, again
Grounding the coil itself
- Not to mains earth. Returning RF current through the house wiring is both interference and a hazard.
- A counterpoise: a large metal sheet under the coil. Roofing aluminium beats foil or mesh because it does not crumple. Round the corners or they corona.
- A short wire to it. The inductance of a long one destroys the point of having it. And not run alongside other objects, because the secondary will fire into them.
- The bottom of the secondary goes to both the counterpoise and the strike ring.
A metal frame under the primary is a shorted turn
An anodised aluminium frame under the primary works as a one-turn secondary. The anodising is insulation, the arc punches through it, and then the frame starts drawing kilowatts and the steel bolts melt.
The cure is nylon bushes and washers, Kapton, spacers, so that each upper rail is grounded at one end only. At kiloamps in the primary, 150 V per side appears easily and two millimetres of gap is not enough. Run the strike rail to RF ground on its own wire rather than through the frame.
And when the arc hits ground, a QCW does the opposite
On an ordinary DRSSTC a strike to ground is a low resistance short across the secondary, and the primary current slams into its limit.
Either way the topload voltage collapses in 100 to 250 nanoseconds, the arc takes the load off the secondary, and the primary rings up, which is why the overcurrent protection usually fires after the event rather than at it.
The earth symbol on the department diagrams answers for itself if you point at it. What it is not connected to is the more important half.