Dual Class E Reverse Engineering/Engineering

[YSPACE Labs]

I found that a lot of the Aliexpress Tesla coils use a topology known as Dual Class E or Push-pull Class E. It's similar to a Push-pull coil (like Tefa's SSTC4), but there is a capacitor in series with the non-tapped primary, two chokes, and two drain-source capacitors. It's basically two single mosfet class E stages glued together. I've seen some DIY coils using this topology (Magneticitist, Ethan's Lab, and some others (none call it dual class E (Magneticitist calls his DCE coil a push pull))), but there are virtually no details on how to tune it. There are component values, but without knowing primary inductance, it's hard to know how it's tuned.

The advantages of this topology seem to be that it's dual resonant (and it produces big sparks for its input voltage and secondary windings number), doesn't require any special floating gate drive, can run at very high frequencies (possibly up to 4MHz if my JavaTC calculations for the secondary are correct), and is pretty tolerant of things that happen to it. I made a PCB (will upload picture later) to experiment with driving low voltage push pull coils like this.

I'm not sure how exactly to tune it, but it seems like either you tune the series cap to resonate with the primary, tune the series cap so that its capacitance in series with one of the DS caps resonates with the primary, or to use the Class E equations and change the DS caps so that it operates in ZVS. Also, according to Magneticitist, the "dirty double hump" waveform is good for getting large sparks (so maybe it should be detuned in that way).

So I just want to see if anyone more experienced or with one of the aliexpress coils can help me research and develop this LVSSTC (Low Voltage Solid State Tesla Coil) design.

[YSPACE Labs]

Here are a few images of the topology and other things:
PCB design
Schematic of topology

[davekni]

Would you call the typical cheap ZVS induction heater circuits class-E?  I don't know the exact definition of class-E.  If gate drive is 50% duty cycle, could be run almost exactly like these ZVS circuits.  Capacitors from drains to ground behaves the same as one capacitor drain-to-drain as long as gate duty cycle is 50% or higher.  (Same for tuning.  Does change FET current, as ground caps add resonant current to FETs.)

Experimenting in simulation (LTSpice or other) is often easier than modifying real parts on a real board.

If you complete a schematic including gate drive, please post.  That could be interesting.

[YSPACE Labs]

OK. Here is the actual driving portion of my board: . This is somewhat pulled off a circuit I found on Aliexpress (lol) (that schematic is what sparked my interest). Based off my sims, the circuit can function in ZVS class E mode and produce some really high primary currents. The gate drive is really simple since it doesn't need any transformers or bootstrapping to drive a floating MOSFET. It probably doesn't even need the resistors and diodes going to the gates.

Those cheap ZVS induction heaters are not Class E. They use a parallel resonant cap and for whatever reason, it's not as good as having a series resonant cap (I read this on a Discord server, and the person who said it said that it was from the HV forum). If anything, those ZVS drivers are something like Push pull Class D^-1 (Based on a slideshow from Berkeley ECE that I found when trying to figure out Dual Class E). Based on my knowledge (I am not an EE), the chokes and DS caps act as "pumps" that smooth and feed the primary through the impedance matching series cap. I could be completely wrong on this theory though. Richie (https://www.richieburnett.co.uk/tesla.shtml) knows something about this topology and calls it Double-ended Class E.

I still have to run some more simulations on it, but I think you can just use the class E equations to tune it (the DS caps need a little adjustment in the sim).

Here's the Aliexpress schematic:

[davekni]

OK. Here is the actual driving portion of my board:

The two U8 inputs labeled "IN" connect to an antenna?  Or to a CT?  Either way, presuming they are wired together, then the gate signals are complimentary 50% duty cycle.  That answers my main question.

It probably doesn't even need the resistors and diodes going to the gates.

Agree, I can't think of any value for the resistor//diode in series with gate.  No dead time is needed for ZVS circuits (whether class E or push-pull or whatever).  If the diodes were reversed for faster FET turn-on, that would guarantee no time when both FETs are off.

Those cheap ZVS induction heaters are not Class E. They use a parallel resonant cap

If FET gate diodes were reversed to avoid any time when both FETs are off, then this circuit would behave the same if C2 and C3 (DS caps) were replaced with a single cap between drains (as in ZVS inductrion heaters).  Either way, I'd consider this circuit half way between series and parallel resonant.  The primary resonant circuit is primary inductance and C1 in series with C2/3.  Drive is in parallel with C2/3, so not in series with the entire resonant circuit, nor in parallel with entire resonant circuit.

I'm not sure how exactly to tune it, but it seems like either you tune the series cap to resonate with the primary, tune the series cap so that its capacitance in series with one of the DS caps resonates with the primary, or to use the Class E equations and change the DS caps so that it operates in ZVS.

I think above three options accurately describe the range possible for this circuit.  Best efficiency should be with your second option of series cap (C1) in series with C2/C3.  Then FET body diodes don't conduct.  Each drain waveform is a half-cycle while off.  For fixed drive frequency, if C is reduced a bit towards your first option of resonant with just series cap C1, drain waveforms will get shorter and higher, looking more like classical class E.  If C1 is reduced further until it alone is resonant with primary, then drain half-cycles would become infinitely narrow and high (if not for real limits of parts).  If C is increased above your middle option, then drain half-cycles don't complete before FET turns on again, causing excess FET stress and power dissipation.  So likely a bit towards class E waveforms is ideal for margin.

They use a parallel resonant cap and for whatever reason, it's not as good as having a series resonant cap

The big advantage of series resonant is higher coil voltage for a given input voltage.  I'm guessing a parallel drive would work with a low inductance single-turn primary (ie. copper sheet bent into a turn) and parallel plates/sheets of copper connecting driver to primary.  Something I hope to try some day.