Hi frequency ringing with GDT

[CarldeLevis]

I'm testing a new half-bridge setup with gate drive transformer. I'm getting high frequency ringing on my mosfet gate signal. What worry me the most is the pronounced negative swing transient that is under -30V.

Here it is under load:


Where the probe is connected (isolated by isolation transformer) :

The gate drive transformer (epcos n30) :

The actual schematic (tryed with another cap in serie with the resistor, bigger dc blocking cap, smaller resistance,....)

With minimum input voltage :

With no load at all, only the starting pulse :

Anyway guys, thanks I dont know what to do with this one.

[Mads Barnkob]

Gate resistor have to be mounted directly on the gate leg, not on a PCB with wires going to the MOSFET/IGBT. That little piece of wire could be the source of your ringing.

Generally you need to tidy up your "power tracks", that is pretty far from low inductance and best practices primary circuit layout. I wrote about parasitic inductances in inverters here: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/busbar-and-primary-circuit/

Richie Burnett made an excellent GDT troubleshooting guide: http://www.richieburnett.co.uk/temp/gdt/gdt2.html

[CarldeLevis]

I removed the resistor across the capacitor and replaced the gate resistors from 10R to 22R. At a frequency of 130khz, the switching time are not to bad. Still a little ringing but acceptable. Managed to crank it up to about 140V in light duty mode without blowing anything up. Trying to keep everything short. I'm going to try faster secondary so I will solder the resistor directly to the gate. Thanks a lot
By the way, I there any way to predict what would be the perfect primary for my setup. Last time, I did class E and the best was about one turn. Now with the half-bridge configuration, I've tryed a few thing but I would like to understand what is happening.
And for the damped wave un my unloaded circuit, It put the fet in activity for quite a long time at the end of each interupter cycle. How can I get rid of it ? I would like to audio modulate it but with that wave, I'm going to have problems !

Thanks again

[davekni]

Where the probe is connected (isolated by isolation transformer) :

Some or even most of the ringing may be signal in the FET source lead inductance.  Connect scope ground lead as close to FET body as possible, and route ground lead adjacent probe to minimize loop area (minimize mutual inductance with power wiring).

If I'm seeing images correctly, FET source connection is soldered close to FET body.  That's great.  Best if GDT connection to source doesn't share much if any distance along the wire conducting FET source current.  In other words, solder GDT source wire close to FET, or on FET source lead separately from power (high current) source connection.  Tighter twisting of wires from GDT to FETs and to driver would help as well.  (IMHO, that is more important than moving the resistor to the FET gate lead.)

Reducing half-bridge power ringing will also help.  In case it's helpful, here's my example of low-parasitic-inductance half-bridge construction:
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795

By the way, I there any way to predict what would be the perfect primary for my setup.

There are a couple discussions on this forum about optimum DRSSTC primary impedance.  Haven't thought much about SSTC.  Mads (Kaiser Power Electronics) web pages may have guides.

And for the damped wave un my unloaded circuit, It put the fet in activity for quite a long time at the end of each interupter cycle. How can I get rid of it ? I would like to audio modulate it but with that wave, I'm going to have problems !

I think you are talking about the lower-amplitude ring-down after half-bridge drive ends.  That is not likely a problem.  Have you zoomed-in to see the ring frequency?  May be at high frequency, resonance of primary inductance and FET capacitance.