Primary coil connection

[flyglas]

Hello,

since a few months I am building a QCW.
Now I am at the point of connecting the cylindrical primary coil to the inverter and the primary capacitor.
Which end of the primary coils should be connected to the primary capacitor?

The voltage swing at the inverter (h-bridge) is only a few hundred volts, but the voltage swing between the primary coil and the primary coil is much higher. Due to the cylindrical design the complete primary coil is at a equal distance to the secondary coil.

Do you have a hint for me?

[Teravolt]

It looks good to me what is its resonant frequency and what circuit are you going to use to drive it?

[flyglas]

Thanks. The measured secondary resonant frequency is 341 kHz. I aim for 327 kHz primary frequency with a coupling of 0.394.
With this data the upper pole should be around 420 kHz.

The driver is my own design (some circuit parts are from U2.7) with an MAX10 FPGA to drive a h-bridge with 8 pcs. FGH75T65SHD.
I realized a phase-shifted pulse pattern with distribution of the hard switching over all IGBTs and the possibility of freewheeling.
The necessary VHDL state machine for the pulse pattern is ready, but there is much coding work to do.

Do you have an idea for my initial question?

[Mads Barnkob]

The voltage envelope between MMC and primary will have the highest amplitude. So the connection to primary coil from inverter will have the lowest voltage and the other from MMC will have the highest. Now lets say you have just 2kV since its a QCW with a low peak current, that is a rather small difference of +/- 300V in one end and +/- 2000V in the other, when its held up against the topload potential of some 50-100kV.

Helical have not given me problems with flash-overs between primary coil and secondary, but racing sparks / flash-overs to the ground rail, which is caused by the high coupling, so I am a little vary of building new coils with such a high coupling and will most likely use the "U" upsidedown shape of primary to get enough inductance and still keep height/coupling down.

[Hydron]

I don't think the connection makes much of a difference - I would pick whichever is easier the manage mechanically.

My QCW coil will actually have both ends of the primary at HV potential - I have split the MMC into two halves and the primary is between them. It will also use phase shift modulation so I am interested in your results. I currently have a prototype running using the UD3 driver but there is still a lot of work to do!

[flyglas]

Thank you for your answers. As soon as I have a first light, I will open a topic to show my creation.