CW coil with terrible gate waves. Need help

[Acid Byte]

Hello!

I am running a cw coil, but running into wild issues on the gate.

So here is my gate drive without running the coil.


But when i turn the power open this starts to happen.


Im running a sstc5 circuit but with ud2.5 gatedrive.
The fet switches are fdl100n50
Im running the coil at approx 260khz
Im currently using 18v gate protection and apply just under 18 volts of gate wave.
Using 6.2 ohms gate resistors with the parallel diode.
My gdt is n30 material and has a 6:4 turn ratio

Have anyone seen this in their builds?
And what might be the cause of this?
I can run the coil at 1kw from mains for 30 seconds with only about 3 to 4 degree change in heat on the bridge.
But somehow im thinking this left over heat is cause by this weird stuff going on.

Greetings jeroen

[T3sl4co1l]

I don't know that that's bad, or an issue.  With all the noise around, I would easily expect to see that from probe effects alone.

Can you show the probe connection?

Tim

[Acid Byte]

Hey tim!
Thanks for responding.
Yeah i can make some pictures after work.
But telling you im measuring the gatewave directly from gate to source with a floating scope ground.

I tried increasing the Rg which took away some heat too.
Lowside gate is running a nice waveform.
Though the highside is looking damped.
Is this normal?

Another shot here with a little less ringing then before with increased Rg


Greeting jeroen

[Mads Barnkob]

But telling you im measuring the gatewave directly from gate to source with a floating scope ground.

A floating ground is the reason for the excessive noise, you either have to isolate your mains supply and ground the negative rails to use the ground clip or invest in a differential probe.

You have a perfect switching gate waveform, so that is properly also true when driving the coil

[Teravolt]

does your mosfet transitioning have any dead time. if your bridge fets are turning on at any time together you can see that type of noise. usaly the UD drivers dont utalize it. if this is happening you will have suring on your line. if you add a lamp in series with your line you might be able to see this. if it were me I would float your mains and earth ground the center tap of your caps if you are using a dublier temoraraly so you can tie your scope to a common at earth ground. usuing a ground lifter adaptor or floteing your scope could be asking for truble.

[Acid Byte]

Thank you all for your response!

A floating ground is the reason for the excessive noise,

So you think its sort of like "regauging" on the switching transistion?
Wich acts out as noise on the scope.

So i did some testing and went upto 17 ohms on the gate. The heat seemed to be less on the bridge.
Also radiate rf was much stronger as the television down stairs didnt like it so much(this was a first for this coil)
Also the switching noise was allot less. Cant it be like cross conduction?
The noise is also there measuring current through ct.


Actually yes i have to buy a mains isolation transformer. But moneys a little tight at the moment.
Then again so far my floating scope hasnt failed on me yet.
I know its not the most ideaal situation. But for now allows me to scope output waves and high side gates.

Heres a picture of the output waveform.


Greetings jeroen