UCC256304: UCC256304

The below are from your message. My question is if the top one is your lower MOSFET Vgs, what is the lower one waveform, and where did you capture your waveform? Based on your reply, the waveform is measured on VT1, so it sounds to me that is Vds of VT1, is it right? Please confirm or clarify which is which and where each did your measure? 

No, I’ve measured Vgs and Vds of VT2 in reference to VT2 source, i.e. oscilloscope ground was connected to VT2 source, yellow probe to VT2 gate and green probe to VT2 drain. It surely opens.

From my point of view it looks pretty OK with VT2 as Vgs high level does drive Vt2 drain low

I have more concerns about VT1 as i see voltage overshot on its drain at the rising edge of Vgs1.

And most of all i wonder how can both Vgs pulses (for VT1 and VT2) come at the same time!..

I need you to clarify your descriptions. Can you plot VT2 with its Vgs and Vds on a same screen. Based on your previous messages, you described, Vds of VT2 was not low when Vgs was high. That means VT2 did not work correctly and you cannot conclude Vgs of VT1 was on. 

So please plot Vgs and Vds of VT2 in the same screen to clarify your description.

This is the plot you asked for. Here's Vgs is yellow and Vds is green

The waveforms cannot conclude Vgs of VT1 is on at the same time. I need you to make another test and to show the three voltage waveforms in a same screen. Please capture share these three waveforms.

90Vdc

Here you are!

Yellow is Vds

Blue is Vblk

Red is Vgs

It looks your circuit missing a connection, UCC256304 HS pin should be connected like the below. But your circuit does not have this connection. So when VT2 Vds is low, VT1 source pulled down and causing VT1 to turn on. So connect HS as below should solve the issue.