Problem with SMPS design for high power ultrasonic

[George]

I'm trying to design a SMPS do power an ultrasonic transducer from the mains. I'm going to have rectified mains converted to around 200Vac @ 25kHz. I've wound a transformer 200 turn CT on primary (guessed) and tried to make a push pull supply. Positive going to centre tap and 800V FET on each side to ground. I made a 555 oscillator and driving FETs with an IR2184 driver, this pulls each one high and low, gives some dead time and has collision ovoidance etc.

Note attachment no load - it's ok - I'm not sure why the huge leading edge spike.

When I turn it on (I'm using low voltage to start with) it goes all swimmingly drawing about 30mA with transducer load, although I'll have to try and keep that big voltage spike in check, - note initial attachment. Then I notice the current start to climb and the wave form changes (the wave form is transformer output with piezo attached) it holds low for a while then begins to climb quite quickly, in 10 seconds or so it's pulling 1 amp and notice the output wave form drop and change. - Just as well I didn't plug it in the mains!!!

I'm assuming my oscillator output isn't giving identical times for each FET and/or I've misscounted a winding or two when I made my transformer so it may be magnetically out of balance. Is it likely what I'm seeing is the core of my Xfmr saturating? If so - how do I fix it? How come the big spike is on one side when I turn the circuit on, then migrates to the other side?

Are there better ways to accomplish what I'm wanting to do? And how do I snub that nasty spike?

I've been working on this for days and it's really annoying me so any help would be very much appreciated. My biggest issue is that I'm a mechanical engineer so my knowledge base on this sort of thing is quite limited.

Many thanks in advance for any help/suggestions