MOSFET gate transformer noise issue
I think you may be exceeding what is known as the volt-microsecond limit for the transformer. Basically this figure tells you how long you can apply a pulse of a certain voltage before the voltage reverses. Real transformers will saturate and the V.\$\mu s\$ limit tells you, in practical terms how far you can push the transformer.

The waveform initially looks good on the rising edge - it attains a peak of 4 or 5 volts (it looks that way from your pictures) but after about 10 us it goes wrong - this means your transformer is probably rated as having a volt-microsecond product of about 50.

Other problems with the circuit are: -


You need more gate drive voltage to adequately and efficiently turn the mosfet on
As you vary mark-space ratio, the peak voltage seen by the gate (from the transformer) will vary considerably - you need to have a clamp circuit on the secondary that keeps the gate drive at a decent level for varying mark-space.
I don't think that transformer is man-enough to provide the current needed by the gate to efficiently switch the mosfet without incurring heavy losses.
The transformer drive circuitry also isn't powerful enough to drive a transformer should you have a transformer (and clamp) that would fit the bill efficiency-wise.


Here is a helpful (in parts) article on the subject. And here is a helpful guide on designing transformer circuits for driving mosfets: -



You can probably see in one of the diagrams (fig 3) the problem you are getting.