core saturation problem in DC/DC converter circuit

[Mithun_K_Das]

I was trying to make a DC/DC converter with the circuit from :

I tried with some modification in transformer, ie: I tried with windings: Primary:secondary: 4T+4T/50T+50T, 5T+5T/50T+50T with some different wire gazes. But in first case, no load current is ok[300mA], but load current is doubled!!!! ie, with 40W load current is 8-12A (in 12V) where it should take around 3.3A. This excess current produces a huge heat and also damages MOSFETs(75n75*4*2)

Fortunately, in second case this is solved. it takes 3.4A at 40W load in the 12V terminal. Seems like no excess loss. After then I made another copy of my PCB (Same as before, almost no change). But unfortunately the second transformer [5T+5T/50T+50T] is taking 8A no load current!!!!! I don't know why? Found no fault, then what may the reason?

Actually what is the problem then? Is this core saturation??

 

[Mithun_K_Das]

I found another problem. When transformer is not connected the oscillator output wave-shape is ok [square wave] but whenever the transformer is being connected in the circuit, the wave shape disorders a lot. just like a noise. But this problem is a new creation for this circuit because I used this circuit many times before, no such problem found then. So what is the source of problem???