Hello,
I am trying to make DC - DC Converter using SG3525 ic. which circuit diagram i mentioned. its work perfectly when the load is under 60W, when i increase load. transformer create noisy sound and waveform of mosfet gate is change ( frequency increase and waveform is unstable). i use a ferrite core transformer which i take from computer power supply. its a EE type core which core area is 0.785 cm2. transformer calculation is attached. primary wire is 0.6mm *10 wire and secoundry side is 0.45mm wire *1. please tell me what changes i need to increase the output power to 350W. i use irf3205 Mosfet instead of irfz44.

 

I would increase the size of the capacitor on the battery and add ceramic capacitors. I would add the rectifier diode in the opposite direction after the rectifier bridge. If you are very interested in increasing the power of this circuit, then I would advise you to simulate this circuit using LTspice. Together with my library collection, you can simulate this circuit, including the ferrite core. It will also be possible to see if the ferrite core is saturated and the power of dispersion of each element, including the core. That's not difficult. There is everything in my collection for this. And I am ready to help.
See http://bordodynov.ltwiki.org