Hi everyone. I am afraid i have started something i cant finish. I am trying to make a heavy duty powersuply using a NE555 timer and a set of 25 IRF740 mosfet. The timer uses a 12 V external supply. I plan to pull out about 90 A at 12-15 V. But i run into some problems. The transistors are running at 25 Khz, and the duty sycles is controlled from 50-75%. All of this is tested and working ok. I am useing a homemade transformer made of 1 mm steel plates (50 pieces) with walls of 50x50 mm. On the primary side i use 280 turns of 1.4 mm wire and on the secondary side i use 23 turns of 2 mm wire, 6 of them in paralell. I am useing a 10 A 700 V bridge and a 2200 uF 400 V cap to make DC. The negative side is feed through the transistors. Ok. So the problem. When I start up this supply, some of my transistors break down, and I get 50Hz on the primary side, makeing my fuses blow. I dont know what happens first, 50 HZ or transistor breakdown. I have a theory that the startup current is so high it makes the transistors brake, feeding my transformer with DC, until the cap gives up, feeding the system with 50Hz. If so, how can i prevent this from happen? Any other ideas about whats wrong? Thankful for all answers.