I found this simple PSU design on the web. I tried to use it but already 5 MOSFETs and 2 npn transistors are blown. I use IRF840 and BC546 or KSD1616AGBU transistors and fixed value resistors in place of VR1. The load is about a 60 Ohm resistance wire and i mesaured the current which is 4 Amps so it's not limited. The voltage is somewhere between 250 and 300 Volts. When the current rises on R2 it's supposed open Q2 transistor that would set the gate and source of the mosfet equal hence it would close it down but it doesn't happen. After the failure i measured the mosfet and the source drain is shorted (I assume the diode is gone) but i'm no sure what's going on. Can anyone help me what may possibly go wrong or just recommend a proper circuit that does the job? I searched through the web and found some circuits but they seem to be just a more complex variation of this one using the same current sensing techinque so i'm a little uncertain. Thank you
The reason your mosfets are shorting is simple. You are pushing WAY too much power through them. Yes they are rated for 400 Volts. And yes, they are rated for 10 amps. But their wattage rating is only 125W. At 300 volts and 4 amps, you are trying to get 1200W of power out of this circuit. At best, shunt regulators like this have 60-75% efficiency, so at 1200W out, that little FET is trying to dissipate 300W or so all by itself. Frankly, I'm amazed the whole thing did not go up in smoke. For what you are trying to do, you probably need 4-8 of those mosfets in parallel to drive that kind of load.