calculating power loss startup resistor smps
Hi,
The destruction of the MOSFET could be due to the fact that your core is not gapped, if the core saturates this can happen. An offline flyback transformer core should really be gapped, it is not easy to calculate the optimum gap without the core parameters, which you do not have but you can easily put a gap into it by inserting a thin piece of high melting point plastic sheet between the two halves of the 'E' core at each contacting face, this will put 0.6mm of gap into the core (2 * 0.3mm as it is in two places), normally the gap calculates somewhere between 0.2 and 1mm. Try about 0.6mm as a start, this will help to ensure that there is no core saturation on voltage peaks. (you may need a few more primary turns with a gapped core incidentally as this does lower the efffective Al value of the core). What switching frequency are you using (I calculate 150kHz from your circuit), if this is too high for the core this will have a high core loss and hence get hot, your MOSFET should not really get too hot with the loads that you have on your circuit.
Regards
Bob.