smps topologies
So did you try to build the transformer as I described? Are you sure the winding starts are correct?
Which voltage goes up to 50V? The voltage at pin 7?
It sounds like the feedback loop is not working. Did you check the output voltage?
Speaking of the loop, the optocoupler is not pulled up by anything other than the IC's pin 1. Ad a 2.7kohm resistor from pin 1 to pin 8. Somwehow I missed that the first time I checked the schematic.
Also, remove that cap from pin 1 to GND, it slows down the loop too much. The loop should be made stable only by the components on the TL431.
Another thing: if you think the IC blows because the voltage at pin 7 gets too high, then temporarily disconnect the feedback winding and power the IC from a lab supply. Make sure you start at about 17-18V and then reduce it to 12V. With the IC powered this way it will not blow up, but the MOSFET still might, if the loop is open.
Until you get this P/S working, it's also a good idea to temporarily limit the duty-cycle. Change the timing components (see datasheet) such that the max duty-cycle is about 60%. That should better protect the MOSFET.
If you have a variac, then it's a good idea to vary the input voltage slowly while watching the waveform at the drain. If you can, please attach waveforms. I can then comment on those. It's very hard for me, not seeing the actual circuit or waveforms, to provide much advice.