MOSFET overheating in LED driver
Designing switched power supplies is not easy. (Not for beginners)

Well, a power transister becomes hot out of two possible reasons:

The current that flows when it switches through multiplied with the voltage that remains between Drain and Source. You can connect an oscilloscope and measure the remaining voltage between drain and source when it is switched through and calulate the current by measuring the the voltage at the resistors RS1 - RS5. Form that you calulate the power that the transistor is consuming. If this power is 1 Watt you already feel the transistor is warm. If it exceeds 3 Watt it will become so hot that you cannot touch it anymore with your finger if there is no cooler.

The other reason for extreme heat (and that is more probable) is that the signal at the gate is not sufficient square. If the voltage rises and falls too slowly you will have a lot of loss in the transistor.

So to answer your question you must provide the oscilloscope signals measured between gate and source and between drain and source and at RS1 - RS5.

If the signal at the gate is OK I suppose that your transformator is not well designed. 

Generally the information you give is very basic. What voltage is DCPOS ? What are you connecting at the output? Does the transistor also become so hot when the output is without load?