First of all, you must specify the output power. Then, we can figure out the expected current draw.
What is the expected current draw? Is 1.4A much much higher than the expected current draw? Which MOSFET are you using?
Your MOSFET could heat up excessively due to spikes when switching off or due to incorrect gate drive or excessive voltage stress when operating or other reasons.
Which topology are you using?
It would be easier to help you if you uploaded the circuit diagram and provided circuit specifications.