{987} why ntc is used? 

great point. when it is cold, at that time it provides resistance and that is the start up time of the capacitor charging when capacitor is charged at a specific level, after that there is not need to limit the current until that time the substrate temperature of the thermistor is increased and its resistance is dropped to allow full rated current (its negligible resistance value remains in circuit, but that doesn't effect on the circuit efficiency)

in that case the fuse will burn out immediately after connecting power

Thanks 🙏

Yes Negative temperature coefficient resistance or thermistor

Broken solder joints or cracks in circuit board? Checking the ic with a meter and pushing down on the ic pins can reveal a loose solder joint on the ic where it is soldered onto the circuit board. If you can't see cracks around the pins with a magnifying glass, try to soldering around all the pins anyway and it might fix cracks in the solder. Be very careful when checking with a meter. VERY easy to short between pins with the meter test leads and destroy the ic! A short (bad capacitor or mosfet) or other problem will cause a fault and the ic will go into a loop and keep trying to reset itself. Check output voltage with a volt meter and see if there's a fluctuation in output voltage and that may show that. Watch Haseeb's MANY smps videos and he'll show this. Good Luck!