Step up SMPS with unrealistic voltage output with a 100W LED COB load
Your not paying attention to the 'dead-time' in the transformer. As the pulse width increases the OFF time decreases, thus more output voltage and current. What you need to do is adjust R2 or R3 so that a usable maximum brightness is achieved-but no more than that. Your paying a penalty as it is by using pull-up resistors which slow down the rise time at the gate of the mosfet, causing it to be less efficient as a switch. The transformer core MUST have a minimum OFF time so the magnetic field can collapse before the next drive pulse. Maximum safe ON time is likely about 95%, but your current design may push it close to 99% or more. Chances are the transformer may be getting warm as well, yet it can run cool while the mosfet gets very hot. With no load the transformer can have large eddy currents, so overheating of the mosfet is no surprize. One thing that works for me is to find out the voltage across the output capacitor when LED's are at maximum brightness, then insert a string of 24 to 50 volt 5W zener diodes across the capacitor so that if no load is present, the zener diodes act as a dummy load and clamp the voltage to just 5 or 10 volts higher than if the LED's were hooked up. This way the circuit always has a load, either the LED's or the diode string.