why Boost converter output voltage decreases at high power?
At a switching frequency of 15 kHz, you need to put

$$\frac{1000 W}{15 kHz} = 66.7 mJ$$

of energy into the inductor on every switching cycle. With a 4 mH inductor, this would be a peak current of

$$\sqrt{\frac{2 \cdot 66.7 mJ}{4 mH}} = 5.77 A$$

However, when you apply 120V to that inductor, the current ramps at a rate of

$$\frac{120 V}{4 mH} = 30 A/ms$$

but you're only giving it

$$\frac{0.68}{15 kHz} = 45.3 \mu s$$

to charge, which only lets it get to

$$45.3 \mu s * 30 A/ms = 1.36 A$$

Therefore, with or without feedback, this design is incapable of converting 1000W.

BTW, at these power levels, I would be looking at building a multi-phase converter, with 3 or 4 sets of coils and switches. They all operate at the same duty cycle, but with staggered timing. This makes the input and output ripple a lot easier to deal with.



EDIT: The above analysis is for DCM (discontinuous conduction mode) only. The converter can be made to work with those parameters in CCM (continuous conduction mode), as shown by the simulation below. (If you run it, note that it takes a few hundred milliseconds for the startup transient to die out. I added R2 to help tame that.)







simulate this circuit &ndash; Schematic created using CircuitLab

The key thing to note here is that the peak current in the coil is almost 13 A, so its saturation current rating must be comfortably above that. The switch and the diode need to be able to handle this current, as well.