So I've been trying to simulate a flyback converter with the full load condition of 10v, 10A (100W) at the output at fs = 80kHz. The design equations I used were from "Pulse-Width Modulated DC-DC Converters" by M. Kazimierczuk. Given that my input range is from 180Vrms to 230Vrms, I wanted the flyback to regulate the output at 10V from 0 to 10A. My simulation circuit is shown below. My output voltage is shown below for both 1ohm load and 100ohm load. 1ohm is to simulate the 10A output while 100ohm is to simulate a much lower load (0.1A). Since V1 is supposedly from the controller IC and there is no feedback here yet, it is an open loop system. I basically adjusted the duty cycle of Q1 gate signal to get my desired output. 325v at the input is assumed to come from the previous rectifier stage with 230Vrms input (high line). At full load (10A), duty cycle is around 30% to achieve voltage regulation (10v). But at 0.1A output, I can't seem to regulate the output, even if I drop the duty cycle as much as I can. The output waveform I've shown for 0.1A has a gate signal for Q1 with 5% duty cycle.
L2 was the magnetizing inductance, and since I want to design the flyback in DCM, I calculated that Lmax = 546.4uH. Anything higher than this can possibly make the flyback operate in CCM. So I chose Lm = 500uH. L1 was just there to simulate some leakage inductance.
The number of turns n = N1/N2 = 16.78. So I chose 17 in this case. The mosfet and diode are chosen such that their ratings are higher than my calculated max current and voltage across them.
Any suggestions on where do I start to fix this?