I don't understand what is happening.

The goal of the project is to have a DC power supply for four output voltages with isolation from each other. I ran the simulation in pSIM but the output voltages are showing a capacitive discharge with time and I don't understand why they aren't just steady values.

Firstly, is to use open loop control which is shown and then to design a feedback closed loop controller.

 

Can you post the waveform, denoting the time and voltage scales?

From the schematic, there is noting to drive your output MOSFET so it is surprising that there is any output, unless for some reason during power-up the opamp/comparator that drives the MOSFET drives the gate low for a moment. That would charge the filter capacitors on the outputs and then the load resistors would discharge them.

By the way, in real life there will be leakage inductance on the primary so you will probably want to put some sort of snubber on the drain of the output MOSFET.

 

Here are the output waveforms after 10 seconds.

 

It would appear that the MOSFET has only switched once during the interval you are showing.
What is the voltage at the MOSFET switch gate?

 

The triangular wave pulse has switching frequency 20kHz. Here is the waveform for the switching signal voltage at the gate. Is this too high a frequency?

 

I modified the converter to include the snubber and move the position of the MOSFET switch.



The outputs now seem stable.

 

I'm puzzled, but it is great that it seems to be working better.

Is your gate drive only 1 volt P-P?

 

Yes, the switching pulse has 1V peak to peak and 40% duty cycle.

I'm puzzled, but it is great that it seems to be working better.

Do you think there is something else problematic in the circuit?

 

With most real-life MOSFETs, 1 volt is not even enough to get it to conduct even a little bit, let alone saturate under a "reasonable" load. Check the gate drive requirements for your MOSFET and expected drain current and I suggest you make sure you supply more than that much gate drive.