A 5V, 20A flyback will require extreme attention regarding the secondary layout, with parallel sync rectifier fets and sufficient output capacitors to handle the ripple current. As a "rule of thumb" estimate, the output capacitor ripple current requirement will be greater than the output current. Polymer output caps are recommended as they are more compact for a given ripple current rating, reducing the secondary loop area. The output will see peak currents in excess of 40A. This is the reason that our software cuts off at 15A output for a 5V output voltage.

A forward converter would be a far better solution for a low output voltage, high current application, as the peak secondary currents are greatly reduced.

For the 5V/20A Flyback, can I do the parallel SRs in the software? And if not, what is the best approach to do it, given that other elements in the circuit accommodate for the change?

For the forward converter topology, is there any RD available here?

By the way, how to obtain the schematic for a PIXL generated design? DO you think I can achieve the same design using PI-Expert? (design attached)

If you are violating the rules imposed by our software, you will be pretty much on your own for your preferred design. A 5V, 20A flyback will be an extremely difficult design due to the high peak/RMS secondary current stress. 

Of course I do not want to go that route, and since a Flyback with these specs is extremely difficult, I don't mind switching to the forward topology. But even for the TopSwitchJX, there's still limit of 12A in PI-Expert. I found another post here where the OP was advised to use TopSwitchJX with PIXL as the 5V/20A (same solution sought as mine) is quite possible with this approach. Can you elaborate more on this?

It seems the only way for me to do it is to generate the schematic using PI-Expert within the acceptable software limits, then update the schematic externally from the parameters obtained using PIXL. This is the only feasible solution that I can think of, or there's a better way to do it?

** update **

I'm attaching the last modified/optimized design for the 5V/15A Flyback. This is still within the software boundaries. The SR has been changed and now the total loss is 3.7W on the SR alone. Please try to take a look at the design and let me know if this one seems ok to proceed with so I can start the layout.

Thank you again for your support!

I've been looking at some of your other posts, and it looks like the 5V/20A output is predicated by your choice of a particular LED strip. What is preventing you from choosing an LED strip with higher voltage requirement, say 12-15V? That way, the output current requirement from the power supply is substantially reduced. 

Good question. @ 12V/100W, output current would be 8.33A. The problem is, the 5V LED strip is a requirement for the project and I'm already stuck with bunch of them. Also, I still need 5V to power certain other sections of the design. Maybe a dual-output 12V/8.33A, and 5V/1A is a best fit for this scenario. Of course, if we want to switch to the 12V strip.