TOPswitch-GX isolated DC-DC converter with 42-56V DC input

1. TOPSwitch is meant primarily for AC-DC applications.

2. For an Input Voltage=48V and Output Voltage=40V, the Duty Cycle will be too high. If you try to reduce the VOR at this point to achieve the required Duty Cycle, then the transformer's flux density will be extremely high.

3. For your design requirements, I would recommend using the DPA switch. http://www.powerint.com/en/products/dpa-switch-family/dpa-switch 

1. I understand that TOPSwitch is designed for AC-DC applications, but all these designs convert the AC to DC before it reaches the TOPSwitch, so why does it matter?  Or is it just that TOPSwitch is designed for a higher input rail?

2. I don't design these things every day, but I would think that a 2:1 winding ratio on the transformer would result in a VOR of 80V and a duty cycle of 50%.  Is this too long?

3. We have existing product (AC-input) and management wants to use the existing PCB design (with an alternate component population) for this application.  I realize it is not the most effifient or cost effective, but the DC-input version is not high volume and they want an immediate solution.

  As for using DPA, I did a brief investigation of that in PIXIdesigner and it says the 40V@2A output "exceeds max power for DPA switch".

I would like to keep with the TOPSwitch-GX and use a forward-converter topology (Ref: AN30).

The forward-converter may impact the duty-cycle too.

My problem is that neither PIXI and PIExpert support this type of design, making the design process more error-prone and tedious.

Any more suggestions?

Neither PIExpert nor PIXls support the forward topology, except for DPA switch. Your design is possible using the TOPGX in PIXls, however, the design will not be an optimal design. I have attached an image of a PIXls document using TOPGX as per your specifications. This is not a design recommended by Power Integrations and is for your reference only. The RMS current is unreasonably high.