25W Stackfet multi output design 110-540Vac Tiny290 or TOP258 which one to recommend?

You have 8 separated  outputs.   Are the isolated from each other or are they  sharing same return?    if they have the same return, you can use  TL431 with opto coupler to regulate your output voltage. With TL431 you can sense  2 outputs by spliting you  feedback sensing signal

They are all isolated from each other through a speical long distance customised housing. I saw examples with multiple outputs but 1 leg will be regulated . Will there be a problem if i have totally isolated 8 outputs. (the outputs are for isolated supplies of the high side drivers of an inverter. They must have a high voltage isolation)

They are all isolated from each other through a speical long distance customised housing. I saw examples with multiple outputs but 1 leg will be regulated . Will there be a problem if i have totally isolated 8 outputs. (the outputs are for isolated supplies of the high side drivers of an inverter. They must have a high voltage isolation)

It depends on your specs for output regulation. Let say you have a flyback design with two outputs, one of the outputs is fully loaded while the second output is unloaded.  Then you have the situation that the controller changes the duty cycle of the switching waveforms to satisfy the demand of power in the loaded output. This output stays in regulation.  The change is waveforms will make the unloaded output voltage to go higher. This is a consequence of flyback topology with multiple outputs.

If you loads are steady and always ON/OFF at the same time, then you would not see this cross regulation issue

We have changed the outputs of each 8 isolated secondary channel from 15VDC 300mA to 20VDC 300mA . We will add then as a secondary regulator a classic 7815 linear regulator to compaansate the load variations and its consequenses to the output voltage of each independent channel.

Is this configuration safe enough to ensure a better distrubution between more loaded and less loaded channels. We plan to insert a higher value capacitor after 20VDC regulation and as a buffer to the 7815 regulator.

In this case do you think that the margin of 5V  (20V to 15V) suffiecient. Or is it better to increase the regulation margin up to 24V in cost of more power dissipation for 7815 @300mA load current.

I think the worst case will happen when one of your outputs ( or more that one ) is fully loaded ( one output with not sensing feedback).  At the same time the output with sensing feedback resistor is unloaded. In this case, this output voltage will go low because is a un-senses output.

So you need to investigate how low will be this output.  It needs to be larger that the minimum required voltage plus the drop of the regulator

Thank you for your suggestions.

As power consumption is not critical in this application , I can calculate statically the least power requiring channel and maybe use some dummy loads(resistors) to compensate . There are also minimum 2 channels I know that they will be loaded up to %80 and will not go under %50 . For these more loaded channels it is optimal to place the feedback circuit. For the rest I can place some dummy resistors according to the power requirements of the channel. All these combined with 7815 will narrow the chance of failure in cost of more power consumption which is not critical.

As new power required it is then a better choice to use TOP258 to allow a power reserve because of the above arrangement.

You need to take in account all the power your system will consume and the temperature of the system.  One oversized controller is good idea but keep in mind you need to set a limit to the power the power supply can deliver.  In other words, you need to set the maximum power to be about 10 % more of the peak power you need for your application