What is your actual intention for this design? Do you really want to run the full range from 19.2 to 373VDC? If you do not need the high voltage capability. a DPASwitch may be a more appropriate choice for your needs. Also, at the lower voltage, you might want to check the drain waveform to make sure that you are not running into the max duty cycle limit.  What does the design spreadsheet tell you about the peak current at low input voltage? That will be the guide for determining whether you need a TOPSwitch with higher peak current rating.

I actually tried running a spreadsheet for this design - after some massaging, I got it to work. What is your target output voltage and operating frequency? Running a TOP266  may be a solution for this design, as the larger mosfet in the TOP266 will have a lower voltage drop at the peak current associated with the output power you are seeking.

1.Yes I need full range 19.2 to 373V,output 12V 5W.

2.It is not crossing maximum duty limit in any operating voltage range.

3.Operating frequency is 132kHZ 

4.Tried with spreadsheet(keeping 25V to 373V as 25 is the lowest possible value it allows to enter) but its showing error of maximum duty reached to 0.932.

You can correct the duty cycle problem by varying the reflected output voltage of the transformer (VOR). I had to crank the VOR all the way down to 50V to get a reasonable duty cycle that was within the maximum limit of the TOPSwitch. Such a low VOR forces you to use an output diode with a relatively high PIV, as this results in a secondary winding with high output voltage at the max limit of your input voltage range. I had to choose a very large input cap and a ridiculously high AC input frequency to get results that would approximate DC input. I have appended a PDF of the spreadsheet in question for your information.

Is there any limitation related to negative switch current (source to drain current )?

In the below picture it is 400mA ? is there any maximum allowable value for it.