I need someone expert or rather familiar with the UC3842 to help clarify.
Based on the following,

http://danyk.wz.cz/uc3842_2.pdf
Page 7

if I were to change my input value from vac 160-270 and output should be 55v 4a(this is separately based on transformer design),what are the values of R and C that should be changed surrounding the 8 pins of UC3842?
Is there any recommended secondary feedback voltage regulator that is optically isolated with example design available?

 

No change to the reference voltage components (R and C at pin 8).

You will need to review the voltage ratings of the bridge rectifier and snubber components and the 56k resistor will probably need to be increased.

For the feedback have a look here:
http://switchingpowermagazine.com/downloads/15%20Designing%20with%20the%20TL431.pdf

 

Hi,what about value of R and C at other pins (Isense, Comparator VFB, ETC.)where do I look to modify the values?

 

Hi,what about value of R and C at other pins (Isense, Comparator VFB, ETC.)where do I look to modify the values?

For Isense you can adapt the current measuring resistor if necessary.

Since you have to rewind your transformer anyway you can choose the feedback winding to have the same output voltage, so you probably will not need to change the comparator input values.

You have to review them anyway because this is just a reference design. You have to test the smps under all possible load conditions, its dynamic, step load response etc, to see if it oscillates.

 

That is alright, but without changing the transformer winding can we get desired voltage?
I mean variation between 2 volt to 20 volt.
can we put variable resistor to get variable voltage 2V to 20V?