Hi,

I want to design a Flyback, no problem with the transformer, Clamp .. etc. Now I want to design the feedback circuit using a TL431 and an Optocoupler.

1) How to calculate the Feedback component values for the UC3842/3?
2) How to regulate several outputs?

 

Using the TL431 formula, Vout =( R1/R2) +1 X 2.5V, where R1 is the top resistor, R2 is the bottom resistor.

So if you want 7.5V out, then, R1 can be 10k, R2 5K, etc...

 

Thanks, but it is not that easy as you described. this feedback should run an optocoupler and in harmony with the chip

 

Then download the datasheet for that chip, and it will have a test circuit,, here is one from the internet using an Opto-coupler..
R1 is 39K, R2 8K2, with these values that's 14V.

 

Input is 12V, then the output can not be higher

 

Input is 12V, then the output can not be higher

If your input is 12VDC, then use a buck regulator instead, like LM2596 or similar..

 

No, I meant in your mentioned circuit, the input voltage is 12V but your calculations show 14 for the output. therefore it can not be correct for this purpose and feedback design is more complex