UCC28742: UCC28742 NON ISOLATED FLYBACK

Abhishek

The transistors Q2 and Q3 are used to mimic the behavior of an optocoupler.  UCC28742 is optimized for isolated designs where an optocoupler is used in the feedback.  One of the ways it's optimized is that it connects directly to the output of an optocoupler, which is a current source.  Since this design is not isolated, Q2 and Q3 are there to convert the feedback voltage signal to a current signal so it can properly interface with UCC28742.

To better understand how the components around these transistors, such as R18, R14 and C12 are derived we recommend you review the following collateral

http://www.ti.com/lit/an/slva662/slva662.pdf

http://www.ti.com/lit/an/slua671/slua671.pdf - note that UCC28600 is similar to UCC28742 and the design for the feedback components is identical

Best Regards,

Eric

hi ERIC ,

So i need to find the transfer function of the dcm flyback and then use the k factor method to find the poles and zeros of the type 2 compensator 

correct me if i am wrong consider flyback with single output to regulate the 5 volts output .

Hi,

how do we decide the start up time for the converter as soft start we will keep rc circuit what is the usual time constant .