Peak current mode control is chosen for this design and a TL431 (U1) acts as the output voltage error amplifier. It has a 2.5-V internal reference and we want to regulate VOUT to 12 V. We set RB, the lower resistor of the output voltage divider chain to 10 kΩ. RA the upper resistor is given by:
It is possible, but not necessary, to add a small resistor, RLOOP, in series with the feedback network as a signal injection point for loop stability tests, RLOOP.
The output of U1 is transferred across the isolation barrier by the optocoupler U2 and fed into the EAP pin of the UCC3895 as a current demand signal. The UCC2895 internal error amplifier is configured as a voltage follower by connecting EAN to EAOUT.