UCC28780EVM-002: How to let UCC28780EVM-002 Vo change to 12V

Hello Kygo, 

Thank you for your interest in the UCC28780 EVM design. 

Most definitely, several other parts will need to be modified!  How many other parts depends on what the new output power level is required to be.

The existing power conversion stage is designed to provide output power of 45W at 20V, 2.25A. 

1. If you wish to lower Vout to ~12V and leave the output current specification at 2.25A, then you need to change the transformer T1 for new turns ratios, current-sense resistors R18-R21 to higher value for lower power limit, R25 to adjust OPP line compensation based on new turns ratio, R29 & R14 on VS pin for Vin start-up and lower OVP thresholds based on new turns ratios, R10 and R15 (for RTZ and RDM) to match the new conditions, and R13 to lower value for proper bias current to U2 (ATL431B).   

2.  If you wish to lower Vout to ~12V and leave the output power specification at 45W, then the output current increases to 3.75A and you'll need to change all the parts listed in #1 (except R18-R21) plus change T1 secondary winding to larger wire size, replace Q1 SR-Mosfet to lower Rds(on) part, C8 & C9 resonant caps to higher value, L3 & L4 to higher current ratings, and possibly C16 output cap to higher ripple current rating.  

It is not a simple matter to change the output voltage of a finished ACF converter because the clamp resonance is matched to the reflected voltage and operating frequency at full power, low line.  As can be seen above, changing the output voltage has a wide-spread effect on many components.

I recommend that you fill out the UCC28780 Excel calculator tool (https://www.ti.com/lit/zip/sluc664) with the new target requirements and see what all the new parameters should be. 

Regards,
Ulrich