UCC28782: UCC28782

Hello Robin, 

Happy New Year!

The calculations for Rvs1 value and the voltage across it (Vrs1_max) are correct.  There no known errors in the Excel tool regarding these calculations. 
Rev C is still the latest version, although recently our versioning system has been revamped and it is now designated as version 01.00.00.0C.

Based on your inputs (Vin_max = 400V, Np = 25, Na = 5),  Vrs1_max = 400V *5/25 = 80V and its polarity (at the winding) with respect to GND will be negative. 
This calculation result is intended to inform the user of the maximum voltage stress across Rvs1 in order to choose a resistor with appropriate rating. 
The polarity of the voltage is perhaps incidental, but is consistent with being negative when the low-side MOSFET is on and positive when that MOSFET is off. 

Aside from that, I noticed your choice for Rvs2 ('Calculations':D97) is much higher than recommended (14kR instead of 10.2kR) which would lead to OVP triggering at a lower voltage than your regulation target.  I assume this is left over from a previous design entry and you did not get to changing it yet after seeing the negative voltage at D88.
For the benefit of others reading this thread, I wish to emphasize that ACF design has a lot of inter-dependent parameters and in many cases you can't change one parameter in isolation without reviewing its effect on other parameters.  It is good practice to review all user choices ( all Green cells) with respect to their recommended values each time a user choice is changed.  

Regards,
Ulrich