UCC28781EVM-053: What does Q100 in EVM do?

Hello Valentinas, 

The sub-circuit involving Q100 on the UCC28781 EVM is used to extend the line feed-forward function at the CS pin beyond the limits of the controller. 

The UCC28781 generates a line-voltage compensation current out of the CS pin at a ratio of 1/25th of the VS input current (Ivsl), which is used to establish a voltage offset across Ropp to compensate for excess peak primary current due to delayed turn-off.  The higher the bulk voltage (based on input line voltage) the higher the excess peak current from turn-off delay.  Ivsl is proportional to the line, so as the input line increases, Ivsl increases, and so does Ics into Ropp to increase the offset with voltage.  

However, for Ivsl > 1.25mA, there is no further increase in Ics, so the compensation function ceases effectiveness at very high input lines.  
Since this EVM works from 50V to 500V, and Ivsl is designed around a lower input starting voltage, it runs out of effectiveness around ~250Vdc.  
This external sub-circuit helps to extend the effectiveness of line compensation up to 500V. 

Ivsl is generated by the negative Vaux voltage impressed across Rvs1 while the primary MOSFET is on, and is a function of Rvs1 value, the Npa turns ratio, and Vbulk.
This sub-circuit has no effect when Vaux < 39V and gradually applies a voltage across R108 (on Q100 emitter) as Vaux increases > 39V+Vbe.  
This gradual increase in voltage across R108 generates a proportional current into the CS node which flows through Ropp to further increase the offset voltage after the internal Ics stops increasing.  

The specifications of the BC807-25LT1G indicate that it is a reasonable substitute for the MMST3906.  Basically the PNP needs to have sufficient hFE at low collector currents so that base current can be considered negligible and does not significantly subtract from the collector current, nor affect the voltage at the base.   

I hope this explains the function and substitution. 

Regards,
Ulrich