UCC28711: UCC28711 in TIDA-00173 Question

Hello user5956241,

Q1 and Q2 are connected in what is referred to as a high voltage cascode switch.
In normal operation Q1 switches off and its drain voltage is limited by the zener voltage Z2 on the Q2 switch.
Q1 switching off opens the source path of Q2.
In the reference design  VDS(Q1) = 680 V and VDS(Q2)  = 975-680 = 295V

The Application Note SLUA315 will give you more details about a cascode circuit
https://www.ti.com/lit/an/slua315/slua315.pdf?ts=1597054157834&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fdocs%252Funiversalsearch.tsp%253FsearchTerm%253Dslua315

Regards

John