HV regulator with foldback current limiting
A TMOS MTM7N45 (Q2) is used as a series pass element in a linear high voltage supply that accepts +275-V unregulated and produces 250 V regulated with foldback current limiting. A 15-V zener, Dlf provides the dc reference for operational amplifier Ul, whose other input is obtained from a fraction of the output voltage. Ul drives Q3, which drives the gate of Q2.
Foldback current limiting is achieved by Rl, R2, R3, R4, Ql, and D2.