Check out my latest contraption, a B+ regulator for tube amps, adjustable from 200 to 300 V. The error amplifier/shunt MOSFET bias servo op-amps are fed 5 V from a 7805-based external board placed right next to the regulator board.
I'd like to know if my protection measures are adequate. I'm relatively new to using op-amps in high-voltage regulators.
I have applied the protections I think are necessary: at the top of the schematic there's D2/R5 for discharge of the last filter capacitor in case of a short circuit, and D3 (UF4007 or the like) at the far right for reverse spikes. Prototypes seem to survive fine without them but of course you want protection in a final design; I admit I didn't try shunting the outputs on the prototypes :-)
The op-amp is nothing fancy, LM324 or similar single supply chip. The LM324 has EMI protection diodes on all pins; is this sufficient or should I fit reverse Schottky diodes from the error amp non-inverting input (TP6) to GND and/or VCC? Schottky diodes would be required because their forward drop is lower than the op-amp input's maximum reverse voltage allowed.
That being said, I'm not sure I like the idea to have reverse voltages shunt to the external regulator, and I can't use Schottky diodes to shunt to the HV rail because the voltage is too high.
In normal operation all other inputs should never see any reverse voltage since they are connected to DC regulators so no reverse voltage protection needed there, right? Last thing, should I fit a Zener diode as a crowbar on the error amp's non-inverting input (TP6) to GND?