PMOS circuit protection

PMOS only turns on if Vsg is greater than threshold. Voltage at source and drain are similar. The drain voltage is the source voltage minus the (on resistance times the pass through current). This means the zener diode by regulating the Vdg is also regulating Vsg to 8V in this case. If a 10V Vsg is allowed in the pmos ratings, then 8V is a good choice of zener voltage with some margin. The resistor to the left of the diode is essential because zener diodes only works if a current is always passing through them. The reistor on the right represents a load. The resistor mentioned first which regulates the zener current must be chosen such to both minimize power consumption (by increasing resistance) and to reject high frequency noise (lower resistance). The proper resistance chosen will hence be a tradeoff between power consumption and noise rejection capability.

published 11 months ago