I was reviewing this circuit and I can't figure out how much current goes through the Zener diode. I understand that it is there to protect the MOSFET gate against high voltages but how much current flows through while charging the CGS capacitor?
I assume that the current that charges the capacitor between gate and source (CGS) should flow through the zener diode, is that correct? But if the impedance of the Zener were as low (as it usually is) as about 5 Ω, it would load the capacitor with a huge current spike that would destroy the Zener, or am I wrong?
What current must the Zener support in this circuit?
Best Answer
The zener is used to protect the gate from over voltage. Vgs has a maximum voltage rating, often around 20V.
This is a reverse voltage protection circuit. The zener's current is limited by the gate bias resistor (R1) as explained in @datenheim's post. The LTspice simulation below shows the various waveforms when the voltage source is swept from -30 V to +30 V. Note the step in the output voltage which shows the body diode forward voltage drop when the FET isn't conducting, and jumps up when the FET is turned on. Note, the X-axis is the input voltage, V(in).