Why we use zener diode across VGS of a MOSFET?

 

A Zener diode is often placed across the gate-source (V$_{\text{GS}}$) of a MOSFET to protect it from excessive gate voltage. Here are the key reasons why:

1. Overvoltage Protection: MOSFET gates are sensitive to high voltages, and exceeding the maximum gate-source voltage (V$_{\text{GS}}$) rating can permanently damage the MOSFET. A Zener diode clamps the V$_{\text{GS}}$ voltage to a safe level, preventing gate breakdown.

2. Transient Suppression: During switching, voltage spikes can occur due to parasitic inductances or other factors in the circuit. A Zener diode helps to absorb these transients, protecting the gate from sudden surges that could exceed the gate oxide breakdown voltage.

3. Ensuring Safe Operation: By clamping the V$_{\text{GS}}$, the Zener diode ensures the MOSFET operates within its recommended voltage range, allowing it to function correctly without overstressing the gate structure.

For example, if a MOSFET has a maximum V$_{\text{GS}}$ rating of 20V, a Zener diode with a breakdown voltage slightly below this (e.g., 18V) can be used to ensure the gate is not exposed to damaging voltages.