When working with MOSFETs in your circuits, one common problem is reverse bias damage. This occurs when the MOSFET is exposed to voltage in the opposite direction of its designed polarity, potentially destroying it. Thankfully, there’s an easy fix using simple components!

The Problem :

MOSFETs are designed to control current flow in one direction. If a voltage is applied in reverse, the MOSFET could fail permanently. This can happen in situations where the power supply is mistakenly connected the wrong way around or during an accidental polarity switch.

The Solution :

To prevent reverse bias damage, you can add a diode in series with the MOSFET’s gate or source terminal. The diode blocks reverse voltage, allowing only correct polarity to reach the MOSFET.

Example : 

Imagine you’re building a simple motor driver circuit. You’ve connected a MOSFET to control the motor, but when powering it up, you accidentally reverse the power supply. Without protection, this could destroy the MOSFET. By adding a diode to block the reverse voltage, you ensure the MOSFET remains safe regardless of power polarity.

Sample Calculation :

Suppose your MOSFET gate needs to be charged through a 10kΩ resistor to fully turn on.

The gate charge is 5V, and your supply is 9V.

The current through the resistor is calculated as:

I = (V_supply – V_gate) / R = (9V – 5V) / 10kΩ = 0.4mA.

This small current ensures safe operation of the MOSFET.

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