The Problem :

MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are essential for switching and control in electronic circuits. However, when exposed to reverse bias, where the voltage is applied in the opposite direction (drain to source for an N-channel MOSFET), it can cause overheating, excessive current, and permanent damage. This often happens when driving inductive loads like motors or relays, which generate back EMF (Electromotive Force) when turned off.

The Solution : 

To protect the MOSFET from reverse bias, a Schottky diode or flyback diode can be connected in parallel with the load. This diode blocks reverse voltage and safely diverts the current, preventing damage. Schottky diodes are ideal due to their fast response and low forward voltage drop.

Practical Example : 

Consider a 12V DC motor controlled by an N-channel MOSFET. When the motor is switched off, it generates a reverse voltage that can damage the MOSFET. To protect it:

• Connect a 1N5819 Schottky diode across the motor terminals.

• This diode clamps the voltage and prevents reverse current flow through the MOSFET.

Sample Calculation :

If the motor draws 2A at 12V, select a diode with at least 20% higher current and voltage capacity. For this case:

• Voltage rating: 1.2 × 12V = 14.4V (minimum)

• Current rating: 1.2 × 2A = 2.4A (minimum)

• A 1N5819 Schottky diode rated at 40V, 3A is a perfect fit.

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