When working with MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) in your projects, you might encounter an issue called the MOSFET body diode effect. This occurs when a MOSFET’s internal diode inadvertently conducts current in the wrong direction, leading to power losses or malfunction in circuits. Fortunately, the problem is fixable, and understanding it can help you improve your designs.

The Problem :

MOSFETs have an intrinsic body diode between the drain and source. In normal operation, this diode should remain off. However, under certain conditions, such as when the MOSFET is exposed to reverse voltage, the body diode can become forward biased, causing unwanted current flow. This can result in heat generation, reduced efficiency, and even failure of your circuit.

The Solution :

The simplest way to handle this issue is by selecting MOSFETs with a body diode designed to have a lower forward voltage drop or using a freewheeling diode in your circuit. In certain cases, adding a diode in series with the MOSFET can block reverse current flow, preventing the body diode from turning on.

Practical Example :

Consider a DC motor circuit where a MOSFET controls the motor’s power. If the motor starts turning the wrong way due to the body diode effect, the unwanted current flow could damage your components. Using a freewheeling diode in parallel with the motor can protect the circuit and prevent this issue.

Sample Calculation : 

If you’re using a 12V DC motor, and the current drawn is 2A, you might choose a diode with a forward voltage drop of 0.7V to handle reverse current.

This ensures minimal power loss. The power dissipated in the diode is:

P=V×I=0.7V×2A=1.4W

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