The Role of Inductive Kickback in BSS138 MOSFET Failures

The Role of Inductive Kickback in BSS138 MOSFET Failures: Causes, Solutions, and Steps to Resolve

Inductive kickback is a common issue in circuits involving MOSFETs like the BSS138, leading to device failure if not properly managed. This phenomenon occurs when an inductor (or a circuit with inductive components) is suddenly de-energized, generating a high-voltage spike that can exceed the voltage ratings of the MOSFET. When this happens, it can cause the MOSFET to fail, either by damaging its gate, drain, or source junctions.

In this article, we will analyze the cause of failures due to inductive kickback, explain the reasons behind the failure, and provide a step-by-step guide on how to prevent and resolve these issues.

Understanding the Cause of BSS138 MOSFET Failures Due to Inductive Kickback

Inductive Kickback Phenomenon: Inductive components, such as coils, relays, or motors, store energy when current flows through them. When the current is suddenly interrupted (e.g., when the MOSFET turns off), the energy stored in the magnetic field of the inductor is released as a voltage spike in the opposite direction (known as inductive kickback). This voltage spike can be much higher than the operating voltage of the MOSFET, often damaging the MOSFET’s internal structure, especially the gate-source junction. Why BSS138 is Vulnerable: The BSS138 MOSFET, like many small-signal MOSFETs, has relatively low voltage and current ratings, typically around 50V. An inductive kickback voltage can easily exceed this threshold, resulting in permanent damage. The MOSFET's gate is particularly sensitive to high-voltage spikes, which may cause it to fail by breaking down the gate oxide.

Factors Leading to BSS138 MOSFET Failures

Circuit Design and Component Selection: Unprotected Inductive Loads: Circuits with inductive loads (such as motors, solenoids, or relay coils) without protection can lead to unexpected voltage spikes. Lack of Flyback Diode : A flyback diode is critical for absorbing the inductive kickback. Without it, the voltage spike can directly affect the MOSFET. Exceeding Maximum Voltage Ratings: The BSS138 MOSFET’s maximum drain-source voltage is 50V. If the inductive kickback exceeds this voltage, the MOSFET may be damaged. Inadequate Gate Protection: If the gate is exposed to a high voltage (via a large inductive spike), it may break down the gate oxide, permanently destroying the MOSFET’s ability to function.

How to Resolve and Prevent MOSFET Failures Due to Inductive Kickback

Step 1: Use a Flyback Diode

Solution:

Place a flyback diode (also known as a freewheeling diode) across the inductive load (in reverse polarity to the MOSFET) to safely dissipate the energy stored in the inductor when the MOSFET turns off.

Procedure:

Choose a diode with a fast recovery time and sufficient current rating (such as 1N4007 ). Connect the anode of the diode to the source of the MOSFET and the cathode to the drain. This allows the diode to conduct when the MOSFET turns off, providing a safe path for the energy from the inductor. Step 2: Increase the Voltage Rating of the MOSFET

Solution:

Use a MOSFET with a higher voltage rating than the maximum inductive kickback voltage.

Procedure:

Select a MOSFET that has a higher Vds (drain-source voltage) rating to ensure it can withstand the higher spikes that may occur in inductive circuits. For example, consider using a MOSFET with a voltage rating of 100V or higher for circuits with larger inductive kickback. Step 3: Add a Snubber Circuit

Solution:

A snubber circuit (a resistor- capacitor network) can be added across the MOSFET to suppress voltage spikes.

Procedure:

Choose an appropriate resistor and capacitor for the snubber circuit based on the characteristics of the inductive load. Connect the snubber circuit across the MOSFET's drain and source terminals. This will help absorb the inductive kickback and limit the voltage across the MOSFET. Step 4: Implement Gate Protection

Solution:

Use a gate resistor or Zener diode to protect the gate from excessive voltage spikes.

Procedure:

Place a Zener diode (rated slightly higher than the MOSFET's maximum Vgs) between the gate and source of the MOSFET. This diode will clamp any excessive voltage and protect the gate. Alternatively, a gate resistor can be used to limit the current flowing into the gate, providing additional protection against high-voltage spikes. Step 5: Use a MOSFET with Built-in Protection Features

Solution:

Consider switching to a MOSFET with built-in protection against inductive kickback or one that features a higher robustness against high-voltage spikes.

Procedure:

Some MOSFETs are designed with integrated protection features like Avalanche Energy Rating or Gate Clamping Diodes , which can help in handling transient voltage spikes more effectively.

Conclusion

Inductive kickback is a significant cause of MOSFET failure in circuits involving inductive loads. By following these steps—such as adding a flyback diode, increasing the MOSFET's voltage rating, adding a snubber circuit, and providing adequate gate protection—you can prevent inductive kickback-related failures and ensure the longevity and reliability of your MOSFETs in the circuit. Always remember to consider the specific requirements of your circuit and choose components that offer adequate protection.