When designing circuits with inductive loads like motors or relays, electronics hobbyists and engineers often face a hidden danger: avalanche energy in MOSFETs. If not managed properly, it can destroy your device in seconds.
The Problem: Avalanche Breakdown
When a MOSFET switches off an inductive load, the collapsing magnetic field generates a voltage spike. If there’s nowhere for this energy to go, it can exceed the MOSFET’s drain-source breakdown voltage, causing avalanche breakdown—which may permanently damage the device.
The Solution: Choose Avalanche-Rated MOSFETs & Use Snubber Circuits
The fix is simple but critical: use a MOSFET with a specified avalanche energy rating (E<sub>AS</sub>), and add snubber circuits or flyback diodes to safely dissipate excess energy. This protects your switch and extends your circuit’s life.
Practical Example: Controlling a Relay with a MOSFET
Suppose you’re switching a 12V relay with an N-channel MOSFET. Without a flyback diode, turning the relay off causes a high-voltage spike. By adding a 1N5819 Schottky diode across the coil, you redirect the spike safely, preventing avalanche damage.
Sample Calculation: Estimating Avalanche Energy
If an inductive load generates a 1A current and takes 10µs to decay:
EAS=21×L×I2Assume L = 100µH⇒EAS=0.5×100×10−6×12=50µJ
Choose a MOSFET rated for more than this energy level.
Product Suggestion: Avalanche-Rated MOSFETs & Diodes
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