MOSFET Avalanche Energy

Eas = 0.5 × L × I² × Vbr/(Vbr − Vcc)

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Result
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Formula

E_as = ½ × L × I² × V_br / (V_br − V_cc)

Description

When a MOSFET turns off an inductive load without a freewheeling diode, the inductor current forces the drain voltage above the supply voltage until the MOSFET enters avalanche breakdown. The MOSFET must absorb the stored inductive energy (½LI²) multiplied by a factor Vbr/(Vbr − Vcc) that accounts for the supply voltage sourcing additional energy during the avalanche event. The MOSFET datasheet specifies a maximum single-pulse avalanche energy (EAS) rating. Exceeding this value causes permanent damage or destruction.

Variables

  • L — Parasitic or load inductance (H)
  • I — Current flowing when the switch opens (A)
  • V_br — MOSFET avalanche breakdown voltage (V)
  • V_cc — Supply voltage (V)

Practical Notes

The result is in joules. Always ensure the calculated energy is well below the MOSFET EAS rating, accounting for temperature derating. EAS ratings decrease significantly at elevated temperatures. For reliable operation, keep the avalanche energy below 25% of the rated EAS. Adding a snubber (RC or RCD) or a TVS diode across the MOSFET can clamp the voltage and prevent avalanche entirely.

Related Formulas

MOSFET Conduction Loss MOSFET Switching Loss BJT Total Dissipation Diode Power Loss Zener Regulation Resistor Transistor Current Gain (β)

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