Snubber Resistor

When a power semiconductor switches off an inductive load, the energy stored in stray inductance (L_stray × I) is released as a voltage spike V = L × dI/dt across the device. Without protection, this spike can exceed the breakdown voltage and destroy the switch — or excite a resonant oscillation between L_stray and the device's output capacitance that radiates EMI and stresses the silicon. The snubber resistor, paired with a capacitor (RC) or capacitor-plus-diode (RCD), absorbs the energy and damps the resonance.

For a simple RC snubber, R is chosen for critical damping: R = √(L_stray / C) where C is selected to limit dV/dt to the device's safe operating area (e.g. < 1 kV/μs for older IGBTs). Power dissipation in the snubber is P = ½ × C × V² × f_sw × 2 (charge + discharge each cycle), which can reach tens of watts in a hard-switched 100 kHz converter. Snubber resistors must therefore handle continuous AC power at the switching frequency, not just the peak voltage.

Construction matters: snubbers see fast voltage transients, so low-inductance resistors are mandatory — bifilar wirewound or thick-film, never standard helical wirewound. Common sizes are 5 – 50 Ω, 5 – 25 W. RCD snubbers (capacitor charges through diode, discharges through resistor) reduce R's continuous loss in high-frequency applications.