RC Snubber Calculator (Switching Node)

RC snubber for a ringing switching node

Every switching node — a MOSFET drain, a diode cathode, a transformer winding — has parasitic inductance (trace, package, winding leakage) and parasitic capacitance (device output capacitance, winding capacitance) that form an unwanted L-C tank. Every switching edge excites it, producing damped ringing at the tank's natural frequency: f_ring = 1 / (2π√(L_parC_par)). If you can't measure L and C directly, a scope capture of the ringing frequency plus an estimate (or datasheet value) of C_par lets you back out L_par from the same formula.

An RC snubber — a resistor and capacitor in series, placed across the ringing node — damps this tank without eliminating the parasitics. The characteristic impedance of the tank, Z₀ = √(L_par/C_par), is also the resistor value that critically damps it: R_snub ≈ Z₀. The snubber capacitor is sized a few times larger than the parasitic capacitance — C_snub = k × C_par, k ≈ 2–4 — so it dominates the node's total capacitance and the RC branch, not the small parasitic C alone, controls the ringing; too small and the snubber barely loads the ring, too large and it slows switching edges and adds loss.

The snubber dissipates real power: every switching cycle it charges and discharges C_snub through R_snub across the voltage swing at the node, burning P_snub ≈ C_snub × V² × f_sw as heat in the resistor. This is the main cost of an RC snubber versus a lossless (or partially lossless) alternative like an RCD clamp — budget the resistor's power rating accordingly, and note that a snubber only damps ringing, it does not clamp peak voltage the way a TVS or RCD clamp does.