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When a switch (MOSFET, IGBT or diode) turns off fast, the circuit's stray inductance resonates with the device capacitance, producing a high-frequency voltage ringing and overshoot that can exceed the device rating and radiate EMI. A snubber tames this. An RC snubber damps the ringing by adding a resistor that dissipates the resonant energy; a turn-off (RCD) snubber diverts the switch current into a capacitor during turn-off, slowing the voltage rise and cutting switching loss in the device.
| Snubber | Design |
|---|---|
| RC capacitor | Cs ≈ 2–4 × Cstray |
| RC resistor (critical damping) | Rs = √(Lstray/Cs) |
| RC resistor power | PRs = Cs·V²·fsw |
| RCD capacitor | Cs = Io·tf/(2V) |
| RCD power | Psnub = ½·Cs·V²·fsw |
Rs = √(L/C) is the characteristic impedance of the stray tank — matching the resistor to it gives near-critical damping so the ringing dies in one cycle. The resistor must be rated for the dissipated power, which rises with the square of the voltage and with switching frequency.
A snubber is a small network (usually an RC or RCD) placed across a switch or diode to suppress voltage spikes, damp ringing, and slow the rate of voltage change (dv/dt) during switching, protecting the device and reducing EMI.
An RC snubber (resistor + capacitor) mainly damps high-frequency ringing and is dissipative. An RCD snubber adds a diode so the capacitor charges during turn-off and discharges through the resistor, diverting switch current to cut turn-off loss and limit dv/dt.
A common rule is Cs = 2 to 4 times the switch/stray capacitance you are trying to damp. Too small has little effect; too large wastes power. Then set the resistor to √(Lstray/Cs).
For near-critical damping, set Rs equal to the characteristic impedance of the stray tank: Rs=√(Lstray/Cs). This makes the ringing die out in about one cycle without over-damping.
An RC snubber resistor dissipates roughly P=Cs·V²·fsw (it charges and discharges once per cycle). An RCD turn-off snubber dissipates about ½CsV²fsw. Rate the resistor above this value.
Use Cs=Io·tf/(2V), where Io is the switched current, tf is the current fall time, and V is the bus voltage. This holds the voltage low while the current falls, reducing turn-off loss in the switch.
Measure the ringing frequency fr on the switch node with and without a known added capacitor. From the two frequencies you can solve for Lstray and Cstray using fr=1/(2π√(LC)).
Choose Rs so the capacitor fully discharges during the shortest on-time: roughly Rs ≤ ton(min)/(2.3·Cs). A smaller resistor discharges faster but dissipates more peak power.
Often a small RC snubber or gate-resistor tuning is enough, because the MOSFET body diode and output capacitance already absorb some energy. Fast SiC/GaN devices with high dv/dt may still need snubbers to control overshoot and EMI.
A dissipative RC/RCD snubber trades some efficiency for reliability and lower EMI, since its resistor burns the captured energy. Non-dissipative (energy-recovery) snubbers return the energy to the supply but are more complex.
A snubber shapes the switching transient (dv/dt, ringing). A clamp (e.g. TVS or RCD clamp) limits the peak voltage to a fixed level. Many flyback designs use an RCD that acts as both a clamp and a snubber for the leakage spike.
As close to the switch (or diode) terminals as possible, with short, low-inductance loops. Placing it far away adds its own stray inductance and defeats the purpose of damping the switching node.
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