RC Snubber Design Calculator
Measure a ringing frequency before and after adding a known test capacitor to estimate the loop parasitics. The calculator turns that estimate into a starting RC pair, nearby resistor values to test, and first-pass electrical stress figures.
Estimated parasitic capacitance (pF)156.67
- Estimated parasitic inductance (nH)
- 101.05
- Ideal snubber capacitor (pF)
- 470
- Ideal snubber resistor (ohms)
- 25.4
- Preferred-value starting pair
- E12: 470 pF; E24: 24 ohms
- Bench resistor sweep
- 18, 20, 22, 24, 27, 30, 33 ohms
- Loaded LC reference frequency (MHz)
- 20
- Estimated resistor dissipation (W)
- 0.43
- Minimum resistor power rating (W)
- 0.87
- Initial resistor pulse current (A)
- 2
Ideal versus preferred capacitor
| Choice | Capacitor (pF) | Loaded LC reference (MHz) | Estimated loss (W) |
|---|---|---|---|
| Ideal calculation | 470 | 20 | 0.43 |
| Nearest E12 capacitor | 470 | 20 | 0.43 |
This is a first-pass, single-LC small-signal model; verify overshoot, ringing, EMI, and component temperature on the completed hardware.
Preferred-value rounding is shown in the comparison table because it changes the predicted reference frequency and loss from the ideal calculation.
How to use this calculator
- Measure the dominant ringing frequency with the final probing method and snubber location in mind.
- Fit a known test capacitor at that location and measure the lower ringing frequency again.
- Choose one to four times the recovered parasitic capacitance as the initial snubber-capacitor multiplier.
- Enter the voltage swing and switching details to estimate resistor loss, then test the displayed resistor sweep on the bench.
How the RC snubber values are calculated
A ringing switch node can often be treated as one parasitic inductance and one parasitic capacitance for a first pass. Measure its original frequency f0, then add a known capacitor Ct at the intended snubber location and measure the loaded frequency f1. The added capacitance lowers the resonance, which lets the calculator recover the unknown capacitance:
Cpar = Ct ÷ ((f0 ÷ f1)^2 − 1)
It then obtains inductance from Lpar = 1 ÷ ((2πf0)^2 × Cpar). The starting resistor is the characteristic impedance, Rsnub = sqrt(Lpar ÷ Cpar). This matches the resistor to the original ringing loop, rather than to the larger snubber capacitor. The starting capacitor is the selected multiple of Cpar; one to four times is a commonly used practical range.
Why the preferred pair matters
Calculated values are seldom orderable values. This tool rounds the capacitor to the nearest E12 value and the resistor to the nearest E24 value, then recalculates the undamped LC reference frequency and loss using the selected capacitor. The comparison table makes that rounding visible. It also lists nearby E24 resistors from about 0.7 to 1.3 times the calculated resistor, because damping is normally adjusted against the actual waveform rather than accepted from one equation.
The resistor-loss estimate uses Ploss = 0.5 × Csnub × V^2 × fs × n, where n is the number of full-voltage capacitor transitions per switching cycle. With two full transitions, it reduces to the familiar Csnub × V^2 × fs. The minimum continuous-power figure simply applies your selected margin. Initial current is Ipeak = V ÷ R; it is a useful stress flag, not a complete pulse-rating check.
What can move the answer
The ratio between the two measured frequencies has the greatest effect. If the test capacitor changes frequency by only a little, small measurement error produces a large error in recovered capacitance. Probe capacitance and ground-lead inductance can create or move ringing, so use a short ground spring, a suitable differential probe, or another measurement method appropriate to the voltage and bandwidth.
What the model leaves out
This is not a complete transient simulation. MOSFET output capacitance, diode capacitance, capacitor ESR and ESL, layout, transformer leakage, and the switching waveform can all vary with voltage and current. Multiple visible ringing modes or a waveform that changes materially after the test capacitor is fitted do not satisfy the single-LC assumption. Confirm voltage rating, dielectric, self-resonance, resistor pulse energy, temperature rise, EMI, and device voltage margin on hardware.
Worked example
With 40 MHz original ringing, a 470 pF test capacitor, and 20 MHz loaded ringing, the frequency ratio is two. The recovered parasitic capacitance is 470 ÷ (2^2 − 1) = 156.67 pF, and the recovered inductance is about 101.05 nH. A three-times capacitor target is 470 pF and the characteristic impedance is about 25.4 ohms, so the preferred starting pair is 470 pF and 24 ohms.
At 48 V, 400 kHz, and two full-voltage transitions per cycle, that 470 pF capacitor gives about 0.433 W of resistor dissipation. With a two-times margin, start by considering at least a 0.866 W continuous rating, then check its pulse and temperature ratings. The 24-ohm initial current estimate is 2 A, and the bench sweep spans 18, 20, 22, 24, 27, 30, and 33 ohms.
Common questions
How do I measure ringing frequency without adding probe-induced ringing?
Use the shortest possible probe ground connection, preferably a ground spring at the measurement point, because a long ground lead adds inductance. A properly rated differential probe can avoid a long ground loop. Compare measurements with and without the test capacitor; if the waveform or apparent mode changes substantially, do not treat it as one LC resonance.
Why should the test capacitor lower the ringing frequency?
For a fixed inductance, resonant frequency falls as total capacitance rises. The test capacitor is placed in parallel with the existing parasitic capacitance, so a valid dominant LC mode should ring more slowly. A loaded frequency equal to or higher than the original does not produce a positive capacitance solution in this model.
Does the loaded frequency have to be exactly half the original frequency?
No. The formula accepts any lower loaded frequency. When it is exactly half, the total capacitance has become four times the original value, so the recovered parasitic capacitance is one third of the added test capacitor; that is only a convenient special case.
Why is the resistor based on sqrt(Lpar/Cpar) rather than sqrt(Lpar/Csnub)?
The resistor is a starting match for the characteristic impedance of the original parasitic ringing loop. The snubber capacitor is then chosen large enough to absorb and reshape that ringing. The final damping depends on the complete physical circuit, so test several nearby resistor values rather than relying on the calculated value alone.
How do I choose capacitor and resistor ratings?
Choose a capacitor with suitable voltage, dielectric, AC behavior, ESR, ESL, and self-resonant frequency for the switching node. Check the resistor’s continuous power, pulse-energy, pulse-voltage, temperature, and mounting limits. The figures here are starting estimates and do not replace the manufacturers’ pulse and thermal data.