HesapHanem

RC Snubber Design Calculator

Enter the ringing frequency and parasitic capacitance to size your RC snubber.

How it is calculated

You enter the measured ringing frequency f in MHz and the estimated parasitic capacitance C in pF of the switching node. The tool converts them to hertz and farads and sets the resistor to the reactance of that parasitic capacitance at the ringing frequency: R = 1 ÷ (2π × f × C). At the ring frequency this equals the characteristic impedance √(L ÷ C) of the parasitic loop, which is why it is a sensible damping value. The snubber capacitor is simply Csnub = 3 × C.

R is shown in ohms to one decimal, the capacitor in whole picofarads, and the frequency echoed to two decimals. Both inputs must be above zero. The 3× factor is a fixed rule of thumb, not something derived from your circuit, so treat the output as a first value to try on the bench. The tool does not calculate the power the resistor must dissipate, so check that rating separately.

Worked example

Suppose a switching node rings at 50 MHz and its parasitic capacitance is estimated at 100 pF. In SI units that is 50 × 106 Hz and 100 × 10−12 F. The resistor is 1 ÷ (2π × 50 × 106 × 100 × 10−12) = 1 ÷ 0.0314 = 31.8 Ω. The snubber capacitor is 3 × 100 = 300 pF. A 33 Ω resistor in series with a 300 pF capacitor is therefore a sensible starting pair across the node.

Try this example →

Common mistakes

  • Entering the ringing frequency in Hz or kHz. The field is MHz, so 50 MHz is 50, not 50,000,000.
  • Entering the parasitic capacitance in nF. The field is in pF, and 0.1 nF is 100.
  • Using the switching frequency of the converter rather than the ringing frequency read from the oscilloscope trace at the node.
  • Treating the capacitor result as exact. It is a fixed three times the parasitic value, so the real optimum may differ.
  • Ignoring the resistor power rating. The snubber converts ringing energy to heat on every switching cycle, and that dissipation is not given here.

About this calculator

In switching power converters, every trace, pad and component lead has a bit of stray inductance and capacitance, and when a MOSFET or diode switches off abruptly, that parasitic LC network rings — the voltage overshoots and oscillates at a frequency set by the parasitic inductance and capacitance before settling. An RC snubber tames this ringing by adding a resistor in series with a capacitor across the ringing node: the capacitor is sized at about three times the parasitic capacitance so it can absorb the oscillating energy, and the resistor is sized as R = 1/(2π × f_ringing × C_parasitic) so it dissipates that energy as heat right at the ringing frequency, damping the oscillation without slowing down the switching edge too much.

Power electronics engineers add snubbers to reduce electromagnetic interference (EMI), protect switching devices from voltage spikes beyond their rated breakdown voltage, and pass EMC compliance testing in designs like flyback converters, motor drives and switch-mode power supplies. Measure the ringing frequency on the scope, estimate the parasitic capacitance, and it suggests the RC snubber values to start with before you fine-tune on the bench.

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Comments (1)

  • Editorial team

    The resistor value moves inversely with both inputs, so the measurement quality matters. With 100 pF parasitic capacitance the tool gives 79.6 ohm at a 20 MHz ring but 39.8 ohm at 40 MHz, and the capacitor stays at 300 pF. A probe with a long ground lead can shift the ring you read, so measure with a short ground spring. The power rating of the resistor still has to be checked separately. What converter topology are you tuning?

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