Snubbers and clamps

Intent

• Tell a clamp, which limits the peak voltage and absorbs the leakage energy, from a snubber, which damps the ringing.
• Find a ringing node’s parasitic capacitance and inductance from two measurements, and size an RC snubber from them.
• Estimate what a clamp and a snubber dissipate.

Theory

Clamps

When a flyback’s switch turns off, its leakage inductance keeps the current flowing and drives the drain above the input voltage plus the reflected output voltage (reflected voltage) until a clamp limits it [Kollman 2024] (clamped switch voltage). The clamp absorbs more than the leakage energy, and the closer its voltage is to the reflected voltage, the longer the leakage current takes to reset (reset time) and the more it dissipates (clamp power) [Kollman 2024]. An RCD clamp’s voltage settles where its resistor dissipates the clamp’s power (RCD clamp voltage). A TVS clamps at its breakdown voltage plus its dynamic resistance times the current (TVS clamping voltage) [STMicroelectronics 2022]. The clamp check sizes both.

Ringing and its parasitics

When a switch or a diode stops conducting, the node’s parasitic inductance and capacitance ring. Their values follow from the ringing frequency measured twice: as built, and with a known capacitor added across the node [Nexperia 2024]:

$$C_\mathrm{par} = \frac{C_\mathrm{add}}{\left(\frac{f_{r0}}{f_{r1}}\right)^{2} - 1}$$

$C_\mathrm{par}$

parasitic capacitance of the node

$C_\mathrm{add}$

capacitance added for the measurement

$f_{r0}$

ringing frequency as built

$f_{r1}$

ringing frequency with C_add

Parasitic capacitance from two ringing frequencies✓ verified by 8 tests

Assumptions, convention and source

Assumptions

• undamped L-C resonance
• the added capacitor does not change the inductance that rings

Notes

an added capacitance comparable to $C_\mathrm{par}$ lowers the frequency clearly (adding three times $C_\mathrm{par}$ halves it); a much smaller one changes it too little to measure

Source

Nexperia 2024, Sec. 3 (Determining C_LK and L_LK): the ringing frequency before and after a known capacitor is added; derived here

Derivation

step-by-step sympy derivation

Equation id

snub.C_par · equations.yaml entry

$$L_\mathrm{par} = \frac{1}{4 \pi^{2} f_{r0}^{2} C_\mathrm{par}}$$

$L_\mathrm{par}$

parasitic (ringing) inductance

$C_\mathrm{par}$

parasitic capacitance of the node

$f_{r0}$

ringing frequency as built

Parasitic inductance of a ringing node✓ verified by 8 tests

Assumptions, convention and source

Assumptions

• undamped L-C resonance

Source

Nexperia 2024, Sec. 3 (Determining C_LK and L_LK): the inductance from the ringing frequency and the capacitance; derived here

Derivation

step-by-step sympy derivation

Equation id

snub.L_par · equations.yaml entry

The RC snubber

A resistor in series with a capacitor, across the node, damps the ring. AN11160 sizes the two in turn [Nexperia 2024]:

• The capacitor. One to two times $C_\mathrm{par}$ is a good starting point, a compromise between the loss and the peak voltage. It must also be large enough to take up the parasitic inductance’s energy, and small enough to discharge within the on-time.
• The resistor. From the damping factor of the parasitic inductance, the resistor and the snubber capacitor in series. A damping factor between 0.5 and 1 gives the least peak voltage; the best value in that range is found by trying a few.

$$R_\mathrm{snub} = 2 \zeta \sqrt{\frac{L_\mathrm{par}}{C_\mathrm{snub}}}$$

$R_\mathrm{snub}$

snubber resistance

$C_\mathrm{snub}$

snubber capacitance

$L_\mathrm{par}$

parasitic (ringing) inductance

$\zeta$

damping factor

RC snubber resistance for a damping factor✓ verified by 8 tests

Assumptions, convention and source

Assumptions

• undamped L-C resonance

Notes

AN11160 takes the damping factor of the parasitic inductance, the resistor and the snubber capacitor in series, and sets it between 0.5 and 1 for the least peak voltage: the resistor lies between $\sqrt{L_\mathrm{par}/C_\mathrm{snub}}$ and twice that, and the best value in that range is found by trials. The node's own capacitance is left out of this damping factor

Source

Nexperia 2024, Sec. 2 (Designing the snubber - theory): the damping factor and the range of the snubber resistor; Sec. 4: the best value by trials

Derivation

step-by-step sympy derivation

Equation id

snub.R · equations.yaml entry

Every edge charges or discharges the snubber capacitor. The loss this adds does not depend on the resistor; the parasitic inductance’s energy is lost with or without a snubber [Nexperia 2024]:

$$P = C_\mathrm{snub} V_\mathrm{snub}^{2} f_s$$

$P$

power

$C_\mathrm{snub}$

snubber capacitance

$V_\mathrm{snub}$

voltage swing at each edge

$f_s$

switching frequency

Loss an RC snubber adds✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• the snubber capacitor charges through its resistor to V at one edge and discharges through it at the other, once per switching period

Notes

charging the snubber capacitor through its resistor loses as much energy as it stores, ½ C V², and the next edge discharges it, through the resistor and the switch that turns on, losing the stored energy: C V² per period, whatever the resistance. The parasitic inductance's energy is lost with or without a snubber. A larger snubber capacitor lowers the peak voltage the best resistor can reach, and loses more

Source

Nexperia 2024, Sec. 2 (Designing the snubber - theory): the average power loss due to the snubber; derived here; Erickson & Maksimović 2020, Ch. 4 (Switch Realization: the energy of a capacitance charged at each switching edge is dissipated)

Derivation

step-by-step sympy derivation

Equation id

snub.P · equations.yaml entry

A larger capacitor lowers the peak voltage the best resistor can reach, and loses more: a snubber is a compromise between damping and loss [Nexperia 2024].

The resistor needs its own rating. At the edge that charges the capacitor it takes as much energy as the capacitor stores, and the parasitic inductance’s energy as well; AN11160 rates it from these [Nexperia 2024]:

$$P_{R,\mathrm{snub}} = \frac{f_s \left(C_\mathrm{snub} V_\mathrm{snub}^{2} + L_\mathrm{par} I_\mathrm{ring}^{2}\right)}{2}$$

$P_{R,\mathrm{snub}}$

dissipation in the snubber resistor

$C_\mathrm{snub}$

snubber capacitance

$I_\mathrm{ring}$

current when the ring starts

$L_\mathrm{par}$

parasitic (ringing) inductance

$V_\mathrm{snub}$

voltage swing at each edge

$f_s$

switching frequency

Dissipation in an RC snubber's resistor✓ verified by 8 tests

Assumptions, convention and source

Assumptions

• the edge that charges the snubber capacitor, once per switching period: the node steps by V_snub while the parasitic inductance carries I_ring; the node's own capacitance is left out (AN11160)

Notes

AN11160 rates the resistor from this edge: the resistor takes as much energy as the capacitor stores and the parasitic inductance's energy. That can exceed the loss the snubber adds (`snub.P`), which leaves out the inductance's energy because it is lost with or without a snubber. Where the switch that turns on discharges the capacitor through the resistor, that edge heats the resistor too, with up to the capacitor's stored energy, and its rating must allow for it

Source

Nexperia 2024, Sec. 4 (Designing the snubber - in practice): the power dissipation in the snubber resistor; Sec. 2: the energy lost in the resistor at turn-off; derived here

Derivation

step-by-step sympy derivation

Equation id

snub.P_R · equations.yaml entry

Where the switch that turns on discharges the capacitor through the resistor, that edge heats the resistor too, with up to the capacitor’s stored energy.

Worked example

An example ringing node and its snubber:

RC snubber example

Symbol	Value	How it is obtained
$f_{r0}$ ringing frequency as built	20 MHz	given
$f_{r1}$ ringing frequency with Cadd	10 MHz	given
$C_\mathrm{add}$ capacitance added for the measurement	150 pF	given
$C_\mathrm{snub}$ snubber capacitance	100 pF	given
$\zeta$ damping factor	0.75	given
$V_\mathrm{snub}$ voltage swing at each edge	100 V	given
$f_s$ switching frequency	100 kHz	given
$I_\mathrm{ring}$ current when the ring starts	500 mA	given
$C_\mathrm{par}$ parasitic capacitance of the node	50 pF	Parasitic capacitance from two ringing frequencies
$L_\mathrm{par}$ parasitic (ringing) inductance	1.27 µH	Parasitic inductance of a ringing node
$R_\mathrm{snub}$ snubber resistance	169 Ω	RC snubber resistance for a damping factor
$P$ power	100 mW	Loss an RC snubber adds
$P_{R,\mathrm{snub}}$ dissipation in the snubber resistor	65.8 mW	Dissipation in an RC snubber's resistor

The ring falls from 20 MHz to 10 MHz with 150 pF added, so the node’s own capacitance is 50 pF and its inductance 1.27 µH. A snubber capacitor of 100 pF, twice $C_\mathrm{par}$, and a damping factor of 0.75, the middle of the range, give a resistor of 169 Ω. On a node that swings through 100 V at 100 kHz, the snubber adds a loss of 100 mW. With 500 mA in the parasitic inductance when the ring starts, the resistor dissipates 65.8 mW at the edge that charges the capacitor, and more if the other edge discharges the capacitor through it.

Try it

▶ Open in the equation explorer: Parasitic capacitance from two ringing frequencies
Preset from the example rc-snubber

Sweep $f_{r1}$ towards $f_{r0}$: a small drop in frequency gives a large $C_\mathrm{par}$, and a small error in either frequency a large error in it. Add enough capacitance to lower the frequency clearly.

The clamp check sizes a flyback’s TVS or RCD clamp from its leakage inductance and peak current.

Bench exercise

Probe the switch node with a short ground spring and read the ringing frequency after the switching edge. Add a C0G capacitor of a few times the expected parasitic capacitance, close to the device, and read the frequency again. Compute $C_\mathrm{par}$ and $L_\mathrm{par}$, choose the snubber capacitor and the resistor for a damping factor in the range, and try a few resistors around it. Build the snubber with a resistor rated for its dissipation (snubber resistor dissipation), and check the peak voltage and the resistor’s temperature.

Gotchas

• Judge a TVS by its clamping voltage at the real current. Its clamping voltage is its breakdown voltage plus its dynamic resistance times the current, not the breakdown voltage alone [STMicroelectronics 2022].
• A clamp voltage close to the reflected voltage wastes power. The clamp then takes much more than the leakage energy [Kollman 2024].
• The snubber’s loss is not its resistor’s rating. The loss the snubber adds is set by the capacitor, the voltage swing and the switching frequency, whatever the resistor [Nexperia 2024]; the resistor also takes the parasitic inductance’s energy (snubber resistor dissipation).

Go deeper

• Application note Designing RC snubbers (AN11160) — Nexperia · retrieved 2026-09-24
  How to find a ringing node's parasitic capacitance and inductance with an added capacitor, then choose the RC snubber's capacitor and its resistor from a damping factor, trading peak voltage against loss.
• Technical article Snubbing the flyback converter (SSZTCV6) — R. Kollman, Texas Instruments · retrieved 2026-09-24
  How a flyback's leakage inductance drives the switch above the reflected voltage, and how the clamp voltage trades against the clamp's loss.
• Book chapter Switch Realization (Ch. 4) — R. W. Erickson, D. Maksimović · retrieved 2026-09-23
  How transistors and diodes realize the ideal switches, and where the switching losses come from.

Quiz

1. A node rings at 20 MHz, and at 10 MHz with $150\,\mathrm{pF}$ added across it. What is its own capacitance?

   1. $150\,\mathrm{pF}$
   2. $50\,\mathrm{pF}$
   3. $75\,\mathrm{pF}$
   4. $37.5\,\mathrm{pF}$
   Answer and explanation

   Correct answer: B Halving the frequency needs four times the capacitance, so the added 150 pF is three times the node's own.

2. How does AN11160 choose an RC snubber's resistor?

   1. As small as possible
   2. As large as possible
   3. For a damping factor between 0.5 and 1 of the parasitic inductance, the resistor and the snubber capacitor in series, then by trying a few values
   4. Equal to the load resistance
   Answer and explanation

   Correct answer: C A damping factor between 0.5 and 1 gives the least peak voltage; the best value in that range is found by trials.

3. An RC snubber of $220\,\mathrm{pF}$ sees a 100 V swing at each edge, at 100 kHz. What loss does it add?

   1. $0.22\,\mathrm{W}$
   2. $0.11\,\mathrm{W}$
   3. $22\,\mathrm{mW}$
   4. $2.2\,\mathrm{W}$
   Answer and explanation

   Correct answer: A Charging and discharging the snubber capacitor each lose half of C V squared: C V squared per period, times the frequency.

4. How does the snubber resistor's value change the snubber's loss?

   1. A larger resistor lowers it
   2. It does not: the capacitor, the voltage swing and the frequency set it
   3. A smaller resistor lowers it
   4. The loss is in the capacitor, not the resistor
   Answer and explanation

   Correct answer: B Charging a capacitor through a resistor loses half of C V squared whatever the resistance, and the next edge loses the stored half.

5. What snubber capacitor does AN11160 take as a starting point?

   1. A tenth of the node's own capacitance
   2. A hundred times the node's own capacitance
   3. One to two times the node's own capacitance
   4. As large as the output capacitor
   Answer and explanation

   Correct answer: C A compromise: a larger capacitor lowers the peak voltage the best resistor can reach, but the loss grows with it.

6. In the TVS model, a TVS with a 150 V breakdown voltage and a dynamic resistance of $0.5\,\Omega$ carries 10 A. What does it clamp at?

   1. $155\,\mathrm{V}$
   2. $150\,\mathrm{V}$
   3. $145\,\mathrm{V}$
   4. $160\,\mathrm{V}$
   Answer and explanation

   Correct answer: A The breakdown voltage plus the dynamic resistance times the current: 150 V plus 5 V.

7. What happens to a flyback clamp's loss if its voltage is set just above the reflected voltage?

   1. It falls to the leakage energy per period
   2. It rises far above the leakage energy per period
   3. It becomes zero
   4. It does not change
   Answer and explanation

   Correct answer: B The leakage current then takes long to reset, and the clamp keeps drawing energy from the transformer meanwhile.

8. At the edge that charges the snubber capacitor, AN11160 has the resistor take as much energy as the capacitor stores. What else does it count in the resistor's rating?

   1. The load's power
   2. Nothing else
   3. The parasitic inductance's energy when the ring starts
   4. The gate-drive energy
   Answer and explanation

   Correct answer: C The energy in the parasitic inductance when the ring starts goes into the resistor as well, so the resistor can dissipate more than the loss the snubber adds.