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Question:

How do you decide the resistor and capacitor value of a snubber to reduce switch-node ringing of a step-down DC/DC converter?

Answer:

You can estimate the value of snubber components by using Equation 1 with the ringing frequency (f_{Spike}) from the switching node waveform and the output capacitance (C_{OSS}) of the low-side FET.

Equation 1

C_{SNB} ≥ K × C_{OSS}, R_{SNB} = (0.1~0.3) / f_{Spike}×C_{OSS}

Where:

C_{SNB}: Snubber capacitance (F)

C_{OSS}: Low-side FET output capacitance (F)

K: Coefficient (The approximate value of K is from 4 to 8. If a higher value is selected, the spike voltage of the switching node is reduced. However, power dissipation by the snubber circuit will increase.)

RSNB: Snubber resistance (Ω)

fSpike: Ringing frequency of switching node (Hz)

An example calculation is shown here.

Condition: C_{OSS }= 80 pF, K = 4, f_{Spike} = 240 MHz

*C*_{SNB} = 4 × 80 pF = 320pF, *R _{SNB}* = 0.2 / 240

*MHZ*×80PF = 10Ω

A ceramic capacitor is the ideal choice for a snubber circuit.

Calculate the rated voltage of the snubber capacitor using Equation 2.

Equation 2

VCSNB ≥ VIN

Where:

VCSNB: Rated voltage of the snubber capacitor (V)

VIN:Power supply voltage of the step-down converter (V)

Calculate the rated power dissipation of the snubber resistor using Equation 3.

Equation 3:

*P _{RSNB}* ≥

*V*×

_{IN}^{2}*C*×

_{OSS}*f*

_{SW}PRSNB: Rated power dissipation of the snubber resistor (W)

VIN: Power supply voltage of the step-down converter (V)

COSS: Low-side FET output capacitance (F)

fSW: Switching frequency of the step-down DC/DC converter (Hz)

Snubber components should be placed near the low-side FET to reduce ESL between the components and drain-source terminals of the low-side FET.

An example of snubber connection for DD3V of S6BP502A (Step-down DC/DC Converter) is shown here.

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