How to Design a Snubber Circuit for Step-Down DC/DC Converter – KBA222118

Version: *A

 

Question:

How do I 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 (fSpike) from the switching node waveform and the output capacitance (COSS) of the low-side FET.

    Equation 1

    CSNB   ≥ K ×  COSS, RSNB =

(0.1~0.3)

fspike×COSS

Where:

CSNB: Snubber capacitance (F)

COSS: 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)

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An example calculation is shown here.

Condition: COSS = 80 pF, K = 4, fSpike = 240 MHz

    CSNB = 4 × 80 pF = 320pF, RSNB

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:

PRSNBVIN 2 × CSNB × fSW

    PRSNB: Rated power dissipation of the snubber resistor (W)

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

    CSNB: Snubber 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.