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

Version: *B

 

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.

Equation 1

    Untitled.png

     Where:

CSNB: Snubber capacitance (F)

COSS: Low-side FET output capacitance (F)

          COSS of the switching FET internal Cypress automotive PMIC is shown below.

Part Number

COSS1

Note

S6BP201A, S6BP202A, S6BP203A

80 pF

Parasitic capacitance between LX1 - PGND1. PVIN=12 V.

S6BP401A: DD1, DD3, DD4

76 pF

Parasitic capacitance between LXx - PGNDx. PVCCx=5 V. (Where: x=1, 3 and 4.)

S6BP401A: DD2

100 pF

Parasitic capacitance between LX2 – PGND2. PVCC2=5 V.

S6BP501A, S6BP502A: DD1V

72 pF

Parasitic capacitance between LX1V – PGND1V. PVCC1V=3.3 V.

1Simulated standard value.

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 ()

fSpike: Ringing frequency from the switching node waveform (Hz)

 

An example calculation is shown below.

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

Untitled.png

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

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

Equation 2

     Untitled.png

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:

         Untitled.png

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 the equivalent series inductance (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 below.