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

Version 27

    Version: *B

     

    Translation - Japanese: ステップダウンDC/DCコンバーターのスナバ回路の設計方法 - KBA222118 - Community Translated (JA)

     

    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

    Untitled.png

    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.