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
Where:
C_{SNB}: Snubber capacitance (F)
C_{OSS}: Low-side FET output capacitance (F)
C_{OSS} of the switching FET internal Cypress automotive PMIC is shown below.
Part Number | C_{OSS}^{1} | 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. |
^{1}Simulated 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.)
R_{SNB}: Snubber resistance (Ω)
f_{Spike}: Ringing frequency from the switching node waveform (Hz)
An example calculation is shown below.
Condition: C_{OSS} = 80 pF, K = 4, f_{Spike} = 240 MHz
A ceramic capacitor is the ideal choice for a snubber circuit.
Calculate the rated voltage of the snubber capacitor using Equation 2.
Equation 2
Where:
V_{CSNB}: Rated voltage of the snubber capacitor (V)
V_{IN}: 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}: Rated power dissipation of the snubber resistor (W)
V_{IN}: Power supply voltage of the step-down converter (V)
C_{SNB}: Snubber capacitance (F)
f_{SW}: 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.
Comments