SA57254-30GW Datasheet, PDF (14/18 Pages) NXP Semiconductors – CMOS switching regulator (PWM controlled)

Philips Semiconductors
CMOS switching regulator (PWM controlled)
Product data
SA57254-XX
Designing a passive snubber
If the switching power supply is generating too much radio
frequency interference (RFI) a passive snubber can be added.
A passive snubber is a series resistor and capacitor placed across
any component that exhibits a resonant “ringing”. This series R-L-C
loop creates a lossy or damped tank circuit that dissipates the
ringing energy. The design is critical, because it introduces another
loss within the converter.
Designing a snubber is an empirical process, mainly because it
involves undefined parasitic capacitances and inductances
contributed by the PCB layout, leakage inductance, and device
capacitances. The snubber should be placed across the major
source of the spike or ringing which is the output rectifier for a boost
converter (see Figure 24) and the primary winding of the transformer
for a flyback transformer.
The usual design process is:
1. Measure the period of the undesired ringing (T₀).
2. Place a very small ceramic capacitor (about 10 pF) across the
output rectifier or primary winding.
3. Re-measure the period of the undesired ringing. The new period
should be about 3 times that of T₀. If it is less than this, place a
slightly larger value of capacitor across the output rectifier or
primary winding.
4. Once the desired increase in the ringing period is achieved with
a capacitance (C₀), place a resistor in series with the capacitor
whose value is approximately:
R_snubber
^
T₀
2pC₀
Eqn. (10)
This should produce a snubber that does not load the circuit and
introduces a very small loss.
V_BATT
PASSIVE SNUBBER
R
V_OUT
SW
FB
V_DD
SA57254-XX
GND
SL01506
Figure 29. Flyback converter with passive snubber.
2003 Nov 11
14