Philips Semiconductors

Product data

SA57254-XX

CMOS switching regulator (PWM controlled)

2003 Nov 11

14

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:

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

slightly larger value of capacitor across the output rectifier or

primary winding.

4. Once the desired increase in the ringing period is achieved with

whose value is approximately:

R

T

0

2

pC

0

Eqn. (10)

This should produce a snubber that does not load the circuit and

introduces a very small loss.

SL01506

SA57254-XX

GND

FB

SW

R

PASSIVE SNUBBER

Figure 29. Flyback converter with passive snubber.