AN4192 datasheet(37/47 Pages) STMICROELECTRONICS

37 / 47 page

AN4192

Low-side FET selection

Doc ID 023820 Rev 1

37/47

3.5

RC snubber network settings

In high frequency converters, designers must choose the right electric topology and device

in order to minimize the switching noise. In particular, when the HS turns on, the LS body

diode that was delivering the whole load current must be in the HOLD state. Due to the body

diode reverse recovery process and stray inductances, an overshoot may appear on the

phase node voltage, affecting the MOSFET reliability. Furthermore, the positive and

negative spikes on the phase node waveforms are also dangerous for other electronic

components surrounding the LS FET.

The simplest way to reduce these switching oscillations is by connecting an RC snubber

network across the LS FET. Theoretically, a simple capacitive snubber would be useful to

smooth the switching noise; but, a series resistance is added to decrease the current

amplitude during the capacitor discharging and to dissipate the energy at turn-off (when an

overvoltage event occurs).

Figure 46.

LS FET with RC snubber network

The right RSNUB and CSNUB selection depends on several factors: overshoot reduction,

damping effect and energy losses in the snubber circuit:

Typically, the snubber resistor is equal to the characteristic impedance of the freely

oscillating circuit L-C:

Equation 8

where Lstray includes the PCB and package inductances. Its value can be extrapolated by

the phase node waveform, measuring the ringing oscillation fRING (Figure 47) and using (9):

AM16489V1

---

SNUB

⋅⋅

SNUB

stray

oss

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