MIC2155 datasheet(27/35 Pages) MICREL

27 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

One method of reducing the ringing is to use a resistor

and capacitor to lower the Q of the resonant circuit. The

circuit in Figure 23 shows the resistor in between the

switch node and ground. Capacitor Cs is used to block

DC and minimize the power dissipation in the resistor.

This capacitor value should be between 5 and 10 times

the parasitic capacitance of the MOSFET COSS. A

capacitor that is too small will have high impedance and

prevent the resistor from damping the ringing. A

capacitor that is too large causes unnecessary power

dissipation in the resistor, which lowers efficiency.

The snubber components should be placed as close as

possible to the low-side MOSFET and/or external

schottky diode since in contributes to most of the stray

capacitance. Placing the snubber too far from the FET or

using etch that is too long or thin will add inductance to

the snubber and diminishes its effectiveness.

A proper snubber design requires the parasitic

inductance and capacitance be known. A method of

determining these values and calculating the damping

resistor value is outlined below.

1. Measure the ringing frequency at the switch node

which is determined by parasitic LP and CP. Define this

frequency as f1.

2. Add a capacitor CS (normally at least 3 times as big as

the COSS of the FET) from the switch node to ground and

measure the new ringing frequency. Define this new

(lower) frequency as f2. LP and CP can now be solved

using the values of f1, f2 and CS.

3. Add a resistor RS in series with

to generate critical

damping.

Step 1:

First measure the ringing frequency on the

switch node voltage when the high-side MOSFET turns

on. This ringing is characterized by the equation:

Where:

CP and LP are the parasitic capacitance and

inductance

Step 2:

Add a capacitor, CS, in parallel with the

synchronous MOSFET, Q2. The capacitor value should

be approximately 3 times the COSS of Q2. Measure the

frequency of the switch node ringing, f2:

)

(

Define f’ as:

Combining the equations for f1, f2 and f’ to derive CP, the

parasitic capacitance:

)

(

−

LP is solved by re-arranging the equation for f1:

()

)

(

Step 3:

Calculate the damping resistor.

Critical damping occurs at Q = 1:

Solving for RS

Figure 23 shows the snubber in the circuit and the

damped switch node waveform.

RDS

LSTRAY1

LSTRAY2

LSTRAY3

LSTRAY4

COSS2

Figure 23. Snubber Circuit

MIC2155 Datasheet(PDF) 27 Page - Micrel Semiconductor

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