MIC2174 datasheet(17/27 Pages) MICREL

17 / 27 page

Micrel, Inc.

MIC2174/MIC2174C

September 2010

M9999-091310-C

OUT

D(avg)

30ns

(26)

The reverse voltage requirement of the diode is:

HSD

DIODE(rrm)

The power dissipated by the Schottky diode is:

D(avg)

DIODE

(27)

where, VF = forward voltage at the peak diode current.

The external Schottky diode is not necessary for the

circuit operation since the low-side MOSFET contains a

parasitic body diode. The external diode will improve

efficiency and decrease the high frequency noise. If the

MOSFET body diode is used, then it must be rated to

handle the peak and average current. The body diode

has a relatively slow reverse recovery time and a

relatively high forward voltage drop. The power lost in

the diode is proportional to the forward voltage drop of

the diode. As the high-side MOSFET starts to turn on,

the body diode becomes a short circuit for the reverse

recovery period, dissipating additional power. The diode

recovery and the circuit inductance will cause ringing

during the high-side MOSFET turn-on.

An external Schottky diode conducts at a lower forward

voltage preventing the body diode in the MOSFET from

turning on. The lower forward voltage drop dissipates

less power than the body diode. The lack of a reverse

recovery mechanism in a Schottky diode causes less

ringing and less power loss. Depending upon the circuit

components and operating conditions, an external

Schottky diode will give a 1/2% to 1% improvement in

efficiency.

Snubber Design

A snubber is used to damp out high frequency ringing

caused by parasitic inductance and capacitance in the

buck converter circuit. Figure 5 shows a simplified

schematic of the buck converter. Stray capacitance

consists

mostly

the

two

MOSFETs’

output

capacitance (COSS). The stray inductance consists

mostly package inductance and trace inductance. The

arrows show the resonant current path when the high

side MOSFET turns on. This ringing causes stress on

the semiconductors in the circuit as well as increased

EMI.

COUT

CIN

LSTRAY1

LSTRAY2

LSTRAY3

LSTRAY4

Sync_buck

Controller

VDC

COSS1

COSS2

–

Figure 5. Output Parasitics

One method of reducing the ringing is to use a resistor

and capacitor to lower the Q of the resonant circuit, as

shown in Figure 6. Capacitor CS is used to block DC and

minimize the power dissipation in the resistor. This

capacitor value should be between two and ten 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.

RDS

LSTRAY1

LSTRAY2

LSTRAY3

LSTRAY4

COSS2

Figure 6. Snubber Circuit

The snubber components should be placed as close as

possible to the low-side MOSFET and/or external

Schottky diode since it contributes to most of the stray

capacitance. Placing the snubber too far from the FET or

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

the snubber and diminishes its effectiveness.

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