MCP8021 datasheet(44/66 Pages) MICROCHIP

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MCP8021/2

DS20006265D-page 44

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The MOSFET body diode reverse recovery situation

occurs when the low-side MOSFETs are turned on

while the motor current is flowing to the positive

source through the high-side MOSFET body diode.

The diode reverse recovery time allows a short circuit

to exist between the positive supply and the low-side

MOSFET drain until the high-side diode is reverse

biased and the reverse recovery time has elapsed.

The first remedies above should be used to slow the

switching speeds of the MOSFETs. Then, a snubber is

added to each MOSFET to fine-tune the phase node

slew rate and eliminate any further transients. Adding

a drain-to-source snubber slows down the slew rate of

the phase node and results in a more controlled excur-

sion of the phase node voltage. The snubber consists

of a resistor and a capacitor connected in series

between the drain and source of the MOSFET. The

resistor is chosen to keep the initial snubber voltage

below a few volts when peak motor current is flowing

through the body diode. The capacitor is then chosen

to provide an RC time constant longer than the

MOSFET body diode reverse recovery time. A 0.1



resistor is typically used, along with a 0.1 μF capacitor

to provide an RC of 10 ns.

The power dissipated by the capacitor is calculated by

applying Equation 5-1.

EQUATION 5-1:

SNUBBER CAPACITOR

POWER DISSIPATION

The capacitor and resistor form factors are chosen to

handle the dissipated power.

5.2.5

MOTOR CURRENT SENSE

CIRCUITRY

A sense resistor in series with the bridge ground return

provides a current signal for feedback. This resistor

should be non-inductive to minimize ringing from high

di/dt. Any inductance in the power circuit represents

potential problems in the form of additional voltage

stress and ringing, as well as increasing switching

times. While impractical to eliminate, careful layout

and bypassing will minimize these effects. The output

stage should be as compact as heat sinking will allow,

with wide, short traces carrying all pulsed currents.

Each half-bridge should be separately bypassed with a

low-ESR/ESL capacitor, decoupling it from the rest of

the circuit. Some layouts will allow the input filter

capacitor to be split into three smaller values and serve

double duty as the half-bridge bypass capacitors.

PDISS = 2    f  C  V2  Dissipation Factor

Where:

f = PWM Frequency

C = Capacitance

V = Motor Voltage

Dissipation Factor = 2

 p  f  C  ESR = ESR/XC

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