AN280

Snubber design considerations

Doc ID 1703 Rev 3

9/15

4

Snubber design considerations

The function of the snubber network is to limit the rate of change of the voltage across the

motor (output terminals of the L6203) when one of the DMOS devices is turned off. Using

the RC snubbing circuit shown in Figure 9, the rate of change of the voltage on the output is

dominated by the capacitor while the resistor is used primarily to limit the peak current

flowing through the power transistor when it turns on.

Figure 9.

RC snubber circuit on output

The time constant of the motor current is much longer than the switching time, due to the

inductance of the motor. At the time of switching the DC motor can be assumed to be a

constant current generator equal to the peak current at switching. If this current is switched

into the snubber, the voltage across the snubber network jumps to a value equal to the

snubber resistance times the motor current. After the initial step, the rate of change is limited

by the motor current charging the snubber capacitor. To properly size the snubber network

the resistor is selected such that the maximum motor current produces a voltage less than

the minimum power supply voltage. If the resistor is larger than this value, the snubber is

ineffective since the capacitor doesn’t limit the voltage rise until the voltage has become

greater than the power supply. For the design example, the maximum resistance for the

snubber is given by the equation:

Equation 2

The snubber capacitor is calculated from the peak current and the target rise time. The

capacitance is given by the equation:

Equation 3

AM15176v1

R

max

V

smin

I

peak

---------------

38V

5A

-----------

7.6 ohm

==

=

C

Ipeak

dt

dv

------

5A

150ns

50V

-----------------

0.015

μF

==

=