Motor drive topologies

AN235

10/23

Doc ID 1679 Rev 2

4

Motor drive topologies

For a stepper motor, the motor current is determined primarily by the drive voltage and the

motor impedance (resistance and inductance). A simple and popular drive topology is to

the current as shown in Figure 7. For simplicity, the single FET in Figure 7, Figure 9 and

Figure 10 represents either a FET and clamping diode for the unipolar motor drive or a

composite of the two diagonal switches of a full-bridge the bipolar drive. A typical motor with

5 V and 1 A on its name plate means that with a 5 V drive the resulting current is 1 A,

corresponding to having a 5

Ω coil resistance.

Figure 7.

Simple L/R drive

As already discussed, the torque of the motor is, among others, proportional to the winding

current. In the full-step sequence, the motor changes the polarity of the winding current in

the same stator winding every two steps. The rate at which the current changes its direction,

in the form of an exponential function, depends on the winding inductance, the coil

higher speeds, the polarity of the stator current is changed more often and the current no

longer reaches its saturating value because of the limited change time. Clearly, the peak

and the area under the current waveform diminish with increasing step rate, reducing torque

and power. This is shown in Figure 8 B.

Rc

Lc

Vs

Ic = Vs/Rc

t = Lc/Rc

AM16505v1