Motor drive topologies

AN235

12/23

Doc ID 1679 Rev 2

set by the voltage reference and the value of the sense resistor so that each time the current

reaches the set peak value the switch is turned off for the remainder of the period. Since the

only losses in this technique are the saturation loss of the switch and the resistive loss in the

sense resistor and coil resistance, the total efficiency is very high.

The average current drawn from the power supply is less than the winding current due to the

chopping. When the transistor is on, current is being drawn from the supply, however, during

the off-time the current is recirculating locally and there is no current from the power supply.

This type of phase current control, that has to be done separately for each motor phase,

leads to the best ratio between the supplied electrical and delivered mechanical energy.

Figure 10.

Switch mode drive implementations

It would make no sense to apply the same principle to a current controlled unipolar circuit,

as additional switches for each phase would be necessary for the shortening out of the

windings during the off-time and thus the number of components would be similar to the

bipolar drive. Moreover, there would be the previously discussed torque disadvantage.

Rc

Lc

Vs

Vref

OSC

S

R

Q

Rsense

Ic = Vref/Rsense

Fixed frequency PWM

Current control

L297

L6506

Rc

Lc

Vs

Vref

T

_

Q

Rsense

Ic = Vref/Rsense

Constant off time FM

Current control

Monostable

L6207, L6208

L6219, PBL3717

AM16508v1