AN2009 datasheet(29/39 Pages) STMICROELECTRONICS

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PWM MANAGEMENT FOR 3-PHASE BLDC MOTOR DRIVES USING THE ST7MC

7 WINDING DEMAGNETIZATION

When the microcontroller switches from one step to the next, the non-powered winding needs

a certain demagnetization time. During this time, the current in the winding continues in the

same direction but decreases to zero. It is not possible to see the BEMF voltage as long as

this current is still present, because during this time the voltage on the winding is tied to 0V or

HV by the free-wheeling diode.

Due to the winding self-induction, the demagnetization time decreases if the reverse voltage

on the winding increases. We want to decrease the demagnetization time of the winding so

that the window to detect the BEMF zero voltage crossing event is wider. So the goal to de-

crease this time and to accelerate the end of demagnetization event is to apply the maximum

reverse voltage on the winding that needs to be demagnetized. To do so, depending on the

previous step configuration as we are going to describe in this section, we can apply either the

PWM on the low side switch or on the high side switch to accelerate the demagnetization.

Let’s look at two examples to illustrate the two possible cases:

7.1 ACCELERATION OF DEMAGNETIZATION WHEN PWM IS APPLIED ON THE LOW

SIDE SWITCH

Figure 18 shows step

Σ4 (switches T2-T3 are ON) with the normal current and the free-

wheeling current (dotted line) during the PWM OFF time of the high side switch T2.

Figure 18. Step

Σ4 (switches T2-T3 are ON)

Normal

current

free-wheeling current

for BEMF reading (ST method)

AN2009 Datasheet(PDF) 29 Page - STMicroelectronics