AN2009 datasheet(31/39 Pages) STMICROELECTRONICS

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

Figure 20. Acceleration of demagnetization during low side switch PWM OFF time

In our example, during step

Σ4 (switches T2-T3 are ON), the VMB voltage on the MB winding

was positive. At the beginning of step

Σ5 (switches T4-T3 are ON), the voltage on terminal B

will be tied to 0V and the shortest demagnetization configuration is the one that provides the

highest voltage at point M to apply the maximum reverse voltage and decrease the demagnet-

ization time.

It is easy to verify that the highest voltage (2HV/3) is obtained when the PWM is applied only

on the low side switch T3. During the OFF time of this PWM signal (configuration shown in

Figure 20), the voltage at point M is 2HV/3. Otherwise if the PWM signal is applied on the high

side switch T4, then during the OFF time of the PWM signal, the voltage at point M is 0V. So,

in this configuration, the PWM signal has to be applied on the low side switch T3 after the com-

mutation event in order to accelerate the demagnetization time during the OFF time of this

PWM signal.

7.2 ACCELERATION OF DEMAGNETIZATION WHEN PWM IS APPLIED ON THE HIGH

SIDE SWITCH

Figure 21 shows step

Σ3 (switches T2-T1 are ON) with the normal current and the free-

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

PWM

Normal

current

Demagnetization

current

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