AN2009 datasheet(30/39 Pages) STMICROELECTRONICS

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

When switching from step

Σ4 (switches T2-T3 are ON) to step Σ5 (switches T4-T3 are ON), a

free-wheeling, decreasing demagnetization current is still present through the B terminal

(dashed line) and an increasing magnetization current appears through the C terminal (dotted

line). Figure 19 shows these two currents during the PWM ON time.

Figure 19. Step

Σ5 (switches T4-T3 are ON) during the demagnetization PWM ON time

This means, during the PWM ON time:

– The potential of B is zero

– The potential of M is close to HV/3

Now depending on the switch we are going to apply the PWM signal, the demagnetization

time of winding B will be decreased and the end of demagnetization event will be accelerated

during this PWM signal OFF time. The numerous dedicated registers in the ST7MC peripheral

allow the flexibility to choose where to apply the PWM signal between the different events. The

maximum reverse voltage to decrease the demagnetization time is achieved during the OFF

time of the PWM signal.

PWM on T2

free wheeling

demagnetization

current

Normal mode

magnetization

current

AN2009 Datasheet(PDF) 30 Page - STMicroelectronics