AN2009 Datasheet, PDF (30/39 Pages) STMicroelectronics – PWM MANAGEMENT FOR 3-PHASE BLDC MOTOR DRIVES USING THE ST7MC

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
HV
T0
D0 T2
D2 T4
D4
B
Normal mode
magnetization
current
A
M
C
F
T3
D3 T5
D5 T1
D1
PWM on T2
free wheeling
demagnetization
current
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.
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