Freescale Semiconductor MC68HC908MR32 - Manual (Page 130)

Pulse-Width Modulator for Motor Control (PWMMC)
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
130 Freescale Semiconductor
Figure 12-17. Dead-Time and Small Pulse Widths
12.5.3 Top/Bottom Correction with Motor Phase Current Polarity Sensing
Ideally, when complementary pairs are used, the PWM pairs are inversions of each other, as shown in
Figure 12-18 . When PWM1 is active, PWM2 is inactive, and vice versa. In this case, the motor terminal
voltage is never allowed to float and is strictly controlled by the PWM waveforms.
Figure 12-18. Ideal Complementary Operation (Dead-Time = 0)
However, when dead-time is inserted, the motor voltage is allowed to float momentarily during the
dead-time interval, creating a distortion in the motor current waveform. This distortion is aggravated by
dissimilar turn-on and turn-off delays of each of the transistors.
3
3
3
3 3
3
UP/DOWN COUNTER
MOUDULUS = 3
PWM VALUE = 2 PWM VALUE = 3 PWM VALUE = 2 PWM VALUE = 1
PWM1 W/
NO DEAD-TIME
PWM2 W/
NO DEAD-TIME
PWM1 W/
DEAD-TIME = 3
PWM2 W/
DEAD-TIME = 3
UP/DOWN COUNTER
MODULUS = 4
PWM VALUE = 1
PWM1
PWM2
PWM VALUE = 2
PWM3
PWM4
PWM VALUE = 3
PWM5
PWM6