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Chapter 14 Pulse Width Modulator with Fault Protection (PMF15B6CV3)
MC9S12ZVM Family Reference Manual Rev. 1.3
532
Freescale Semiconductor
Figure 14-54. Deadtime Distortion
During deadtime, load inductance distorts output voltage by keeping current flowing through the diodes.
This deadtime current flow creates a load voltage that varies with current direction. With a positive current
flow, the load voltage during deadtime is equal to the bottom supply, putting the top transistor in control.
With a negative current flow, the load voltage during deadtime is equal to the top supply putting the bottom
transistor in control.
Remembering that the original PWM pulse widths were shortened by deadtime insertion, the averaged
sinusoidal output will be less than the desired value. However, when deadtime is inserted, it creates a
distortion in motor current waveform. This distortion is aggravated by dissimilar turn-on and turn-off
delays of each of the transistors. By giving the PWM module information on which transistor is controlling
at a given time, this distortion can be corrected.
For a typical circuit in complementary channel operation, only one of the transistors will be effective in
controlling the output voltage at any given time. This depends on the direction of the motor current for that
pair. See
. To correct distortion one of two different factors must be added to the desired PWM
value, depending on whether the top or bottom transistor is controlling the output voltage. Therefore, the
software is responsible for calculating both compensated PWM values prior to placing them in an odd-
numbered/even numbered PWM register pair. Either the odd or the even PMFVAL register controls the
pulse width at any given time. For a given PWM pair, whether the odd or even PMFVAL register is active
depends on either:
•
The state of the current status input, IS, for that driver
•
The state of the odd/even correction bit, IPOLx, for that driver if ICC bits in the PMFICCTL
register are set to zeros
•
The direction of PWM counter if ICC bits in the PMFICCTL register are set to ones
To correct deadtime distortion, software can decrease or increase the value in the appropriate PMFVAL
register.
DESIRED
DEADTIME
PWM TO TOP
POSITIVE
NEGATIVE
PWM TO BOTTOM
POSITIVE CURRENT
NEGATIVE CURRENT
LOAD VOLTAGE
TRANSISTOR
TRANSISTOR
LOAD VOLTAGE
LOAD VOLTAGE
CURRENT
CURRENT
V+