mc9s12e256 Freescale Semiconductor, Inc, mc9s12e256 Datasheet - Page 360

• Manufacturer Part Number: mc9s12e256
• Description: Hcs12 Microcontrollers 16-bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

Chapter 11 Pulse Width Modulator with Fault Protection (PMF15B6CV2)
11.4.5.1
In complementary mode, either the top or the bottom transistor controls the output voltage. However,
deadtime has to be inserted to avoid overlap of conducting interval between the top and bottom transistor.
Both transistors in complementary mode are off during deadtime, allowing the output voltage to be
determined by the current status of load and introduce distortion in the output voltage. See
On AC induction motors running open-loop, the distortion typically manifests itself as poor low-speed
performance, such as torque ripple and rough operation.
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 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
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.
360
Figure
Top/Bottom Correction
NEGATIVE CURRENT
POSITIVE CURRENT
11-53. To correct distortion one of two different factors must be added to the desired PWM
PWM TO BOTTOM
LOAD VOLTAGE
LOAD VOLTAGE
LOAD VOLTAGE
PWM TO TOP
TRANSISTOR
TRANSISTOR
DEADTIME
DESIRED
Figure 11-53. Deadtime Distortion
MC9S12E256 Data Sheet, Rev. 1.08
V+
CURRENT
NEGATIVE
CURRENT
POSITIVE
Freescale Semiconductor
Figure
11-53.