MC9S12E128CFUE Datasheet, PDF (364/606 Pages) Freescale Semiconductor, Inc – MC9S12E128 Data Sheet

Chapter 11 Pulse Width Modulator with Fault Protection (PMF15B6CV2)
11.4.5.1 Top/Bottom Correction
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 Figure 11-53.
On AC induction motors running open-loop, the distortion typically manifests itself as poor low-speed
performance, such as torque ripple and rough operation.
DESIRED
LOAD VOLTAGE
DEADTIME
PWM TO TOP
TRANSISTOR
V+
POSITIVE
CURRENT
PWM TO BOTTOM
TRANSISTOR
POSITIVE CURRENT
LOAD VOLTAGE
NEGATIVE
CURRENT
NEGATIVE CURRENT
LOAD VOLTAGE
Figure 11-53. 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 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 Figure 11-53. 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.
MC9S12E128 Data Sheet, Rev. 1.07
364
Freescale Semiconductor