PXS20RM Datasheet, PDF (670/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Flexible Motor Control Pulse Width Modulator Module (FlexPWM)
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 25-22. 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 where 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 the 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 25-22. 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 the VALx registers. Either the VAL2/VAL3 or the VAL4/VAL5 register pair controls the pulse width at
any given time. For a given PWM pair, whether the VAL2/VAL3 or VAL4/VAL5 pair is active depends
on either:
• The state of the current status pin, PWMx, for that driver
• The state of the odd/even correction bit, IPOL, for that driver
To correct deadtime distortion, software can decrease or increase the value in the appropriate VALx
register.
• In edge-aligned operation, decreasing or increasing the PWM value by a correction value equal to
the deadtime typically compensates for deadtime distortion.
25-24
PXS20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor