MC68HC708MP16 Datasheet, PDF (144/398 Pages) Freescale Semiconductor, Inc – Microcontrollers

Pulse Width Modulator for Motor Control (PWMMC)
When complementary operation is used, two additional features are
provided:
• Dead-time insertion
• Separate top/bottom pulse width correction to correct for
distortions caused by the motor drive characteristics.
If independent operation is chosen, each PWM has its own PWM value
register.
9.6.2 Dead-Time Insertion
As shown in Figure 9-12, in complementary mode, each PWM pair can
be used to drive top-side/bottom-side transistors.
When controlling DC-to-AC inverters such as this, the top and bottom
PWMs in one pair should never be active at the same time. In Figure 9-
12, if PWM1 and PWM2 were on at the same time, large currents would
flow through the two transistors as they discharge the bus capacitor. The
IGBTs could be weakened or destroyed.
Simply forcing the two PWMs to be inversions of each other is not always
sufficient. Since a time delay is associated with turning off the transistors
in the motor drive, there must be a “dead-time” between the deactivation
of one PWM and the activation of the other.
A dead-time can be specified in the dead-time write-once register. This
8-bit value specifies the number of CPU clock cycles to use for the dead-
time. The dead-time is not affected by changes in the PWM period
caused by the prescaler.
Dead-time insertion is achieved by feeding the top PWM outputs of the
PWM generator into dead-time generators, as shown in Figure 9-13.
Current sensing determines which PWM value of a PWM generator pair
to use for the TOP PWM in the next PWM cycle. (See 9.6.3 Top/Bottom
Correction.) When output control is enabled, the odd OUT bits, rather
than the PWM generator outputs, are fed into the dead-time generators.
(See 9.6.5 Output Port Control.)
Technical Data
144
MC68HC708MP16 — Rev. 3.1
Pulse Width Modulator for Motor Control (PWMMC) Freescale Semiconductor