Справочник Пользователя для Motorola MC68HC908MR16
Pulse-Width Modulator for Motor Control (PWMMC)
Output Control
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
Advance Information
MOTOROLA
Pulse-Width Modulator for Motor Control (PWMMC)
149
When complementary operation is used, two additional features are
provided:
provided:
•
Dead-time insertion
•
Separate top/bottom pulse width correction to correct for
distortions caused by the motor drive characteristics
distortions caused by the motor drive characteristics
If independent operation is chosen, each PWM has its own PWM value
register.
register.
9.6.2 Dead-Time Insertion
As shown in
, 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
bottom PWMs in one pair should never be active at the same time. In
, 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.
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.
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.
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
PWM generator into dead-time generators, as shown in
.
Current sensing determines which PWM value of a PWM generator pair
to use for the top PWM in the next PWM cycle. See
to use for the top PWM in the next PWM cycle. See
. When output
control is enabled, the odd OUT bits, rather than the PWM generator
outputs, are fed into the dead-time generators. See
outputs, are fed into the dead-time generators. See
.