PIC18F87K22 Datasheet, PDF (269/548 Pages) Microchip Technology – 64/80-Pin, High-Performance, 1-Mbit Enhanced Flash Microcontrollers with 12-Bit A/D and nanoWatt XLP Technology

PIC18F87K22 FAMILY
20.4.2.1 Direction Change in Full-Bridge
Mode
In Full-Bridge mode, the PxM1 bit in the CCPxCON
register allows users to control the forward/reverse
direction. When the application firmware changes this
direction control bit, the module will change to the new
direction on the next PWM cycle.
A direction change is initiated in software by changing
the PxM1 bit of the CCPxCON register. The following
sequence occurs prior to the end of the current PWM
period:
• The modulated outputs (PxB and PxD) are placed
in their inactive state.
• The associated unmodulated outputs (PxA and
PxC) are switched to drive in the opposite
direction.
• PWM modulation resumes at the beginning of the
next period.
For an illustration of this sequence, see Figure 20-10.
The Full-Bridge mode does not provide a dead-band
delay. As one output is modulated at a time, a
dead-band delay is generally not required. There is a
situation where a dead-band delay is required. This
situation occurs when both of the following conditions
are true:
• The direction of the PWM output changes when
the duty cycle of the output is at or near 100%.
• The turn-off time of the power switch, including
the power device and driver circuit, is greater than
the turn-on time.
Figure 20-11 shows an example of the PWM direction
changing from forward to reverse, at a near 100% duty
cycle. In this example, at time, t1, the PxA and PxD
outputs become inactive, while the PxC output
becomes active. Since the turn-off time of the power
devices is longer than the turn-on time, a shoot-through
current will flow through power devices, QC and QD
(see Figure 20-8), for the duration of ‘t’. The same
phenomenon will occur to power devices, QA and QB,
for PWM direction change from reverse to forward.
If changing PWM direction at high duty cycle is required
for an application, two possible solutions for eliminating
the shoot-through current are:
• Reduce PWM duty cycle for one PWM period
before changing directions.
• Use switch drivers that can drive the switches off
faster than they can drive them on.
Other options to prevent shoot-through current may
exist.
FIGURE 20-10:
Signal
EXAMPLE OF PWM DIRECTION CHANGE
Period⁽¹⁾
Period
PxA (Active-High)
PxB (Active-High)
PxC (Active-High)
PxD (Active-High)
Pulse Width
(2)
Pulse Width
Note 1:
2:
The direction bit, PxM1 of the CCPxCON register, is written any time during the PWM cycle.
When changing directions, the PxA and PxC signals switch before the end of the current PWM cycle. The
modulated PxB and PxD signals are inactive at this time. The length of this time is:
(1/FOSC) • TMR2 Prescale Value.
 2010 Microchip Technology Inc.
Preliminary
DS39960B-page 269