PIC18F45K80-I Datasheet, PDF (277/622 Pages) Microchip Technology – 28/40/44/64-Pin, Enhanced Flash Microcontrollers with ECAN and nanoWatt XLP Technology

PIC18F66K80 FAMILY
20.4.2.1 Direction Change in Full-Bridge
Mode
In the Full-Bridge mode, the P1M1 bit in the CCP1CON
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 P1M1 bit of the CCP1CON register. The following
sequence occurs prior to the end of the current PWM
period:
• The modulated outputs (P1B and P1D) are placed
in their inactive state.
• The associated unmodulated outputs (P1A and
P1C) 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 P1A and P1D
outputs become inactive, while the P1C 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
P1A (Active-High)
P1B (Active-High)
P1C (Active-High)
P1D (Active-High)
Pulse Width
(2)
Pulse Width
Note 1:
2:
The direction bit, P1M1 of the CCP1CON register, is written any time during the PWM cycle.
When changing directions, the P1A and P1C signals switch before the end of the current PWM cycle. The
modulated P1B and P1D signals are inactive at this time. The length of this time is:
(1/FOSC) • TMR2 Prescale Value.
 2010-2012 Microchip Technology Inc.
DS39977F-page 277