PIC16F631/677/685/687/689/690
The Full-Bridge mode does not provide dead-band
delay. As one output is modulated at a time, dead-band
delay is generally not required. There is a situation
where dead-band delay is required. This situation
occurs when both of the following conditions are true:
11.4.2.1
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.
1. The direction of the PWM output changes when
the duty cycle of the output is at or near 100%.
2. The turn off time of the power switch, including
the power device and driver circuit, is greater
than the turn on time.
A direction change is initiated in software by changing
the P1M1 bit of the CCP1CON register. The following
sequence occurs four Timer2 cycles prior to the end of
the current PWM period:
Figure 11-13 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 output P1A and
P1D become inactive, while output P1C 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 11-10) for the duration of ‘t’. The same
phenomenon will occur to power devices QA and QB
for PWM direction change from reverse to forward.
• 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.
See Figure 11-12 for an illustration of this sequence.
If changing PWM direction at high duty cycle is required
for an application, two possible solutions for eliminating
the shoot-through current are:
1. Reduce PWM duty cycle for one PWM period
before changing directions.
2. 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 11-12:
EXAMPLE OF PWM DIRECTION CHANGE
(1)
Period
Period
Signal
P1A (Active-High)
P1B (Active-High)
Pulse Width
P1C (Active-High)
P1D (Active-High)
(2)
Pulse Width
Note 1: The direction bit P1M1 of the CCP1CON register is written any time during the PWM cycle.
2: 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 four Timer2 counts.
© 2007 Microchip Technology Inc.
DS41262D-page 139
MICROCHIP [ MICROCHIP ]
