Microchip Technology DV164136 Data Sheet
PIC18F8722 FAMILY
DS39646C-page 198
© 2008 Microchip Technology Inc.
FIGURE 18-7:
EXAMPLE OF FULL-BRIDGE APPLICATION
18.4.5.1
Direction Change in Full-Bridge Mode
In the Full-Bridge Output 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
assume the new direction on the next PWM cycle.
CCP1CON register allows users to control the forward/
reverse direction. When the application firmware
changes this direction control bit, the module will
assume the new direction on the next PWM cycle.
Just before the end of the current PWM period, the
modulated outputs (P1B and P1D) are placed in their
inactive state, while the unmodulated outputs (P1A and
P1C) are switched to drive in the opposite direction.
This occurs in a time interval of (4 T
modulated outputs (P1B and P1D) are placed in their
inactive state, while the unmodulated outputs (P1A and
P1C) are switched to drive in the opposite direction.
This occurs in a time interval of (4 T
OSC
* (Timer2
Prescale Value)) before the next PWM period begins.
The Timer2 prescaler will be either 1, 4 or 16, depend-
ing on the value of the T2CKPSx bit (T2CON<1:0>).
During the interval from the switch of the unmodulated
outputs to the beginning of the next period, the
modulated outputs (P1B and P1D) remain inactive.
This relationship is shown in Figure 18-8.
The Timer2 prescaler will be either 1, 4 or 16, depend-
ing on the value of the T2CKPSx bit (T2CON<1:0>).
During the interval from the switch of the unmodulated
outputs to the beginning of the next period, the
modulated outputs (P1B and P1D) remain inactive.
This relationship is shown in Figure 18-8.
Note that in the Full-Bridge Output mode, the ECCP1
module does not provide any dead-band delay. In gen-
eral, since only one output is modulated at all times,
dead-band delay is not required. However, there is a
situation where a dead-band delay might be required.
This situation occurs when both of the following
conditions are true:
module does not provide any dead-band delay. In gen-
eral, since only one output is modulated at all times,
dead-band delay is not required. However, there is a
situation where a dead-band delay might be required.
This situation occurs when both of the following
conditions are true:
1.
The direction of the PWM output changes when
the duty cycle of the output is at or near 100%.
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.
the power device and driver circuit, is greater
than the turn-on time.
Figure 18-9 shows an example where the PWM direc-
tion changes from forward to reverse at a near 100%
duty cycle. At time, t1, the outputs P1A and P1D
become inactive, while output P1C becomes active. In
this example, since the turn-off time of the power
devices is longer than the turn-on time, a shoot-through
current may flow through power devices QC and QD
(see Figure 18-7) for the duration of ‘t’. The same
phenomenon will occur to power devices QA and QB
for PWM direction change from reverse to forward.
tion changes from forward to reverse at a near 100%
duty cycle. At time, t1, the outputs P1A and P1D
become inactive, while output P1C becomes active. In
this example, since the turn-off time of the power
devices is longer than the turn-on time, a shoot-through
current may flow through power devices QC and QD
(see Figure 18-7) 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, one of the following requirements
must be met:
for an application, one of the following requirements
must be met:
1.
Reduce PWM for a PWM period before
changing directions.
changing directions.
2.
Use switch drivers that can drive the switches off
faster than they can drive them on.
faster than they can drive them on.
Other options to prevent shoot-through current may
exist.
exist.
PIC18F6X27/6X22/8X27/8X22
P1A
P1C
FET
Driver
Driver
FET
Driver
Driver
V+
V-
Load
FET
Driver
Driver
FET
Driver
Driver
P1B
P1D
QA
QB
QD
QC