PIC18F2585_07 Datasheet, PDF (182/482 Pages) Microchip Technology – Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology

PIC18F2585/2680/4585/4680
FIGURE 16-7:
EXAMPLE OF FULL-BRIDGE APPLICATION
V+
PIC18FX585/X680
P1A
FET
QA
Driver
QC _FET
Driver
P1B
Load
FET
Driver
FET
Driver
P1C
QB
QD
V-
P1D
16.4.5.1 Direction Change in Full-Bridge Mode
In the Full-Bridge Output mode, the EPWM1M1 bit in
the CCP1CON register allows the user 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 TOSC * (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 T2CKPS 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 16-8.
Note that in the Full-Bridge Output mode, the CCP1
module does not provide any dead-band delay. In
general, 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%.
2. The turn-off time of the power switch, including
the power device and driver circuit, is greater
than the turn-on time.
Figure 16-9 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 16-7), 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:
1. Reduce PWM for a 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.
DS39625C-page 180
Preliminary
© 2007 Microchip Technology Inc.