Datasheet

 2011 Microchip Technology Inc. DS39932D-page 265
PIC18F46J11 FAMILY
18.5.6 PROGRAMMABLE DEAD-BAND
DELAY MODE
In half-bridge applications, where all power switches
are modulated at the PWM frequency, the power
switches normally require more time to turn off than to
turn on. If both the upper and lower power switches are
switched at the same time (one turned on and the other
turned off), both switches may be on for a short period
until one switch completely turns off. During this brief
interval, a very high current (shoot-through current) will
flow through both power switches, shorting the bridge
supply. To avoid this potentially destructive
shoot-through current from flowing during switching,
turning on either of the power switches is normally
delayed to allow the other switch to completely turn off.
In Half-Bridge mode, a digitally programmable
dead-band delay is available to avoid shoot-through
current from destroying the bridge power switches. The
delay occurs at the signal transition from the non-active
state to the active state. See Figure 18-16 for
illustration. The lower seven bits of the associated
ECCPxDEL register (Register 18-3) sets the delay
period in terms of microcontroller instruction cycles
(T
CY or 4 TOSC).
FIGURE 18-16: EXAMPLE OF
HALF-BRIDGE PWM
OUTPUT
FIGURE 18-17: EXAMPLE OF HALF-BRIDGE APPLICATIONS
Period
Pulse Width
td
td
(1)
PxA
(2)
PxB
(2)
td = Dead-Band Delay
Period
(1) (1)
Note 1: At this time, the TMR2 register is equal to the
PR2 register.
2: Output signals are shown as active-high.
PxA
PxB
FET
Driver
FET
Driver
V+
V-
Load
+
V
-
+
V
-
Standard Half-Bridge Circuit (“Push-Pull”)