PIC18F23K20-ESLASHML datasheet(185/456 Pages) MICROCHIP

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DS41303G-page 185

PIC18F2XK20/4XK20

16.4.6

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 of time until one switch completely turns off.

During

this

brief

interval,

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.

Half-Bridge

mode,

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 16-12 for

illustration. The lower seven bits of the associated

PWM1CON register (Register 16-3) sets the delay

period in terms of microcontroller instruction cycles

(TCY or 4 TOSC).

FIGURE 16-12:

EXAMPLE OF

HALF-BRIDGE PWM

OUTPUT

FIGURE 16-13:

EXAMPLE OF HALF-BRIDGE APPLICATIONS

Period

Pulse Width

(1)

P1A(2)

P1B(2)

td = Dead-Band Delay

Period

(1)

Note 1:

At this time, the TMR2 register is equal to the

PR2 register.

Output signals are shown as active-high.

P1A

P1B

FET

Driver

FET

Driver

V+

V-

Load

Standard Half-Bridge Circuit (“Push-Pull”)

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