PIC18F6X2X datasheet(170/386 Pages) MICROCHIP

170 / 386 page

PIC18F6X2X/8X2X

DS39612A-page 168

Advance Information

 2003 Microchip Technology Inc.

FIGURE 17-7:

EXAMPLE OF FULL-BRIDGE APPLICATION

17.4.5.1

Direction Change in Full-Bridge Mode

In the Full-Bridge Output mode, the P1M1 bit in the

CCP1CON

allows

user

control

the

forward/reverse direction. When the application firm-

ware 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 17-8.

Note that in the Full-Bridge Output mode, the CCP1

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:

The direction of the PWM output changes when

the duty cycle of the output is at or near 100%.

The turn off time of the power switch, including

the power device and driver circuit, is greater

than the turn on time.

Figure 17-9 shows an example where the PWM direc-

tion changes from forward to reverse at a near 100%

duty cycle. At time t1, the output 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 17-7) for the duration of ‘t’. The same phenom-

enon 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:

Reduce PWM for a PWM period before

changing directions.

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.

PIC18F6X2X/8X2X

P1A

P1C

FET

Driver

FET

Driver

V+

V-

Load

FET

Driver

FET

Driver

P1B

P1D

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