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PIC18F6585/8585/6680/8680

DS30491C-page 182

2004 Microchip Technology Inc.

FIGURE 16-8:

EXAMPLE OF FULL-BRIDGE APPLICATION

16.2.5.1

Direction Change in Full-Bridge Mode

In the Full-Bridge Output mode, the P1M1 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 mod-

ulated 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, depending 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 16-9.

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:

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 16-10 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 16-8) 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.

PIC18FXX80/XX85

P1A

P1C

FET

Driver

FET

Driver

V+

V-

Load

FET

Driver

FET

Driver

P1B

P1D

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