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PIC18F2X1X/4X1X

DS39636D-page 146

2009 Microchip Technology Inc.

FIGURE 15-7:

EXAMPLE OF FULL-BRIDGE APPLICATION

15.4.5.1

Direction Change in Full-Bridge Mode

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

CCP1CON register allows 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 15-8.

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:

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 15-9 shows an example where the PWM

direction 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 15-7), 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:

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.

P1A

P1C

FET

Driver

FET

Driver

V+

V-

Load

FET

Driver

FET

Driver

P1B

P1D

PIC18F4X1X

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