PIC16F631 datasheet(141/294 Pages) MICROCHIP

141 / 294 page

DS41262D-page 139

PIC16F631/677/685/687/689/690

11.4.2.1

Direction Change in Full-Bridge

Mode

In the Full-Bridge mode, the P1M1 bit in the CCP1CON

direction. When the application firmware changes this

direction control bit, the module will change to the new

direction on the next PWM cycle.

A direction change is initiated in software by changing

the P1M1 bit of the CCP1CON register. The following

sequence occurs four Timer2 cycles prior to the end of

the current PWM period:

• The modulated outputs (P1B and P1D) are placed

in their inactive state.

• The associated unmodulated outputs (P1A and

P1C) are switched to drive in the opposite

direction.

• PWM modulation resumes at the beginning of the

next period.

See Figure 11-12 for an illustration of this sequence.

The Full-Bridge mode does not provide dead-band

delay. As one output is modulated at a time, dead-band

delay is generally not required. There is a situation

where dead-band delay is 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 11-13 shows an example of the PWM direction

changing from forward to reverse, at a near 100% duty

cycle. In this example, at time t1, the output P1A and

P1D become inactive, while output P1C becomes

active. Since the turn off time of the power devices is

longer than the turn on time, a shoot-through current

will flow through power devices QC and QD (see

Figure 11-10) 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, two possible solutions for eliminating

the shoot-through current are:

Reduce PWM duty cycle for one 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.

FIGURE 11-12:

EXAMPLE OF PWM DIRECTION CHANGE

Pulse Width

Period(1)

Signal

Note 1:

The direction bit P1M1 of the CCP1CON register is written any time during the PWM cycle.

When changing directions, the P1A and P1C signals switch before the end of the current PWM cycle. The

modulated P1B and P1D signals are inactive at this time. The length of this time is four Timer2 counts.

Period

(2)

P1A (Active-High)

P1B (Active-High)

P1C (Active-High)

P1D (Active-High)

Pulse Width

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