INTEDG datasheet(230/478 Pages) MICROCHIP

230 / 478 page

PIC16(L)F1946/47

DS41414D-page 230

 2010-2012 Microchip Technology Inc.

23.4.2.1

Direction Change in Full-Bridge

Mode

In the Full-Bridge mode, the PxM1 bit in the CCPxCON

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 PxM1 bit of the CCPxCON register. The following

sequence occurs four Timer cycles prior to the end of

the current PWM period:

• The modulated outputs (PxB and PxD) are placed

in their inactive state.

• The associated unmodulated outputs (PxA and

PxC) are switched to drive in the opposite

direction.

• PWM modulation resumes at the beginning of the

next period.

See Figure 23-12 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 23-13 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 PxA and

PxD become inactive, while output PxC 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 23-10) 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 23-12:

EXAMPLE OF PWM DIRECTION CHANGE

Pulse Width

Period(1)

Signal

Note 1: The direction bit PxM1 of the CCPxCON register is written any time during the PWM cycle.

2: When changing directions, the PxA and PxC signals switch before the end of the current PWM cycle. The

modulated PxB and PxD signals are inactive at this time. The length of this time is four Timer counts.

Period

(2)

PxA (Active-High)

PxB (Active-High)

PxC (Active-High)

PxD (Active-High)

Pulse Width

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