PIC16F684-ESLASHST datasheet(85/164 Pages) MICROCHIP

85 / 164 page

 2004 Microchip Technology Inc.

Preliminary

DS41202C-page 83

PIC16F684

FIGURE 11-9:

EXAMPLE OF FULL-BRIDGE APPLICATION

11.3.5.1

Direction Change in Full-Bridge

Mode

In the Full-bridge Output mode, the P1M1 bit

(CCP1CON<7>)

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, depending on the

value of the T2CKPS<1:0> bits (T2CON<1:0>). During

the interval from the switch of the unmodulated outputs

to the beginning of the next period, the modulated out-

puts (P1B and P1D) remain inactive. This relationship is

shown in Figure 11-10.

Note that in the Full-bridge Output mode, the ECCP

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 11-11 shows an example where the PWM

direction 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 11-9) 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 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.

PIC16F684

P1A

P1C

FET

Driver

FET

Driver

V+

V-

Load

FET

Driver

FET

Driver

P1B

P1D

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