Enhanced Pulse-Width Modulation (PWM) Dead-Band Control

In Half-Bridge or Full-Bridge Enhanced Pulse-Width Modulation (PWM) applications, if both the upper and lower power circuits are switched at the same time (one turned on, and the other turned off), due to a delay in the circuitry, both switches may be on for a short period of time. During this brief interval, a very high current (shoot-through current) will flow through both power switches, shorting the bridge supply.

To avoid this potentially destructive shoot-through current from flowing during switching, Dead-Band Delay can be implemented to allow one current path to completely turn off before the other current path is turned on.

The delay occurs at the signal transition from the non-active state to the active state and the delay is adjustable.

The lower seven bits of the associated PWMxCON register set the delay period in units of instruction clock cycles (Fosc/4).

For example, a device running at 4 Mhz will have a 1 Mhz instruction clock or 1 microsecond period.

Therefore the value in the PWMxCON register will create a delay between pulses in increments of 1 microsecond.

The accompanying image shows the Dead-Band Delay graphically as the value td.