The pulse phaser cannon, unlike a conventional phaser array, fired bolts of energy rather then a beam discharge. This points to a considerably different design. They were more powerful than a normal phaser, due to the destructive energy being delivered onto the target all at once, instead of a steady beam like a conventional phaser, yet seem to have shorter range than traditional, ship mounted phaser arrays.
The weapon was substantially more effective than conventional phaser arrays at piercing shielded objects, and was originally designed to defend against a Borg attack. (DS9: "The Search, Part I", "Defiant")
One of the first Starfleet ships to use such weapons was the Defiant-class. These arrays seem to be mounted in pairs on the port and starboard sides of the ship, and are fired in unison.