If you're hitting your irons straight, but you're still not getting the kind of explosive power you want, you're probably not creating enough lag in your downswing. Lag is produced when you maintain the angle between your arms and the clubshaft almost all the way down to impact. Here's how to do it.
Grab an iron by the hosel and make the widest backswing possible while keeping both arms straight. Stop when you feel tension in the left side of your torso. Now drop your hands down toward your right pocket as fast as you can. If you do it right, you should notice immediately how your swing arc transitions from wide in the backswing to narrow in the downswing, and how this move produces maximum wrist hinge.
Practice with the club upside-down until you groove the motion, then flip it over and make actual practice swings. If you find yourself hitting behind the ball, you're not swinging your hands and arms fast enough in your transition. The faster you swing your hands, the more your wrists will hinge and the more lag you'll create.