Dell now seems to use 2 types of barrel connectors, the older ones are 7.4mm and the newer ones are 4.5mm, usually for the thinner (e.g. XPS, Precision 55xx) laptops. Fortunately, you can get 4.5mm-to-7.4mm converter dongles and velcro them to the older power cords. As a side note, Dell power supplies also come in flavors of 130W, 180W and 230W. The 230W power brick is a monster.