A novel concept for the analysis of planar devices with a field limiting ring(FLR) is presented which allows analytic expressions in a normalized form for the potential distributions of FLR structure. Based on the method of image charges the main and ring junctions with identical cylindrical edges are kept to be two different equipotential surfaces. The potential relations between main and ring junction of the FLR structure are compared with 2-dimensional device simulation program. MEDICI. A good accordance is found. Comparisions with experimental data reported for the optimum ring spacing and the relative improvement of the breakdowm voltages in the FLR sturcture show the validity of the concept. The normalized expressions allow a universal application regardless to the junction depths and background doping levels.