We designed two different types of explosively-driven ferromagnetic generators based on NdFeB magnets; cylinder- and annulus-types. We investigated the output characteristics of those generators while varying design features; length and diameter of magnets, pitch or turns of generator coils, and inductance of loads. We performed hydrodynamic simulations extensively for shockwave propagation in magnets for both types of generators. Based on experimental and calculational results, we optimized the designs to maximize the energy output. The maximum energy obtained was 0.9 J with an energy conversion efficiency of 6.7% for the cylinder-type generator with magnets of 50 mm in diameter and 25 mm in length, and 1.5 J with an efficiency of 9.4% for the annulus-type generators with magnets of 50 mm in diameter and 40 mm in length.